Green roof installation: how a sedum roof is built
Green roofs are chosen for looks, wildlife, rainwater attenuation and to soften the view from upstairs windows onto an extension or garden room. They are a real roofing system with its own design checks, not a layer of turf laid on an ordinary roof.
A green roof is a flat or low-pitched roof finished with living planting, usually sedum, over a fully waterproofed deck. The layers are: a sound structure, a root-resistant waterproof membrane, a drainage and protection layer, a lightweight growing substrate and the planting. The waterproofing underneath does the real work; the plants protect it.
Extensive and intensive green roofs
Extensive green roofs use a thin, lightweight substrate planted with hardy sedums, mosses or wildflowers. They need little maintenance and suit most domestic extensions, garages, garden rooms and bin stores. Intensive roofs are deep, heavy and planted with lawns, shrubs or even trees; they behave like a garden and need structural design from the outset. Nearly all domestic green roofs are extensive, and that is what this guide describes.
Checking the structure first
A saturated green roof weighs far more than a felt or rubber roof, so the joists, deck and supports must carry that load when wet, plus snow and maintenance access. New extensions can be designed for it from the start. Retrofitting onto an existing roof needs the structure checked, and sometimes strengthened, before anything else is decided. This is the step that most often determines whether a green roof is practical.
Waterproofing and root protection
The membrane must resist roots as well as water. Some single-ply and reinforced bitumen systems are certified root-resistant; others need a separate root barrier over them. Upstands at walls and edges are taken higher than on a plain flat roof so water held in the substrate cannot reach the top of the detail. Outlets, rooflights and any penetrations are detailed before the roof is covered, because finding a leak under planting is far harder than on a bare membrane.
Drainage, falls and vegetation-free zones
A drainage and protection layer sits over the membrane to let excess water escape and protect it from damage. The deck still needs falls to its outlets. Around outlets, edges and rooflights, a strip of pebbles or gravel is left clear of planting so outlets can be inspected and kept clear, and so fire and wind edge details are met. Planning these zones early keeps the finished roof neat and serviceable.
Substrate and planting
The growing medium is a lightweight engineered substrate, not garden soil, which would be too heavy and would hold too much water. Pre-grown sedum blankets give instant cover; plug planting or seeding is cheaper in materials but takes longer to establish. Sedums cope with drought and shallow depths, which is why they dominate domestic roofs. Wildflower mixes suit deeper substrates and a less manicured look.
Maintenance after installation
An extensive roof needs occasional visits rather than regular gardening: clearing outlets and the gravel strips, removing weeds or tree seedlings before roots develop, and topping up bare patches. A newly laid roof may need watering in a dry spell while it establishes. Safe access for these visits should be considered at design stage, especially on higher roofs.
Planning and neighbours
A green roof on a flat extension roof generally raises the same planning questions as the extension itself. It becomes a different matter if the roof is intended as a usable terrace, because that can raise overlooking and balustrade issues. Party walls, boundary edges and drainage onto neighbouring land deserve attention before work begins.
Is a green roof right for your building?
It suits a structure that can carry the weight, a waterproofing system chosen for it, and an owner happy with light seasonal maintenance. If the structure cannot be strengthened economically, a conventional EPDM, GRP or felt roof is the practical alternative, and some owners add planters instead. A survey is the sensible first step.
FAQs
Sometimes. The structure must carry the saturated weight and the waterproofing must be root-resistant, so both are checked first.
Usually only while newly laid or in a prolonged dry spell. Established sedum copes well with drought.
Occasional visits to clear outlets and gravel strips, pull out weeds or seedlings and top up any bare patches.
Yes. The planting and substrate shield the waterproofing from sunlight and temperature swings, which helps it last.
Related reading
More on materials & systems and the jobs that usually come with it.
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- Roof valleys: the highest-risk detail on a pitched roofMaterials & systems
- Garage and outbuilding roof replacementCosts & budgeting
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