To size gravity rainwater outlets for a flat roof, calculate design flow using Q = A × r × C. For an impermeable surface in Ireland, TGD Part H assumes 75mm per hour, equal to 0.0208 litres per second per square metre, and the run-off coefficient is normally 1.0. Multiply effective catchment by 0.0208, then select enough drainage points to exceed that flow at the manufacturer's tested head of water. Check the arrangement again with one blocked, and calculate emergency provision separately. Design Assumptions Used Throughout This Guide Input Assumption Jurisdiction Republic of Ireland Drainage type Gravity, not siphonic Roof surface Impermeable Primary intensity 75mm per hour Run-off coefficient 1.0 Capacity basis Tested discharge at stated head Resilience check One primary path unavailable Emergency provision Separate project-specific calculation Get this wrong, and the consequence is not a damp patch. Standing water weighs a tonne per cubic metre, and a deck holding 50mm across 200 square metres carries ten tonnes it was never designed for. Outlet sizing is one of the most important technical decisions on a flat roofing project, yet it frequently gets settled by habit rather than arithmetic: two per bay, 100mm bore, done. What follows is the method, the Irish inputs, and the tested evidence that shows why bore alone tells you almost nothing. The Calculation In Short Standard requirement. Everything rests on one equation from I.S. EN 12056-3, the standard TGD Part H points to for gravity roof drainage: Q = A × r × C Q is design flow in litres per second. A is effective catchment in square metres. The letter r represents design rainfall intensity in litres per second per square metre. C is the run-off coefficient, taken as 1.0 for impermeable surfaces, so it quietly drops out of most sums. The arithmetic is never the difficulty. Getting A and r right, then matching the answer to products whose tested capacity you can evidence. Effective catchment Design flow at 75mm per hour 50 m² 1.04 l/s 100 m² 2.08 l/s 200 m² 4.17 l/s 300 m² 6.25 l/s 400 m² 8.33 l/s 500 m² 10.42 l/s 1,000 m² 20.83 l/s These figures show the total flow for the entire surface only. They do not select anything on their own, because selection depends on tested capacity at the available head. Working Through It, Step By Step Step One: Establish The Effective Catchment Plan footprint is the starting point, not the finished answer. Standard requirement. Annex NC of the standard recommends that 50% of exposed vertical area counts towards catchment, up to a maximum height of 10 metres above the surface being drained. Include: The drained plan footprint Half the exposed area of walls rising above and shedding onto it, to 10 metres. Plant enclosures, lift overruns and stair cores Any upper terrace or balcony discharging onto the surface below Adjoining pitched planes above 70 degrees, treated as vertical Miss the wall contribution, and you have sized on footprint alone. On stepped and courtyard buildings, that omission is routinely the difference between a system that copes and one that does not. Step Two: Choose The Design Intensity Irish regulatory guidance. TGD Part H paragraph 1.1.3.2 assumes 75mm per hour for roof surfaces, dropping to 50mm per hour for paved areas. That suits ordinary situations. Project engineer decision. Raise it where failure would be expensive or dangerous, meaning internal drainage above occupied space, no safe overflow route, or sensitive contents below. The standard structures this through four design categories based on storm return period, so a longer return period buys headroom. Step Three: Convert To A Design Flow Divide millimetres per hour by 3,600 for litres per second per square metre. So 75 becomes 0.0208. Multiply by catchment. A Worked Example Assumptions: effective catchment 400 m²; intensity 0.0208 l/s/m²; coefficient 1.0; four primary drainage points; blocked case assumes three operating. The design flow is 400 × 0.0208, which is 8.33 litres per second in total. Across four positions, that is 2.08 l/s each in normal service. With one blocked, the remaining three carry 2.78 l/s each. Manufacturer recommendation. Alumasc advises adding a 10% factor of safety rather than designing to absolute capacities, which lifts the requirement to 3.06 l/s per operating position. The answer is therefore not "100mm". It is that each operating position must pass at least 3.06 l/s at the design head, which the next section shows several nominally 100mm products cannot do. Step Four: Read The Tested Discharge Data Capacity is not a function of bore. It depends on the head of water above the grating, on whether discharge runs vertically or turns horizontally, on grating pattern, and on whether the arrangement is gravity or siphonic. So the question is never "what size do I need?" It is "at what head does this deliver 3.06 litres per second, and where was that tested?" Step Five: Check Falls, Sumps And The Drained Result Standard requirement. A correctly sized product in a badly formed sump still ponds. BS 6229:2025 sets a minimum 1:80 finished fall for general areas and internal gutters, and introduced defined sump dimension limits so drainage points do not sit in flat plateaus of their own making. Our post on what the BS 6229 revision changed for Irish projects covers the wider design implications. Working to a live drawing? QBM's technical team can check the calculation against currently available products before your schedule is issued. Call +353 1 839 1170. Why Nominal Bore Does Not Tell You Capacity Here is the evidence. Every row below is nominally 100mm. All values come from Alumasc's published flow-rate sheet FR001DR, tested at Salford University's Department of Civil Engineering, quoted at a 35mm head and held within one-third of rainwater pipe capacity, as the standard requires. Configuration Code Tested at 35mm head Vertical spigot AV400 10.71 l/s 45 degree spigot 445 10.66 l/s 90 degree spigot 490 7.19 l/s Insulated vertical spigot 1001 6.53 l/s Two-way vertical 4TW 6.00 l/s Two-way horizontal 4TW 2.05 l/s The spread runs from 2.05 to 10.71 litres per second. That is a factor of more than five across products a schedule would describe identically. Against our worked requirement of 3.06 l/s, the vertical spigot passes with enormous margin while the two-way horizontal fails outright, despite sharing a nominal size. Two details deserve particular attention. The 35mm test head matches the design head the standard adopts for flat roofs, so these figures are directly comparable to your calculation. And insulated bodies carry roughly 39% less than their uninsulated equivalents, which matters because deep warm deck build-ups need them. If you are working above the deck, budget for that reduction at design stage rather than discovering it at order stage. The sheet carries a November 2010 issue date. Treat the pattern as instructive and confirm current figures against a live datasheet before ordering. How Many, And Where They Go Deciding the number Manufacturer recommendation. Alumasc advises incorporating a backup drainage path to prevent failure due to blockage, even where a single position would carry the flow. QBM recommendation. Provide at least two drainage paths per isolated bay wherever blockage of one could create structural or internal flooding risk. Confirm the final arrangement with the project engineer against the applicable standard. Beyond that, the count follows from three things pulling against each other: flow per position, the fall geometry you can build, and the drainage layout below the deck. Position them at low points the falls actually create Verify those low points by survey on any refurbishment Keep clear of upstands, parapet junctions and rooflight kerbs Allow safe access for clearing, since anything unreachable stops being maintained Confirm each sits over a viable pipework route beneath Placing them on the roof Falls get set out first, positions derived from them, rather than the reverse. Tapered insulation makes this straightforward, since falls are built into the boards rather than into the structure. Watch the thin point: schemes that taper down towards drainage often thin below the required thermal performance exactly where water collects. Standard requirement. Backfalls are not acceptable in any circumstance. On refurbishment, a level survey comes first, because decks deflect and a fall drawn at 1:80 twenty years ago may now run backwards. Overflows, The Second LINE Of Defence Standard requirement. Clause 7.4 of I.S. EN 12056-3 covers emergency outlets, so this is not an optional extra that could overload the structure if blocked. Sizing The Secondary System Common practice, not a cited requirement. Emergency provision is frequently sized at roughly double the primary intensity, and the secondary invert is often set around 50mm above the primary invert. Both figures are widely used, but I have not been able to source either to a clause, so treat them as a starting point for the project engineer to confirm rather than a rule. The structural check on water depth is the part that genuinely governs. That height difference does give you something useful. Water discharging from an overflow indicates the primary route is blocked right now, is visible from ground level, and is reportable the same day. Discharge point is where a soaking gets noticed, not somewhere hidden The route through parapet or wall does not compromise the waterproofing Structural capacity is verified for the water depth the secondary invert allows Provision exists for every drained bay, including small isolated ones Matching Products To Deck And Membrane Deck Type Bodies, clamping rings and fixings differ by substrate. Harmer Roof AV vertical outlets suit in-situ cast concrete, timber and lightweight metal deck construction. Confirm before ordering: Deck material and thickness at the penetration Insulation depth, since deep build-ups need insulated bodies Whether discharge runs vertically or must turn Clearance beneath for pipework and any trap Structural provision for the penetration itself An uninsulated body dropped through 160mm of insulation is a thermal weak point with a hole through the middle, and it surfaces years later as damp around the sump. Membrane Compatibility Manufacturer requirement. The clamping arrangement must suit whatever it seals against. Harmer roof outlets are designed for connection to continuous waterproofing systems using single-ply membranes, mastic asphalt, high-performance built-up felt, hot-melt, and wet-applied systems. Where detailing gets awkward, liquid systems can dress around bodies that sheet materials struggle to follow. Sizing Mistakes That Surface Later Five that recur: Sizing on plan footprint while ignoring walls shedding onto the surface Treating nominal bore as capacity, without reading tested data at head No allowance for a blocked position in the primary calculation Emergency provision omitted, or discharging where nobody will see it Tapered scheme thinning below required thermal performance at the drainage point None of these is visible at handover. All of them are visible three winters later. Frequently Asked Questions How to make a drainage system for a flat roof? Set falls first, at a minimum finished 1:80 under BS 6229:2025, then locate collection points at the resulting low points. Calculate the design flow from the catchment and intensity, select components with published, tested capacity to suit, and add emergency provision. Confirm the components suit both the deck and the membrane. QBM supplies drainage elements, tapered insulation, and waterproofing as a single compatible package, with technical support for selection. What is the angle for a flat roof runoff? BS 6229:2025 requires a minimum finished fall of 1:80, roughly 0.7 degrees, measured on the completed surface rather than the drawing. Because decks deflect and tolerances accumulate, designers work to a steeper design fall so the finished result still achieves it; 1:60 is a common starting point. Backfalls are never acceptable. Tapered insulation is the usual way to build falls in, and QBM can arrange a scheme against your survey. What is the formula for roof drain sizing? Design flow Q, in litres per second, equals effective catchment A in square metres multiplied by design rainfall intensity r in litres per second per square metre, multiplied by run-off coefficient C. For impermeable surfaces, C is 1.0. Irish guidance assumes 75mm per hour, giving r of 0.0208. Match the result to tested discharge at the available head. QBM can supply the manufacturer performance data behind that selection. How many drains are needed for a flat roof? At least two per isolated bay, so no single blockage floods the structure; the manufacturer makes the same recommendation even where one would carry the flow. Beyond that, divide the total design flow by the tested capacity at the available head, add a 10% safety factor, then recheck, assuming one is blocked. Fall geometry and pipework layout usually further constrain the answer. Ask QBM to run it against current components. Products And Technical Support QBM supplies Harmer rainwater outlets across vertical, detail, two-way, balcony, and insulated configurations, and b/s/t flat roof drainage systems and accessories. A manufacturer working in flat roof accessories since 1962, it offers a free calculation service for emergency drainage plus an online tool for the discharge capacity of gargoyles and emergency overflows. Because the full range also covers insulation, cover boards and membranes, compatibility across the whole build-up can be confirmed from one source rather than assembled from three suppliers' assumptions. Talk to QBM before the schedule is issued Send through: Roof plan with drained bays and any walls shedding onto them Deck type and build-up depth Membrane or finish already specified Whether emergency provision has been designed Email info@qbm.ie, call +353 1 839 1170, or use the enquiry form. Related Article Standing Seam vs Profiled Metal Sheet Roofing Vapour Control Layers Explained: When You Need One Warm Roof vs Cold Roof Construction: What Irish Specifiers Should Decide First Fire Ratings for Flat Roof Systems: BROOF(t4) and Reaction to Fire Explained

A vapour control layer is normally required on the warm side of insulation wherever warm, moisture-laden indoor air could reach a cold surface and condense inside the construction. It may be omitted only where the complete build-up, the airtightness strategy and a condensation risk calculation together demonstrate that it is unnecessary. Standards checked during preparation: BS 5250:2021, BS EN 13984, BS 6229:2025, I.S. EN ISO 13788, EN 1931. Project-specific condensation analysis may still be required. A common assumption on site is that vapour control layers are always required. The codes are more careful than that, and specifying one by reflex can occasionally harm, particularly in solid masonry. What follows covers what the membrane does, how to read the numbers, and where the decision changes across the build-ups QBM supplies. What Is A Vapour Control Layer? A thin sheet material used on the warm side of insulation to reduce moisture flow into the structure; in most build-ups, it also forms the primary airtight line. Products are covered by BS EN 13984, the product standard for plastic and rubber vapour control layers. Position and use fall under BS 5250:2021. Strictly, nothing is a true barrier. Every material allows some moisture to pass, which is why the older term vapour barrier' has largely given way to VCL in current practice. Both names still describe the same thing: a high-resistance sheet, usually polythene or foil-faced, installed inboard of the insulation. What does a VCL do? Two jobs, and they get confused all the time. Limiting vapour diffusion Moisture migrates through materials from warm humid conditions towards cold dry ones. A high-resistance sheet slows that migration so vapour never reaches a surface below dew point inside the fabric. Restricting moisture-laden air leakage Bulk air movement carries far more moisture than diffusion ever will. A draught through an unsealed lap deposits more water in one night than diffusion manages in a season. Because polythene and foil resist both air and moisture, the same sheet does both jobs, which is why specifications increasingly say AVCL, air and vapour control layer. When is a VCL required? The general rule: in any heated, insulated building where internal air can reach a cold interface within the construction. That covers the overwhelming majority of Irish flat roofing and framed wall work. The exceptions are real but narrow, and each needs a calculation rather than an opinion. They are set out further down. Where does it go? Warm deck flat roofing System requirement. A warm deck will normally need one directly on the structural deck, beneath the insulation. Final specification should follow the system manufacturer's requirements and the project condensation risk assessment. Our post on warm-roof and cold-roof construction sets out the position for each arrangement. Sits on the deck, below the insulation, never above Laps sealed to the manufacturer's stated overlap Turned up and terminated at every upstand and parapet Sealed around outlets, rooflight kerbs and service penetrations Fixings passing through it accounted for in the risk calculation Cold Deck Flat Roofing Also normally required, at ceiling level, and this is exactly where it tends to fail. Installed at fit-out and then perforated by services, the sheet ends up compromised where continuity matters most. Position at ceiling level, warm side of the quilt Plan the sealing detail for services before fit-out begins Ventilated void above must remain clear Access hatches need perimeter sealing Treat recessed light fittings as a design problem, not an afterthought Rainscreen And Façade Build-ups Here the picture inverts. A ventilated rainscreen wall normally wants a permeable membrane outboard of the insulation and, depending on the frame, a VCL inboard. Confirm whether the airtight line sits internally or externally Identify which interface is coldest and most at risk Coordinate with the rainscreen insulation build-up Verify reaction to fire classification for that wall Allow for cladding fixings passing through the plane What do MNs/g and sd values mean? Resistance to moisture gets quoted two ways, and datasheets mix them freely. Converting between the two MNs/g, mega-newton seconds per gram, is the resistance of a specific thickness sd, in metres, is the equivalent still-air thickness Multiply MNs/g by 0.2 to get sd in metres, a conversion based on the vapour permeability of still air as set out in BS 5250 Annex E The 0.2 factor is a UK convention, so check whether a European datasheet has used it 500 gauge polythene VCLs are typically declared above 250 MNs/g when measured to EN 1931, giving roughly 50m sd Thickness in microns tells you almost nothing alone. Two products of identical gauge can behave very differently, so specify against the declared figure, never the gauge. VCL Versus Vapour Permeable Membrane Membrane type Primary purpose Typical position Behaviour Vapour control layer Limits moisture entering the construction Warm side of insulation High resistance Air and vapour control layer Controls air leakage and diffusion together Continuous warm-side plane High resistance, airtight when sealed Variable diffusion VCL Resists in winter, allows inward drying in summer Warm side, mainly moisture-sensitive refurbishment Changes with humidity Vapour permeable membrane Allows outward drying, resists wind and liquid water Cold side or outboard Low resistance BS 5250 defines the permeable end by an sd value, with a breathable membrane below 0.12m, roughly 0.6 MNs/g, and a low-resistance pitched underlay at 0.25 MNs/g or lower. Never substitute one family for the other. A permeable sheet installed where a VCL was specified removes the moisture control entirely, and the failure surfaces years later as saturated insulation. When can a VCL be omitted? Project engineer decision, supported by calculation. Defensible in specific cases, never as a default: Warm frame or hybrid build-ups where the dew point sits outside the structure Elements where an external airtight membrane delivers the airtight line Solid masonry where inward drying matters more than outward resistance Unheated structures with no meaningful vapour drive Any case where condensation risk analysis to I.S. EN ISO 13788 confirms it The humidity class question BS 5250:2021 sorts buildings into humidity classes, and the class drives the answer. A lightly occupied office and a commercial kitchen impose very different loads, so a build-up safe under one may not be under the other. Omission needs a calculation behind it. Where analysis is marginal, the membrane is cheap insurance against an expensive failure. Common Vcl Installation Failures The product rarely fails. The continuity does. Laps left untaped or overlapped short of the stated dimension Sheet stopped at the perimeter instead of turned up and sealed Services, hatches and downlighters punctured without remedial sealing Damage during follow-on trades left unrepaired Junctions between horizontal and vertical planes not bridged Frequently Asked Questions Do you need a vapour control layer? Most insulated, heated buildings require one on the warm side of the insulation. Whether a specific build-up needs it depends on internal humidity, airtightness strategy, material arrangement and the condensation risk assessment. Can you install two vapour control layers? Two high-resistance sheets should not normally be added without a calculated design, because moisture can become trapped between them with no route to dry in either direction. What is a vapour control layer? A high-resistance membrane, usually polythene or foil-faced, installed inboard of insulation to limit moisture movement into colder parts of the fabric and to form the airtight plane and covered by BS EN 13984. What thickness should a vapour control layer be? Specify by declared resistance, durability and system compatibility rather than thickness. Two products of identical gauge can perform quite differently, so the MNs/g or sd figure is what matters. What is the difference between a VCL and an AVCL? An AVCL is a VCL that also serves as the airtightness line. Most polythene and foil products do both, but the airtight function only exists if laps and penetrations are properly sealed. Which side of the insulation does a VCL go on? The warm side, meaning the side with the higher vapour pressure. In the Irish climate, that is the internal face for most of the year. Does a cold roof need a vapour control layer? Normally yes, at ceiling level. The greater practical difficulty is maintaining continuity through fit-out, since services routinely puncture it. Can OSB act as a vapour control layer? OSB has meaningful resistance and is sometimes used as part of a strategy. Still, it is not a substitute for a dedicated sheet without a condensation risk calculation confirming the build-up works. Should a VCL be taped at every joint? Yes, where it serves as the airtight line. Untaped laps leave the airtight function unperformed regardless of the resistance figure on the datasheet. Is a VCL the same as a breathable membrane? No, they are opposites. A breathable membrane is designed to let moisture out; a VCL is designed to keep it from getting in. Substituting one for the other removes the control entirely. Products And Technical Support QBM supplies vapour control layers as part of complete flat roof insulation packages, alongside insulation boards, cover boards and compatible fixings. Hence, the membrane suits the system rather than being bought separately and hoped for. Unsure whether your build-up needs one? Email info@qbm.ie, call +353 1 839 1170, or use the enquiry form. Architects can request the RIAI-approved CPD session covering deck options, insulation and waterproofing. Related Articles How to Size Rainwater Outlets for a Flat Roof: A Specifier's Method Standing Seam vs Profiled Metal Sheet Roofing Warm Roof vs Cold Roof Construction: What Irish Specifiers Should Decide First Fire Ratings for Flat Roof Systems: BROOF(t4) and Reaction to Fire Explained

A warm roof places insulation above the structural deck, so the deck remains close to the internal temperature, reducing thermal bridging and lowering the risk of interstitial condensation. A cold roof places insulation below that deck and depends on a continuously ventilated void above it. For most new Irish flat roofing, the above-deck arrangement is the lower-risk starting point. Below-deck construction is generally considered only where build-up height is severely restricted, and a ventilation route can be demonstrated. Last technically reviewed: July 2026. Checked against TGD Part L (2022), BS 6229:2025, BS 5250:2021 and I.S. EN 13501. Technical reviewer: [name and role to be inserted by QBM]. That decision usually gets made before anyone has picked a membrane, and it quietly governs what follows: build-up depth, condensation risk, threshold levels, and which certified components will work together. Get it wrong on an Irish project, and the consequence turns up as damp plasterboard two winters later, not on handover day. Below you will find how each construction behaves, what the standards expect, how to choose between them, and how to assemble a compliant section. Cold Roof Construction Explained How a cold deck is built Everything hinges on position. Insulation sits between the joists, beneath the structural deck, with waterproofing laid directly onto that deck. Working upwards from the ceiling, a typical arrangement includes: Plasterboard ceiling finish Vapour control layer at ceiling level Quilt or rigid board between the joists A ventilated air gap over the insulation Structural deck carrying the waterproofing Its appeal is obvious enough. Nothing gets added above the deck, so finished height barely moves, and that matters where a door opens onto a terrace. Why The Rafters Cost You Timber conducts heat far better than mineral wool or PIR does. Where structural members interrupt the layer, heat takes the short path, and on a thermal image the joist positions read straight through the ceiling below. That effect is thermal bridging, and in an element insulated only between rafters it removes a meaningful slice of the performance your calculation promised. Where The Ventilation Argument Breaks Down Cross-ventilating the void is the traditional answer. It needs a clear route, open at both ends, correctly sized and kept that way for decades. NBS technical guidance states that cold-deck flat-roof construction should be avoided because ventilating restricted voids is often impractical in a humid temperate climate. On a sheltered Dublin site hemmed in by parapets, plant and neighbouring buildings, that route is frequently theoretical. Failure points that recur: Insulation pushed tight to the underside, closing the air path Downlighters and hatches puncturing the vapour barrier Upstands or parapets leaving no route for air to cross Voids too deep or too shallow for the intended airflow Vents blocked, painted over or built out during later works Warm Roof Construction Explained How A Warm Deck Is Built Order changes here. Insulation moves above the structural deck, the vapour control layer goes immediately beneath it, and waterproofing runs over the top. Inside out, a build-up usually reads: Structural deck: concrete, profiled metal or timber-based Vapour control layer, bonded or loose laid Rigid insulation, flat or tapered Cover board, where the system calls for one Waterproofing membrane Ballast, paving or a planted finish where required Deck material shifts the detailing considerably. Concrete brings mass, dimensional stability and adhesive fixing options; timber brings movement and a moisture history worth surveying; profiled metal brings fastener pull-out calculations into play early. Continuous Cover And The Dew Point Because that layer runs unbroken, nothing structural repeatedly cuts through it. Warm roof insulation ensures the entire roof structure is insulated, pushing the dew point out into the insulation rather than leaving it at the underside of the deck. Damp internal air meets the VCL before reaching anything cold enough to condense; no ventilation void is required, and nothing blocks it. Correctly designed and installed, with continuity maintained at the VCL and around penetrations, this arrangement substantially reduces the risk of interstitial condensation rather than removing it by default. Practical gains beyond moisture control tend to be what win the argument on-site. Falls can be built in with tapered boards instead of firrings. The deck stays warm and dry, which suits timber. And because the layers come from one manufacturer's tested system, the guarantee conditions stay legible. Where The Extra Height Bites Depth is the trade-off. Adding 140mm to 180mm above the structure lifts the finished surface, and that has to come from somewhere: the parapet height, the threshold level, or the upstand of a rooflight. On new build it is a drawing exercise. During refurbishment it can be the single constraint that decides the job. Warm Roof Vs Cold Roof: The Comparison That Matters Feature Above-deck (warm) Below-deck (cold) Insulation position Over the structural deck Between the joists Ventilation Not required Required, and hard to sustain Vapour control layer On the deck, continuous At ceiling level, easily punctured Thermal bridging Minimal Significant at every member Interstitial condensation Low risk when detailed and installed correctly Recognised long-term risk Added depth 140mm to 180mm typical Almost none Tapered falls Straightforward Not practical Best suited to Most new flat roofing and major refurbishment Height-restricted cases where cross-ventilation can be demonstrated Main design constraint Increased finished build-up depth Ventilation continuity and VCL integrity Main failure risk Poorly sealed VCL, penetrations, incompatible components Condensation, blocked ventilation, repeated bridging Irish specification status Standard for new work Rarely recommended The two lines above are reported backwards elsewhere, so they are worth stressing. Ventilation belongs to the cold deck, not the above-deck section. A vapour control layer is needed in both cases, just in different positions and with very different odds of surviving the fit-out intact. Weighing the two on a live scheme? Pull U-value support and component data from the QBM technical resources library before the section gets frozen. Which Roof Construction Should Irish Specifiers Choose? A working sequence, roughly in the order the questions arise: Start from an above-deck arrangement for most new flat roofing Test threshold, parapet and rooflight heights early, since depth is the usual blocker Consider a below-deck build-up only where height genuinely prevents the alternative Where you do, require a demonstrable ventilation route plus a condensation risk calculation Avoid unassessed hybrid constructions, meaning insulation left in the void beneath a new above-deck layer Confirm insulation, VCL, cover board, membrane, outlets and fixings form one compatible, tested system Internal humidity class drives how much of this matters. A commercial kitchen, a pool hall or a busy apartment block generates far more vapour than a lightly occupied office, and BS 5250 sets out the classes used to size that risk. An inverted arrangement is a third option worth knowing about, with insulation placed above the waterproofing rather than below it. It suits ballasted and podium decks, and BS 6229:2025 revised its thermal requirements, so treat it as a separate design conversation. Building The Above-deck Section Layer By Layer The Deck Concrete, metal and timber substrates each impose different fixing regimes and tolerances. Confirm these first: Substrate type and pull-out resistance for mechanical fixings Surface regularity, plus a level survey on refurbishment work Span direction and where falls will be formed Capacity for ballast, paving or rooftop plant The Vapour Control Layer This single layer decides whether the design works. Laps sealed, continuity maintained at upstands and around every penetration. One that stops 100mm short of a parapet is not a barrier; it is a suggestion. The Insulation Two routes are worth considering. ROCKWOOL HARDROCK Multi-Fix dual-density boards are non-combustible, dimensionally stable, comply with I.S. EN 13162, and suit torch, pour-and-roll, single-ply EPDM, liquid-applied, and green finishes, per the manufacturer's flat roofing brochure. That breadth makes them a broadly compatible option, subject to the membrane manufacturer's tested system requirements. PIR gives a lower-declared lambda instead, which is useful when depth binds. Our post on how much insulation a roof actually needs covers the relationship between thickness and target performance. The Cover Board Where a membrane needs a harder surface beneath it, the Elevate ISOGARD HD cover board protects against foot traffic and dropped tools, and works with EPDM, PVC and liquid systems. Certain high-density boards eliminate the need for a separate layer entirely, which is worth checking against the manufacturer's conditions rather than assuming. Ask for the datasheet and the system warranty conditions together. The Waterproofing Elevate RubberGard EPDM and EPDM SA cover most adhered and self-adhesive routes. Around plant, awkward geometry and tight detailing, Polyroof Protec Evolve and Mariseal Detail handle what sheet materials struggle with. Request the certification and substrate compatibility notes for the exact build-up. Outlets and Penetrations Harmer rainwater outlets come in insulated versions made for deep build-ups, which matters because a standard outlet dropped into 160mm of insulation is a cold bridge with a hole through it. Fakro flat roof lights need upstands designed around the same depth. Have the deck type and a target U-value but no layer schedule? Ask for specification support, or call into the Baldoyle trade counter, Monday to Thursday 7 am to 5 pm and Friday until 4 pm. What The Standards Expect On An Irish Project Thermal Performance and Part L TGD Part L sets the backstop U-values, while the DEAP or NEAP calculation sets what the building genuinely needs; both govern. BS 6229:2025 also strengthened design requirements, and the NFRC's summary notes a minimum 1:80 finished fall for general areas and internal gutters. Our post on what the revision means in Ireland covers the wider changes. Moisture, Fire And Wind Moisture risk is managed in accordance with BS 5250:2021, with interstitial condensation assessed using the calculation method in I.S. EN ISO 13788. Reaction to fire classifies to I.S. EN 13501-1 and external fire performance to I.S. EN 13501-5. QBM lists BROOF(t4)- rated systems within its flat roofing range; request the classification report covering your intended build-up, as the rating applies to a tested assembly rather than to any single product. Wind uplift follows I.S. EN 1991-1-4 with its Irish National Annex, and that calculation drives the fixing patterns throughout the section. Converting A Cold Deck To An Above-Deck Build-up This happens regularly, and it is usually the right move once a covering has reached the end of its service life anyway. Simply laying boards over what already exists, however, is not the job. Check before committing: Whether the existing void gets sealed or stays ventilated, then detail accordingly Condition and moisture content of the existing substrate New finished height against thresholds, parapets and upstand terminations Additional dead load on the supporting structure Where the new VCL sits and how it laps into surrounding walls Whether Building Regulations trigger a thermal upgrade across the element Leaving old material in an uninsulated or partially filled void beneath a new layer creates a hybrid. Removal is often preferred, but the final approach should be based on a moisture survey and a condensation risk assessment rather than a rule of thumb. Specification Mistakes That Quietly Undo The Design Six that surface repeatedly at the counter: Calling a build-up insulated between members a warm design because it feels like one Vapour barrier stopped at the perimeter instead of turned up and sealed Fixings driven through that barrier without allowing for it in the risk assessment Standard uninsulated outlets dropped into a deep section Cover board omitted where the membrane manufacturer requires one, which usually voids the guarantee Tapered scheme thinning below the required performance near drainage points None of these is reliably visible on a walkover inspection at handover. Frequently Asked Questions Is a warm roof more efficient than a cold roof? Generally yes, and the gap is wider than the headline U-value suggests. Insulating between joists causes timber to bridge the layer repeatedly, so measured performance falls short of the calculated figure. Continuous cover over the structure avoids that. Depth is the price you pay. QBM can model both options against your target and confirm which board thickness reaches it, using either stone wool or PIR depending on the space available. Can you change a cold roof to a warm roof? It is a common refurbishment route and normally makes sense once the covering needs replacing anyway. The existing material usually comes out, a new vapour control layer is applied to the cleaned substrate, and rigid boards plus membrane build up over it. Height at thresholds and added load are the two things that catch people. Send QBM your existing section and target performance for a component list and compatible system options. What is a warm roof in construction? Any construction placing its thermal layer above the structural deck and directly beneath the waterproofing, with the vapour control layer immediately under the insulation. Because the deck stays close to internal temperature, no ventilated void is needed. Codes of practice treat this as the default arrangement for new flat roofing in Ireland. QBM supplies every layer, from the barrier through ROCKWOOL or PIR boards and cover boards to the membrane itself. What is the difference between a hot roof and a cold roof? "Hot roof" is informal shorthand, used mostly in North American sources, for what Irish and UK practice calls a warm deck: unventilated insulation above the structure. A cold roof is insulation installed below the structure, with a ventilated gap. The terminology matters when reading manufacturer literature from abroad, because installation guidance written for a different climate and code will not always transfer. Ask QBM for the certification that applies here. Talk to QBM before the section is frozen Materials, technical backup and system compatibility from one Dublin supplier, with nationwide delivery and collection from Baldoyle. Send through: Deck type and span Target U-value or your DEAP/NEAP output Threshold and parapet constraints Intended membrane or surface finish Email info@qbm.ie, call +353 1 839 1170, or use the enquiry form. Architects can also request a place on the RIAI-approved CPD session covering deck options, insulation and waterproofing. Related Articles How to Size Rainwater Outlets for a Flat Roof: A Specifier's Method Standing Seam vs Profiled Metal Sheet Roofing Vapour Control Layers Explained: When You Need One Fire Ratings for Flat Roof Systems: BROOF(t4) and Reaction to Fire Explained

Rockpanel façade panels are durable, lightweight rainscreen cladding boards made from a basalt/stone-wool based core, designed for modern external wall finishes. Available in a wide range of colours and textures, they’re easy to fabricate on site and suitable for both new-build and refurbishment projects where a clean, contemporary façade is required. Rockpanel Rockpanel façade panels are durable, lightweight rainscreen cladding boards made from a basalt/stone-wool based core, designed for modern external wall finishes. Available in a wide range of colours and textures, they’re easy to fabricate on site and suitable for both new-build and refurbishment projects where a clean, contemporary façade is required. < Back Next > Rockpanel stones Download Rockpanel metals Download Rockpanel woods Download Frequently Asked Questions What are Rockpanel boards made from? Rockpanel boards use a compressed stone-wool-based core with product-specific surface finishes. They are lightweight and can be cut and drilled using appropriate standard tools, subject to the installation guidance. Are Rockpanel boards A2 fire rated? Many Rockpanel A2 products are classified A2-s1,d0, but classification depends on the exact product, thickness, coating, substructure, cavity and fixing arrangement. The complete façade must be checked against the project fire strategy and Irish requirements. Can Rockpanel be used on curved façades? Selected board types and thicknesses can be curved to manufacturer-stated minimum radii. The designer must verify the permitted radius, fixing centres, joint arrangement and substructure for the proposed product. Why are the boards used in ventilated façades? They are intended for use as the external panel within suitable ventilated rainscreen assemblies. The cavity, insulation, membranes, rails, cavity barriers and flashings perform separate functions and must be designed as a complete façade. How long are Rockpanel boards expected to last? Relevant Rockpanel ETAs are based on an assumed intended working life of 50 years. The assessment explicitly states that this is not a manufacturer guarantee, and actual performance depends on design, exposure, installation and maintenance.

Roofing Supplies Delivered across Wexford from QBM If you are sourcing roofing materials for a project in Wexford, QBM can supply and deliver what you need directly from our warehouse. We are your local specialist roofing supplies company based in Baldoyle, Dublin 13, delivering nationwide across Ireland with next working day dispatch on orders placed before noon. If your project involves a flat roof extension, a commercial re-roofing, or a domestic repair, our team can help you specify the right system and get the materials to your site. QBM is a supplier, not a roofing contractor. We do not install or repair roofs. Since 2012, our focus has been on supplying high-performance roofing materials to contractors, builders, architects, engineers, and experienced specifiers, backed by technical guidance and reliable nationwide delivery. Everything ships from our Baldoyle warehouse in Wexford, which is how we maintain consistent stock levels and product availability for customers right across Ireland. What Roofing Products Does QBM Supply? Our product range covers everything you may need for your next roofing project in Wexford. This includes, but is not limited to; Single Ply Membranes : EPDM rubber membranes (Elevate RubberCover) and PVC systems (FDT Rhenofol) for flat roofing on residential and commercial buildings. Reinforced Bitumen Systems : Modified bitumen membranes from Nord Bitumi, suitable for traditional flat roof construction and refurbishment. Liquid Waterproofing : Seamless liquid-applied systems for complex roof geometries where sheet membranes are difficult to detail. Standing Seam Metal Roofing : Kalzip and Vestis metal systems for pitched and architectural roofing projects. Roof Drainage and Accessories : Rainwater outlets (b/s/t and Harmer), edge trims, pipe flashings, roof vents, primers, adhesives, and sealants. Insulation : Insulation boards suitable for use within flat roof build-ups, available to specification. Professional Roofing Tools : Specialist hand tools by Freund, hot air welding equipment, and sheet metal folding machines for on-site fabrication. Facade Systems : Kalzip FC rainscreen facade panels and airtight membranes from Riwega. One of the practical advantages of ordering from QBM is the ability to source a complete roofing system from one supplier. Membrane, insulation, drainage, trims, adhesives, and tools all come from the same place, which helps ensure material compatibility and avoids warranty issues that can arise from mixing components from different manufacturers. Approvals and certifications vary by product and system. Where required for building control submissions, tender documentation, or specification work, we can supply the relevant data sheets and certification information for the specific products you are using. Delivering Across Wexford from Our Baldoyle Warehouse We ship our roofing products across Wexford and the rest of Ireland from our warehouse in Baldoyle, Dublin 13. Orders placed before midday are dispatched the same day for next working day delivery. For larger construction projects requiring multiple loads or phased deliveries, our logistics team can coordinate scheduled drops to keep your site supplied as the programme progresses. Our trade counter in Baldoyle is also open for collection if a project timeline is tight or you happen to be passing through Wexford. Contractors, builders, and specifiers working in Wexford choose to source from QBM rather than a general builders merchant for many reasons. Stock availability . One of the more frustrating parts of any construction project is waiting on materials. We hold substantial stock at our Baldoyle warehouse across the main product categories, which means fewer delays and less time spent chasing back orders. For Wexford-based contractors, that translates to materials arriving when they are supposed to. Technical support and system guidance . Our team can help with product selection, material compatibility, and full system specification. That includes advising on the right membrane for a given exposure or build-up, recommending compatible accessories, and helping you avoid the kind of mixed-system issues that cause problems later. If you are tendering or pricing a job in Wexford, we can assist with take-offs. Documentation for submissions . We can supply the data sheets, certification information, and product documentation needed for building control submissions, tender packages, and specification work. If you need something for compliance, ask us, and we will get it to you. Nationwide delivery from a single Wexford hub . Operating from one warehouse means consistent stock visibility and a single point of contact for your orders, wherever in Ireland your site is. Frequently Asked Questions Does QBM deliver roofing materials directly to sites in Wexford? Yes. We deliver across Wexford, from our warehouse in Balydoyle, Dublin 13. Orders confirmed before noon go out the same day for next working day arrival. Larger projects requiring coordinated or phased deliveries can be arranged through our logistics team. You can also collect from our trade counter in Baldoyle if that suits your schedule better. Where is QBM Located? QBM operates from a single warehouse and trade counter in Baldoyle, Dublin 13. We stock, supply and deliver a wide range of roofing materials and products to all local customers in Wexford. Can QBM recommend a roofing contractor in Wexford? QBM is a materials supplier and does not directly employ roofing contractors. That said, we work with installers across Ireland who are familiar with the systems we supply. If you need a steer towards experienced roofers in Wexford for a specific product, our team can usually help point you in the right direction. What roofing system is best suited for a project in Wexford? The right system depends on the building type, roof geometry, exposure, and the performance requirements of the project. EPDM rubber membranes perform well across a wide range of Irish conditions because they stay flexible and resist UV degradation. However, PVC single-ply systems are often specified for commercial buildings where particular fire or chemical resistance requirements apply. The best approach is to talk to our technical team about your specific project so we can recommend a suitable system. Request a Quote or Speak to Our Technical Team If you have an upcoming or ongoing project and are looking to request a quote or speak to our technical team about the right specification for your job, call us on 01 839 117 and send us an email at info@qbm.ie . We supply contractors, builders, specifiers, and homeowners across Dublin and the rest of Ireland with technical support and reliable next-day delivery from Dublin 13. Get in Touch

ELAPRO synthetic liquids are free of harmful contents. They do not contain any solvents or pollutants, which makes them particularly environmentally friendly. The ideal, simple and sustainably safe solution for every challenge. ELAPRO synthetic liquids ELAPRO synthetic liquids are free of harmful contents. They do not contain any solvents or pollutants, which makes them particularly environmentally friendly. The ideal, simple and sustainably safe solution for every challenge. < Back Next > ELAPRO specialises in synthetic liquids that are non-hazardous to health, environmentally friendly and developed and manufactured in Germany. The systems can be used for lasting waterproofing of flat roofs, balconies, terraces and buildings in new builds as well as when renovating and are easy, quick and reliable to use. The multifunctional ELAPRO synthetic liquids with the ETA005 W3 test certificate fulfil the highest possible requirement specifications for all areas of application. Elapro Products Download Frequently Asked Questions What are ELAPRO synthetic liquids used for? They are used for lasting waterproofing of flat roofs, balconies, terraces and buildings, suitable for both new builds and renovation projects. Are ELAPRO synthetic liquids environmentally friendly and safe to use? Yes, they are free of harmful contents and contain no solvents or pollutants, making them non-hazardous to health and particularly environmentally friendly. Do ELAPRO products meet recognised quality or safety standards? Yes, they hold the ETA005 W3 test certificate, fulfilling the highest requirement specifications for all areas of application.