How to Size Rainwater Outlets for a Flat Roof: A Specifier's Method
Updated: Sep 9
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.
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