The short answer
Often yes, but not without checks. A sedum blanket adds weight, holds water and needs a covering that resists root and moisture, so the roof has to be assessed before anything is laid. A structural check is wise when the deck is old, timber or already showing ponding. Sedum is among the lighter green roof options, and it still needs regular upkeep.
What a sedum roof is made of
Sedum is a family of small, fleshy plants that store water in their leaves and cope with thin soil and dry spells. On a roof they are usually supplied as a pre-grown mat or blanket, rolled out over a built-up system. The usual layers, from the top down, are the plants and their thin growing medium, a filter sheet, a drainage layer that holds some water and lets the rest run away, a protection layer and then the waterproof covering on the deck.
Each layer has a job. The growing medium feeds the plants. The drainage layer stops the roof becoming a bath. The protection layer keeps roots and stones away from the membrane. The membrane below is the roof's real defence, and everything above it is there to leave it undisturbed. If one of those upper layers is skipped or squashed, roots or trapped water can reach the membrane, and the whole point of the build-up is lost.
Loading: the question to answer first
A green roof is heavier than a bare one, and it gets heavier when wet, because the layers hold water. The roof must carry that load as a permanent weight, together with snow and anyone who maintains it.
We do not quote a weight in kilograms here, because it varies with the system and the depth of the build-up, and the manufacturer's figures for the chosen system are what count. The way to settle it is a check against Approved Document A, which flags a big rise in roof loading and observes that a structure short of capacity may reveal it as overstressed members or ponding. Before work on a heavier covering, the Planning Portal advises speaking to a structural engineer or surveyor, and the same sense applies to a planted roof.
On an older timber roof, on a roof with a history of ponding or on joists that look slender for their span, a structural check should come before ordering plants.
The waterproofing under the plants
Roots and constant moisture are hard on a waterproof layer, and a leak under a green roof is expensive to trace, since the planting has to be lifted. The covering below should be one the manufacturer approves for use under green roofs, and its seams and upstands must be sound before anything is laid on top.
That is why a green roof is often best added at the same time as a new covering, not onto an old one of uncertain age, where nobody can say how much life the membrane has left and the cost of lifting the planting to repair it later would be high. Adding sedum over felt that is already blistering simply buries the faults. Our answer on how long a flat roof replacement takes sets out the stages, and a green roof adds its own layers at the end.
The fall still matters. A drained green roof is designed to shed excess water, so the finished slope should reach the recommended 1:80 minimum, designed at 1:40 as BS 6229 advises, because sedum dislikes sitting in standing water.
Looking after sedum once it is down
Sedum is low-maintenance but not no-maintenance. In the first season the plants need watching until they establish, and in a dry spell a new mat may need watering. Roofers who fit green roofs also tend to agree that neglect is what spoils them: a clogged outlet or a patch of invasive weed left for a season is much harder to put right than one dealt with at the start. After that, a few jobs come round each year.
- Clear leaves, silt and weeds from drainage outlets, gutters and the edges, because a blocked outlet floods the roof.
- Pull out tree seedlings and tall weeds before their roots reach the membrane.
- Check for bare patches and fill them, since gaps invite weeds.
- Keep a clear border of gravel or hard surface around the edges and rooflights.
Access matters here, too. You will need to reach the roof safely, which links back to our answer on whether you can walk on a flat roof. A planted roof is not a lawn, and boots can damage plants and the layers beneath. Plan one fixed route to the outlets and edges, and keep tools, ladders and bags of waste off the planting while you work.
When sedum is the wrong answer
A green roof is not for every flat roof. It suits a roof with a sound deck, a decent fall, a modern membrane and safe access. It is a poor fit for a roof with a soft deck, a roof that is already leaking or a roof where the structure cannot take the weight.
Planning is a separate matter, and a roof that changes how the building looks may need permission. Ask Stroud District Council's planning team, especially if the house is listed or in a conservation area.
If a pitched roof is what you really want, our answer on converting a flat roof to a pitched one compares the route, and for the covering options start at our flat roofing page or the flat roof materials guide.
On a survey we photograph the deck and membrane, check the falls and the access, and note whether a structural opinion is needed. The written quote then covers the covering, the green roof layers and the scaffold, and our written 10-year guarantee covers the workmanship.