Question answered
Can you put a sedum green roof on a house flat roof?
The short answer
You can put a sedum green roof on a house flat roof provided the joists can take the added weight and the waterproofing below is sound and suitable. Sedum is a planted mat laid over a drainage layer, a root-resistant membrane and a proper fall. A structural check comes first, and a heavier covering can bring Building Regulations into play.
Often you can, but a sedum roof is a layered system sitting on top of your waterproofing, not a cover for an old roof. The structure, the membrane and the drainage all have to be ready for it.
What a sedum roof is made of
Sedum is a family of low, drought-tolerant succulents. On a house they usually arrive as a pre-grown mat that is unrolled over a thin layer of growing medium. The plants store water in their leaves, which lets them cope with the dry spells that a shallow roof-top soil produces.
Underneath the mat sits a stack of layers, each with a job. A waterproof membrane stops water reaching the building, a protection layer shields it, a drainage layer carries away excess water and a filter fabric stops the growing medium from washing into the outlets. The planting is the visible part; the layers beneath are what make it work.
This is why a sedum roof is a design decision, not a finish. You cannot simply add plants to an old felt roof. The build-up is chosen together with the covering beneath and the edge details, usually as part of a replacement, so the whole sequence is worked out before anything is ordered.
Weight and what the structure must carry
A wet sedum build-up is far heavier than a bare membrane, because the drainage layer and growing medium hold water. The roof structure has to carry that dead load, plus the weight of anyone maintaining it, plus the allowances for snow and wind.
The Planning Portal advises getting a structural engineer or surveyor to look at the roof before you change to a covering that is significantly heavier. Approved Document A treats a load increase of more than 15% as a significant change, while the Planning Portal puts the approval trigger at a replacement material that makes the covering 15 per cent heavier or more. A planted system is a much heavier covering than a bare membrane, so raise it with building control early and let the structural check settle the figures.
A structural survey tells you whether the joists need strengthening. We would rather learn that at the survey than after the plants arrive on site.
Waterproofing and falls beneath the planting
Sedum is not waterproof. The membrane below is, and it must be suitable: a layer that resists root penetration and is protected from the drainage stack above it. A covering that is merely borderline, or that is close to wearing out, makes a poor base, because reaching it later means lifting the entire green roof.
Falls still matter. IKO, citing the 2018 edition of BS 6229 (since revised), advises designing a flat roof to a 1:40 fall so that at least 1:80 remains once the roof is built. Planted roofs hold water longer than bare ones, so a roof that ponds without plants will behave worse with them.
Where the roof is also being re-insulated, a warm-roof layout puts the insulation under the waterproofing, as described in warm versus cold flat roofs. Part L can apply to the same work, as insulating a flat roof when replacing it explains. Membrane, insulation and planting should be designed together.
Drainage, edges and access
A green roof still has to shed water. Outlets are kept clear of growing medium by a gravel strip or a purpose-made inspection chamber so that they stay visible and reachable. A hidden, buried outlet is a leak waiting for the first leaf fall.
The edges need a barrier too. A gravel or paving border separates planting from the roof rim, keeps debris out of the gutters and gives a safe place to stand when the roof is inspected. At a wall, the upstand is carried up past the finished surface so that water cannot track behind it.
Access deserves a plan. A roof that nobody walks on needs little more than a route to the outlets, but one used for sitting out needs a different build, as the page on walking on a flat roof explains. Sedum mats tolerate only light, occasional foot traffic, and bare patches appear along any regular path.
Looking after a sedum roof
Sedum has a name for being low-maintenance, and compared with a lawn it is. It still needs attention. Check the outlets and edges after leaf fall and again in spring, pull out weeds and seedlings that take hold, and fill any bare patches before they spread. Dry spells in the first year may call for watering until the plants establish.
Keep a record. Photograph the roof at each visit, since changes in colour or bare patches often show a drainage problem before it reaches the ceiling.
Repairs are harder than on a bare roof, because the planting must be rolled back before the membrane can be reached. That is the real cost of a green roof: not the plants, but the effort of getting at the waterproofing underneath. If you are weighing it against a plain covering, our flat roofing page lists the choices, and the price comes as a written quote after we have surveyed your roof.
Sedum on a house roof: further questions
Does a sedum roof need planning permission?
Not necessarily, but the look of the roof changes, so ask the Adur & Worthing planning team if your home is listed, in a conservation area or covered by an Article 4 direction. A heavier covering is a Building Regulations matter as well.
Can sedum go on a roof that already has a felt covering?
Only where the felt is sound, suitable and has plenty of life left, which is rare on a roof being planted. The usual route is to renew the waterproofing with a root-resistant layer first, then build the green roof on top.
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