What the kerb has to clear on a sedum roof
On a green roof the finished surface is the top of the substrate, so the upstand is measured from there. That can add 100mm or more to the kerb compared with the same rooflight on a plain membrane roof.
A green roof is a waterproof roof with a garden laid on top. Above the membrane sit a root barrier (sometimes built into the membrane itself), a protection fleece, a drainage and water storage board, a filter fleece, then the growing medium and plants. On an extensive sedum roof, the kind usually seen on Cambridge garden rooms, rear extensions and the flat roofs of newer homes around Eddington and Trumpington Meadows, that build-up is typically 80 to 150mm deep. Wildflower and biodiverse roofs are often deeper, and an intensive roof with shrubs or lawn can be several times that.
The general flat roof guidance of a 150mm minimum upstand still applies, but the reference point moves up. If the kerb is set out against the membrane, the substrate can swallow most of its height. We work from the top of the growing layer at the lowest side of the rooflight, then add the margin detail around it. The companion answer on how high a flat rooflight upstand should be explains the measurement itself, and the guide to flat rooflight upstands covers how the kerb is built.
| Roof type | Typical build-up above membrane | Kerb height to plan for |
|---|---|---|
| Sedum mat | 80 to 120mm | 250 to 300mm |
| Biodiverse or wildflower | 100 to 200mm | 300 to 400mm |
| Intensive (lawn, shrubs) | 200mm and more | Designed case by case |
These are guide figures from the membrane surface. The real height comes from the green roof supplier's section and the rooflight maker's fitting instructions, and on a new extension it should be drawn before the roof is built.
The gravel margin and where the water goes
Plants and soil must stop short of the rooflight. A strip of washed gravel or pebbles around the kerb, held back by a perforated edge trim, keeps the frame clean, lets water drain and gives a firebreak between the planting and the glazing.
The Green Roof Organisation's code of practice for green roofs in the UK asks for vegetation-free zones around upstands, outlets and penetrations. Around a rooflight the margin is commonly 300 to 500mm wide, and wider on the side that faces the fall of the roof. It does several jobs at once. Sedum spreads, and without a margin it will creep up against the frame and hold moisture at the membrane turn-up. Wind-blown seed from the Cambridge fields finds any strip of soil against a kerb. Dry sedum and grasses can also carry fire in a hot spell, and the gravel break keeps the rooflight and its seals clear of that.
Drainage matters as much as planting. A green roof holds water by design, so the rooflight should never sit where the roof ponds, and never on the downhill side of a run of substrate with no way for water to pass it. The drainage board runs under the margin so water flows round the kerb towards the outlet. Where the rooflight sits near a gutter or outlet, the outlet needs its own inspection chamber and gravel surround so it stays clear. How the membrane is dressed up the kerb on EPDM, single-ply or bitumen is set out in flat rooflights and roof coverings.
Weight, structure and getting onto the roof
A green roof is heavy when wet, and cutting an opening for a rooflight changes how that weight reaches the walls. The structure has to be checked for both at once.
A saturated extensive sedum roof typically weighs somewhere between 60 and 150kg per square metre, depending on depth and substrate, and deeper roofs weigh a good deal more. Where joists are trimmed around a rooflight opening, the trimmers carry a share of that load as well as the unit itself. On an existing roof, and on any roof with a larger or long linear unit, an engineer may need to size the new members. The answer on whether a flat roof needs strengthening for a rooflight goes through when that is needed.
Access is the other practical point. A sedum roof needs a visit or two a year to weed, feed and clear outlets, and the person doing it should not step on or lean against the glass. Most flat rooflights are designed to be non-fragile under the UK test standard, but a walk-on route along the margin and away from the glazing is still the sensible layout. If the rooflight is the only way onto the roof, an opening access hatch is worth considering.
Retrofitting a rooflight into a green roof that already exists
It can be done, but it is a bigger job than on a plain roof, because the planting and drainage layers have to come up before the roof can be opened.
The substrate and sedum are lifted back from an area well beyond the opening and set aside, the drainage board is cut back, and the membrane is opened and re-detailed around the new kerb. The kerb is built, insulated and dressed, the rooflight fitted, then the gravel margin and edge trim go in before the planting is relaid. Sedum mats usually recover once relaid, though the margin will look bare for a season. On a warm deck roof the insulation also has to run up the kerb so there is no cold bridge, which is covered in the answer on rooflights on warm deck flat roofs.
What a taller kerb does to the light in the room
A deeper upstand makes a deeper light well. That trims a little low winter sun and narrows the patch of light on the floor, which is why a green roof rooflight is often sized or splayed slightly differently.
In Cambridge the midwinter sun is only about 14 degrees above the horizon at noon, and it rises past 60 degrees in June. A 300mm kerb plus the roof depth below can mean a reveal of 500mm or more from glass to ceiling. In summer that barely matters, as the sun comes steeply through. In winter the deep sides of the opening catch much of the direct sun, and the room gets mostly sky light. Splaying the internal reveal outwards, or going a size up, gives back a good share of that light and spreads it wider across the ceiling. Glass choice shifts too: self-cleaning coatings help less on near-level glass, and pollen and dust from the planting settle on it, as the answer on self-cleaning flat rooflights explains. How we model this for your room is set out in how we plan the light, and the wider options for this kind of skylight are on our flat roof rooflight installation page.
Questions people also ask
Can the sedum grow right up to the rooflight frame?
No. Plants and substrate should stop at the gravel margin, commonly 300 to 500mm from the kerb. Planting against the frame traps moisture at the membrane turn-up, lets roots find the seal and brings dry growth close to the glazing in summer.
Does a green roof change whether I need planning permission for a rooflight?
The rooflight is judged on its own terms, such as how far it projects above the roof plane and whether the house is in a conservation area. A taller kerb can push a unit past permitted development limits on an existing roof, so it is worth checking; see whether a flat rooflight needs planning permission.
Can I add a green roof later around an existing rooflight?
Only if the kerb is tall enough for the new build-up. A rooflight on a 150mm kerb over a bare membrane will be too low once substrate goes down, so the upstand usually has to be raised, or the rooflight renewed on a taller kerb, at the same time.
Is an opening rooflight a good idea on a green roof?
It can be, for ventilation or roof access, as long as the opening sash clears the margin and nothing growing can foul the hinge or motor. Rain sensors on electric units close the sash before water reaches the room.
If you are planning a green roof extension with a skylight in it, book a light survey and we will draw the kerb, margin and light together.
