Green roofs explained: extensive vs intensive

PiP Architects
3 days ago
4 min read

From rising temperatures to heavier rainfall, UK homes are under growing pressure to perform. We need buildings that stay comfortable in extreme weather, handle downpours and support biodiversity.

Green roofs (layers of planting installed on top of a roof structure) offer a powerful way to achieve all three. But not all green roofs are the same, and there are important structural and maintenance implications that homeowners need to understand.

In this guide, we explore the main types of green roofs, and when and why we recommend them.

What is a green roof?

A green roof is a flat (or near flat) roof that’s been designed to support plant life.

Instead of looking out onto bare membrane or felt, you see greenery - anything from a thin carpet of sedum to a fully planted roof garden with wildflowers and shrubs.

Every green roof is built up from a few essential layers:

A waterproof membrane that keeps the building dry
A drainage and root barrier layer
A growing medium (like a specially engineered soil)
The plants themselves

Beyond the visual appeal, these layers help to minimise temperature swings at roof level, absorb and slow rainwater (easing pressure on drainage systems) and create small but valuable pockets of habitat in otherwise hard landscapes.

How well a green roof performs in each of these areas depends on which type you choose.

What are the two types of green roof?

Green roofs are often described as extensive or intensive. In simple terms, that translates to sedum mats versus roof gardens.

Extensive: sedum roofs – lightweight and low maintenance

Sedum roofs use a shallow layer of growing medium with a pre‑grown sedum mat rolled out on top, much like turf. The result is a thin, continuous carpet of planting that is relatively light and straightforward to install.

Because the build‑up is slim, sedum roofs are easier to accommodate structurally and are genuinely low maintenance. Sedums are tough succulents, so they cope well with dry periods and typically only need extra irrigation during extreme heatwaves. Most of the time, care is limited to occasional inspections and light weeding.

While they don’t store as much water or support as many species as deeper systems, sedum roofs still transform how a building feels and performs. They offer a far better outlook than bare membrane, add some cooling and rainwater attenuation, and provide a robust way to green-up otherwise ‘dead’ roof surfaces.

We often specify sedum systems where the roof is mostly seen from above rather than used as a terrace, where there are structural limits on weight, or where the priority is a clean, green backdrop rather than intensive gardening.

Intensive: roof gardens – a real haven above your head

At the other end of the spectrum are deeper build-ups that behave much more like a conventional garden. Here, the green roof becomes a real outdoor space.

With a greater depth of growing medium, these roofs can support a diverse mix of perennials, grasses and wildflowers, along with small shrubs and, in some cases, even small trees. The extra depth brings richer biodiversity, greater ability to store and slowly release rainwater, and far more texture, colour and seasonal change – especially when you experience the space up close.

That performance comes with real demands. A roof garden is significantly heavier than a sedum mat, so the structure must be designed from the outset to carry saturated soil, planting, people and furniture. Architectural detailing around thresholds and levels becomes more complex, and the planting itself needs ongoing gardening and reliable irrigation.

For that reason, we typically recommend deeper roof garden systems where people will be on or beside the roof (for example, a terrace outside a living space) and where planting is central to the user experience, not just a visual extra.

When do we recommend a green roof?

At PiP, we won't specify a green roof just because it ‘looks nice’ on a drawing. We match the system to how the space will be used.

Overlooked flat roofs

If you have upstairs rooms looking down onto a ground-floor extension, a sedum roof dramatically improves that view.

In these cases we typically choose a light and low maintenance sedum system, which residents will see as a clean, continuous green surface.

Roof terraces and close-up spaces

Where you’ll be standing on or right next to the roof, we look at deeper options.

Here, a roof garden can extend your usable outdoor space and bring seasonal planting right to the heart of the home. The intensive green roof creates a valuable amenity area rather than a token balcony.

Integrating house and landscape

On some projects, we use green roofs to blur the line between architecture and garden.

Wrapping a single-storey wing and adjacent planting in a continuous sweep of greenery can help a new development sit more comfortably in an established landscape.

The choice always comes back to three questions:

How will this roof be seen and used?
What can the structure reasonably carry?
How much maintenance is realistic for the owner?

How PiP can help

Green roofs work best when they’re part of the conversation from day one.

At PiP Architects we:

Help you decide if a green roof is appropriate for your brief and budget
Coordinate with engineers to ensure your structure is designed for the right loads
Select systems that suit how the roof will be used and viewed
Work closely with reputable suppliers so that membranes, build-ups and warranties all align

The result is a roof that quietly improves comfort, value and biodiversity – while giving you something far more uplifting to look at than a bare felt roof.