Green Roof Types Compared: What Every Building Owner Should Know Before Choosing

Green roofs have moved a long way from being a novelty. You see them on commercial buildings, housing developments, public transport hubs, and increasingly on smaller residential projects too. The environmental case is well established: reduced stormwater runoff, better insulation, improved air quality, extended roof life. But once someone decides they want a green roof, the next question is almost always the same: intensive or extensive?

It sounds like a simple distinction. In practice, the two systems are quite different in terms of construction depth, structural demands, vegetation options, maintenance requirements, and cost. Getting that choice wrong at the design stage is an expensive mistake to correct.

This guide sets out what separates these two approaches, where each one performs well, and what to think about before your project goes any further.

Side-by-Side Comparison

Feature	Extensive	Intensive
Substrate depth	60 to 200 mm	200 mm to 1,000+ mm
Saturated weight	60 to 150 kg/m²	200 to 500+ kg/m²
Plant types	Sedums, mosses, grasses	Shrubs, trees, perennials
Maintenance level	Low	High
Human accessibility	Rarely	Commonly
Installation cost	Lower	Higher
Structural requirement	Suitable for most buildings	Requires significant load capacity
Retrofit suitability	Generally feasible	Often requires new build
Irrigation required	Rarely	Usually

What Are Extensive Green Roofs?

Extensive green roofs are considered the simpler version of the two systems, and that's not a criticism. Simplicity, in this context, is genuinely useful. The substrate depth is shallow, typically between 60 mm and 200 mm, which keeps the overall weight low. Extensive green rooftops usually have six inches of growing medium or less, which limits plant choice but also limits structural requirements considerably.

Because the substrate layer is thin, the vegetation needs to be tough. Sedums are the go-to choice: low-growing, drought-tolerant, and well suited to the variable conditions a rooftop environment produces. Wildflowers, mosses, and certain grasses are also used, depending on the biodiversity goals of the project.

Key characteristics of extensive systems:

• Substrate depth: typically 60 mm to 200 mm

• Weight when saturated: roughly 60 to 150 kg/m²

• Vegetation: sedums, mosses, grasses, wildflowers

• Maintenance: minimal, often one or two visits per year

• Accessibility: generally not designed for regular human use

• Cost: lower than intensive options, both for installation and ongoing care

Extensive roofs are more affordable to install, easier to retrofit onto existing buildings, and suit a wider range of structures because of their lighter load. For many commercial and residential projects, particularly where the roof is not intended as an amenity space, an extensive system delivers everything required.

What Are Intensive Green Roofs?

Intensive green roofs involve intense landscaping at roof level, and that distinction matters. The substrate depth can range from around 200 mm to well over a metre in some cases, which means the structural load increases substantially. Simple intensive green roofs have substrate depths starting around 200 mm; more ambitious designs push considerably deeper.

Intensive green roofs provide greater design options precisely because of that depth. Trees, shrubs, perennial plants, herbs, and ornamental grasses all become possible. Some installations include seating areas, pathways, raised planters, or even small water features. These are, in practical terms, rooftop gardens.

Intensive green roofs have a higher layer structure than extensive green roofs across every component: deeper drainage, deeper substrate, and often a more complex irrigation setup. That complexity comes with a cost, both financially and in terms of ongoing maintenance. Regular watering, pruning, fertilising, and general horticultural care are expected.

Key characteristics of intensive roofs:

• Substrate depth: 200 mm and above, sometimes exceeding 1,000 mm

• Weight when saturated: can exceed 500 kg/m² in deeper designs

• Vegetation: shrubs, trees, perennial plants, grasses, herbs

• Maintenance: regular, often requiring a professional gardener or groundskeeper

• Accessibility: typically designed for human use as an amenity space

• Cost: significantly higher than extensive systems

For new-build commercial projects where the structural capacity is planned in from the outset, intensive systems are a realistic and impressive option. Retrofitting an intensive roof onto an existing building is far more difficult and sometimes simply not viable without major structural work.

How to Choose Between Them

Start With the Structure

The single most important factor is what the building can actually carry. An intensive green roof on a structure that wasn't designed for it is not a compromise; it's a structural risk. Before any vegetation or substrate decisions are made, a structural engineer needs to confirm the load-bearing capacity of the roof.

Extensive systems are far more forgiving here. Their lighter construction means they can often be added to existing buildings without modification. Intensive systems, because of the weight involved, are almost always best suited to new construction where the load is factored in at the design stage.

Think About Maintenance Realistically

Perhaps the part that gets underestimated most often is what happens after installation. An extensive roof, once established, needs relatively little attention. Weeding occasionally, checking drainage outlets, perhaps a visit from a specialist once or twice a year. For most building owners or facilities managers, that's manageable.

An intensive roof is a different matter. Plants need water, especially in dry periods. Trees and shrubs need pruning. The substrate needs attention. If there isn't a clear plan for who maintains the space and how often, an intensive installation can deteriorate quickly. I think it's fair to say that many intensive green roof projects run into problems not because of poor installation, but because ongoing care wasn't properly budgeted for.

Consider the Purpose

What is the roof actually for? If the answer is environmental performance, insulation, and stormwater management, an extensive system almost certainly does the job at a fraction of the cost. If the answer involves people using the space, whether as a communal garden, a corporate terrace, or a recreational area, then intensive is the only realistic option.

There's also a middle ground worth mentioning. Semi-intensive systems sit between the two, with substrate depths typically between 100 mm and 250 mm, and a wider plant palette than extensive but without the full weight and maintenance commitment of intensive roofs. For some projects, that's the most practical answer.

Frequently Asked Questions

Can an extensive green roof be converted to an intensive one later?

Converting an existing extensive green roof to an intensive system is rarely straightforward. The primary obstacle is structural: intensive roofs require significantly greater load-bearing capacity than most buildings originally designed for an extensive system can provide. In some cases, structural reinforcement is possible, but this involves considerable cost and disruption. It is far more practical to decide on the intended system type at the design stage and ensure the building's construction reflects that requirement from the outset. A structural engineer should always assess feasibility before any conversion is considered.

What plants work best on an extensive green roof in Ireland?

Sedums are the most reliable choice for extensive green roofs in Ireland's climate, given their tolerance of both wet winters and dry summer periods. Varieties such as Sedum acre, Sedum album, and Sedum spurium are commonly used. Wildflowers including thyme, stonecrop, and certain grasses also perform well at shallow substrate depths. The key requirement is drought tolerance combined with the ability to survive in thin, low-nutrient growing media. Pre-grown sedum blankets and modular tray systems are often used to establish vegetation quickly and reduce the establishment risk on new installations.

Do green roofs require planning permission in Ireland?

In many cases, green roofs on existing buildings fall within permitted development rights and do not require formal planning permission, particularly where the installation does not materially alter the roofline or external appearance. However, this depends on the building type, location, and the scale of the proposed installation. Intensive systems, given their greater visual impact and structural implications, are more likely to require a planning application. It is always advisable to check with your local authority before proceeding, particularly for buildings in conservation areas or those with protected status.

How do green roofs affect a building's energy performance?

Green roofs provide a degree of thermal insulation by adding a layer of vegetation and substrate above the roof structure. This reduces heat loss in winter and, perhaps more significantly, limits heat gain during summer, which can reduce the demand for mechanical cooling in commercial buildings. Extensive systems contribute to this effect, though the insulation value is proportionally lower than intensive roofs given the thinner substrate. Research consistently shows that green roofs reduce rooftop surface temperatures substantially compared with conventional membranes, which also extends the life of the underlying waterproofing layer.

What is a semi-intensive green roof?

A semi-intensive green roof sits between extensive and intensive systems in terms of substrate depth, typically between 100 mm and 250 mm. It supports a wider range of vegetation than an extensive roof, including low shrubs and more varied perennial plants, without the full structural demands of a deeply planted intensive system. Maintenance requirements are moderate: more than an extensive roof but considerably less than a fully intensive installation. Semi-intensive systems are a practical option for projects where some design flexibility is wanted but structural or budget constraints rule out a full intensive approach.

QBM supplies green roofing systems across Ireland. For product information and technical support, visit the QBM green roofing systems page.