Yes, and green roofs and rooflights sit together well, because both belong to the same build-up. The one thing that changes everything is the datum. On a bare roof the upstand is measured from the waterproof covering. On a green roof it is measured from the top of the growing medium, because that is the level water and soil actually reach. A 150mm kerb that would be right on a single ply roof becomes a 150mm kerb plus the full depth of the substrate, and on a biodiverse roof that can be another 150mm again.
Measuring the upstand from the top of the planting
Everything above the waterproofing on a green roof is ballast: a root barrier, a drainage and reservoir layer, a filter fleece, the growing medium and the plants themselves. Together they are a layer of saturated material sitting against whatever passes through the roof.
So the kerb has to stand clear of that whole assembly, not just of the membrane. In practice we work to a minimum of 150mm of visible upstand measured from the finished surface of the substrate, which on a sedum blanket system means roughly 230mm to 250mm above the waterproofing, and considerably more on a deeper scheme. Getting this wrong buries the termination of the covering in wet soil, which is the one place it must never be.
Sedum blanket, extensive and biodiverse depths
The three common systems differ enough to change the kerb, and it is worth knowing which one is being specified before the rooflight is ordered.
| System | Typical build-up above waterproofing | Saturated weight |
|---|---|---|
| Sedum blanket | 60mm to 100mm | 60 to 100 kg/m2 |
| Extensive substrate | 80mm to 150mm | 100 to 180 kg/m2 |
| Biodiverse or wildflower | 150mm to 250mm | 180 to 350 kg/m2 |
| Intensive or roof garden | 300mm upwards | 400 kg/m2 upwards |
Those weights are the reason the structural question comes before the rooflight question on any roof that was not designed green from the outset.
The vegetation-free margin round the unit
Planting is not brought up against the kerb. A margin of washed rounded gravel, typically 40mm aggregate, is laid in a band around the rooflight, held back from the substrate by an edge restraint or a perforated trim.
That band does three jobs. It gives water a free-draining route around the obstruction, it acts as a fire break between areas of dry summer vegetation and the unit, and it keeps roots and moisture away from the point where the membrane terminates. The usual width is 300mm to 500mm, and it is a design decision rather than a detail left to whoever lays the substrate. The same band is normally run at the roof perimeter and at any outlet.
Root barriers and where they stop
A green roof needs a root-resistant layer above the waterproofing, either a separate barrier sheet or a membrane that carries root resistance in its own specification. Around a rooflight that layer has to be carried up the kerb and terminated above the substrate line, in the same way as the waterproofing.
A root barrier that stops at the base of the upstand is the commonest defect on a retrofitted green roof, and the failure it produces takes five years to appear. Roots track sideways, find the vertical face, and work into any lap or fixing at the base of the kerb. Where the barrier is a separate sheet it is welded or taped to itself at the corners, not just overlapped.
Drainage, and why the kerb must not act as a dam
Water in a green roof travels through the drainage layer under the substrate, not across the surface, so the obstruction the rooflight creates is a sub-surface one. Blocking that layer produces a saturated area upslope of the unit that stays wet all year and kills the planting there before it does anything else.
The answer is the same in principle as on a bare roof and different in execution. The drainage board is cut round the gravel margin so flow continues past the unit on both sides, and on a wide rooflight a channel is formed behind the upslope face to collect and carry water round. The falls, drainage and ponding page covers how those routes are set out.
The structural question, and who answers it
Cutting an opening in a roof removes joists and transfers their load onto trimmers either side, and those trimmers now carry the weight of the green roof as well as the rooflight. On a biodiverse build-up that is a meaningful additional load and it is not something we sign off.
Where a rooflight goes into a green roof that already exists, or where a green roof is going onto a roof that already has a rooflight, a structural engineer looks at the joist size, span and condition and specifies the trimmers. That is a modest cost and it is the right way round. We are happy to open up and take measurements to give an engineer what they need before anybody commits to the unit.
Getting to the glass to clean it
A rooflight in the middle of a wildflower roof is harder to reach than one on a bare membrane, and walking on planting compacts the substrate and kills it. So access is designed in rather than improvised.
Usually that means a run of paving slabs on adjustable supports or rubber pads from the access point to the gravel margin, forming a path that does not sink and does not crush the growth. On a roof where the only access is a ladder at the eaves, be realistic about how often the glass will actually get cleaned, and specify a self-cleaning outer pane and a slope towards the nearest edge to make the intervals longer.
Growth against the glass and the light it takes
Sedum will climb a gravel margin given a few seasons, and wildflower species will lean over anything at their own height. On a rooflight sitting only 150mm above the substrate, a stand of summer growth around three sides is enough to noticeably darken the room below by August.
Two things deal with it. A slightly taller kerb than the minimum, 200mm to 250mm above the substrate rather than 150mm, keeps the glass clear of the seasonal growth. And the annual cut back, which every green roof needs anyway, is set out to include the margin round the unit. It is worth agreeing who does that before the roof is planted.
Retrofitting into an established green roof
This is the harder version of the job and it is entirely doable. The planting and substrate are lifted back in a wide zone around the opening and set aside on boards, sedum blanket in rolls if it is a blanket system. The opening is cut, trimmed and kerbed, the waterproofing and root barrier are made good and lapped into the existing, the gravel margin is formed, and the growing medium is reinstated.
The realistic outcome is that a blanket system relays and knits back within a season, while a mature wildflower roof will show where the work happened for a couple of years. That is worth knowing in advance. We install and replace rooflights across Oxford and up to twenty five miles by road, the roof is never left open overnight, and our workmanship carries a ten-year guarantee.
Helpful reading on this
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