A flat rooflight sits on a kerb, and the kerb is dressed into whatever waterproof covering the roof already has. On GRP the kerb is laminated in with the roof itself. On EPDM the rubber sheet is bonded up the kerb and finished beneath the rooflight's cill. On felt the layers are lapped up and capped. The rooflight can be the same in all three cases. What changes is how the roof and the kerb are joined, and that join decides how the opening behaves for decades.
This page takes the three coverings one at a time, then deals with the part nobody sees from the roof: the build-up under the kerb, and the lining and plaster that meet the ceiling in the room below. If you are choosing between a flat rooflight and a lantern first, the flat roof rooflights page covers the wider choice.
Find out what your roof is made of first
Most people know they have a flat roof and not much else about it. Before anyone can quote for a rooflight, the covering has to be identified, because each one is cut, lifted and rejoined differently.
A smooth, slightly glossy surface with no joins that looks like fibreglass is GRP. A black or grey rubber sheet, matt, with few joins and neat folded corners, is EPDM. A mineral-chip or dark bituminous surface with visible overlaps, or lines where sheets meet, is felt. Some roofs have been covered twice, and the survey has to find that out.
The age of the covering matters as much as the type. A rooflight is a long-lived product, and cutting a hole in a roof that is near the end of its life invites a conversation about doing the roof first. A sensible survey notes the covering, its condition around the opening and whether the deck beneath is sound.
GRP: one skin, one kerb
GRP is glass-reinforced plastic, usually called fibreglass. It is laid as resin over glass matting in situ, and it cures into a single rigid skin with no joins. It suits small roofs with awkward shapes and a lot of edges.
For a rooflight, the neatest answer on GRP is a kerb that becomes part of the skin. A timber upstand is fixed around the opening, glass matting and resin are taken up and over it, and the finished kerb is topcoated in the roof colour. There are also factory-made GRP kerbs that are laminated onto the roof deck and bonded in the same way. Either way the outcome is a continuous waterproof surface from roof to kerb, with no lapped joint at the base where water might sit.
GRP asks for dry conditions and a window of settled weather, because resin needs to cure properly. That is a scheduling matter, not a reason to avoid it. It also asks the installer to be a GRP laminator or to work with one: the join between the old skin and the new lamination has to be keyed, cleaned and overlapped, not just painted over. A tidy GRP kerb has a rounded, confident edge and a topcoat that matches the roof beside it, with no visible line where new met old.
EPDM: a rubber sheet bonded up the kerb
EPDM is a synthetic rubber membrane, supplied in large sheets and often laid as one piece across a whole roof. It is bonded to the deck or insulation with adhesive, folded neatly at the corners, and held at the edges with trims. Its main strengths are flexibility and a long service life, and it handles movement without cracking.
Around a rooflight, the sheet is lifted up the face of the kerb and secured, then covered by the cill or the rooflight's own skirt. Corners are folded or finished with preformed corner pieces rather than cut and joined, because every cut is a joint, and every joint is a place that depends on adhesive and tape. The best EPDM work on a rooflight kerb has very few of them.
Because EPDM is flexible, it needs a firm, smooth, clean surface beneath, and it suits a kerb with a flat face. A rough timber kerb, a sharp edge or a stray nail head can mark the rubber over time. For that reason the kerb is faced with a smooth board and the arrises are eased. The rubber itself is dressed up and over, then the rooflight frame sits on top of the upstand and is fixed through the kerb, not through the membrane.
If your roof is already rubber, ask for the sheet maker and age to be recorded in the quote, so the new join follows the right system.
Felt: layers lapped up and capped
Felt roofs are built from layers. A traditional built-up felt roof has two or three bituminous sheets, laid with staggered laps, each one hiding the joints of the one below. Modern high-performance felt is more durable than the old three-layer roofs many Devon houses still carry, and is usually laid in two layers, with a heat-bonded cap sheet on top.
At a rooflight kerb the layers are turned up the face of the kerb in sequence: a base layer first, then a cap sheet over it, each lapped over the one below so water can only run over joints, never into them. The top edge is then covered with a flashing, a metal capping or the rooflight's own upstand skirt. Where felt is applied with a torch, the kerb and the timber behind it must be protected from the flame, which is one reason the build-up and fire safety on the day are planned in advance.
Felt tolerates the slightly uneven, older roof better than the other two coverings, and it is often the right one to match when a flat roof is already felted. Bituminous felt does age, and its surface can blister or craze in sun over many years. That's why a rooflight fitted into an old felt roof should be treated as the moment to review the covering round the opening, not just to cut a hole in it.
Kerb height, falls and the build-up beneath
Whatever the covering, the kerb needs a height. The membrane is turned up the face, and the water on the roof surface needs to stay well below the point where the rooflight meets the kerb. Guidance generally asks for the waterproofing to rise a minimum distance above the finished roof surface, and the exact figure depends on the covering and the manufacturer's system, so it is one to confirm for your roof rather than assume. If you want the reasoning in more depth, see our guide to flat rooflight upstand heights.
A flat roof is never completely flat. It needs a fall, so water drains to an outlet or an edge, and the rooflight should not sit in the path of a pond. Where the deck has little fall, a small tapered insulation board, called a cricket, is set on the uphill side of the kerb, so water is steered round it and not pushed against it. It stops a puddle collecting against the kerb.
Underneath the covering is the roof build-up, and there are two main types:
- A warm roof has the insulation above the deck, under the waterproof layer. The deck stays warm, and the kerb is raised on top of the insulation, so the insulation continues up inside the kerb walls.
- A cold roof has the insulation between or below the joists, with a ventilated void above it. The kerb is fixed to the joists, and the void has to be kept clear and ventilated around the opening.
The roof build decides how the kerb is framed, how much of it is hidden, and how the lining below is formed.
The kerb is the part that does the work
A flat rooflight is only as good as what it stands on. Two kinds of kerb are in common use. A site-built timber kerb is made from treated timber and plywood around the opening, insulated and clad in the roof covering. A factory kerb comes with the rooflight as a prefabricated upstand, usually of insulated GRP or timber and metal, built to fit the unit.
A factory kerb is consistent, quick to fit and designed with the rooflight. A site-built kerb can be formed to any size, which matters when the opening is unusual.
Whichever is chosen, it is fixed to a structure that can carry it. The roof joists around the opening are doubled or trimmed to take the load, because cutting a joist without a trimmer to carry it is a structural alteration, which is why it falls under Building Regulations. The rooflight frame is then fixed to the kerb and to nothing else.
Matching the rooflight to the covering
The kerb upstand and the glass are two separate choices, but they have to agree. Fixed and opening units sit on the same kind of kerb, and an opening unit adds hinges, an actuator or a manual opening stay, which want a kerb that is square and stiff. Our guide to fixed or opening flat rooflights explains when opening is worth it.
| Point | GRP | EPDM | Felt |
|---|---|---|---|
| How the kerb is dressed | Laminated into the skin | Sheet bonded up the face | Layers lapped and capped |
| Joints at the kerb | None in the skin | Few, with folded corners | Several, staggered laps |
| Best on | Small roofs with many edges | Large, simple roofs | Older timber-decked roofs |
| Weather needed | Dry spell for curing | Dry deck for adhesive | Dry deck, fire care with a torch |
| Kerb surface wanted | Sound timber or GRP kerb | Smooth board, eased edges | Firm, clean timber face |
Zinc, lead and green roofs are detailed differently again, and the survey records which you have.
Planning and Building Regulations for a flat roof opening
A house rooflight is usually permitted development when it rises no more than 150mm off the roof plane and stays under the roof's top level. A flat roof rooflight sits on a kerb, so it is worth checking that the kerb and glass together stay within that limit. Flats and maisonettes do not have householder permitted development, listed buildings always need consent, and in a conservation area or national park rights can be restricted, so check with the local planning authority where any of those apply. Our answer on adding a flat rooflight to an existing roof covers the questions to ask first.
A new rooflight in an existing roof is building work. Cutting or trimming the structure is a structural alteration, and the new unit has to meet Part L of the Building Regulations, which covers conservation of fuel and power. Replacing a whole rooflight, frame and glass, is also notifiable. We handle the Building Control notification where the work needs it. Replacing only a sealed glass unit in an existing frame is not notifiable.
The inside: lining the kerb and meeting the ceiling
From the room, you will never see the kerb dressing, the membrane or the cricket. You will see the ceiling opening, the lining that runs up through the joist depth and the inside face of the kerb. That surface has to be formed, insulated and finished, and on a flat roof it is often deeper than people expect.
Joists are commonly 150 to 225mm deep, and the kerb adds a further stretch above the deck. The lining therefore runs up through the joists and then through the thickness of the kerb walls. A straight, square lining is simple and neat. A splayed lining, wider at the ceiling than at the glass, takes more of the daylight into the room, and on a deeper flat roof it is the better choice. The flat roof rooflight lining and the plastering around it are what you will look at every day, and the guide to how a rooflight is finished inside goes through each layer.
Insulation matters here as it does outside. The lining faces sit inside the warm envelope on a warm roof, but in a cold roof void they are surrounded by cold air, and the insulation has to continue down them so the lining is not the coldest surface in the room. A vapour control layer is lapped onto the kerb so warm, damp air cannot reach the cold side. Skip either and condensation shows first at the lining, at the sill, and it comes back through paint.
The ceiling itself deserves thought. Older ceilings under flat roofs on Georgian and Victorian houses may be lath and plaster, and cutting those needs support and a clean edge. Where the house has lime plaster, we finish the opening in lime. Each of these is recorded in the finish schedule that comes with every quote: the room, the ceiling and roof build, how the lining is formed, how it is finished, and how dust is managed.
Replacing an existing flat rooflight
A kerb in good order, of the right height and fixed to a solid structure, can often take a new unit, with the covering turned up fresh to meet it. An old kerb with soft timber, a low upstand or no insulation is better rebuilt. If the new unit is a different size, the opening is re-formed and the ceiling is touched. If it matches, the lining can often stay, with its edge cut back and its paint line redone.
What to ask for in a quote
A good flat rooflight quote names the covering and says how the kerb will be dressed into it. Ask for these things in writing:
- the roof covering as found, its age and its condition around the opening;
- the kerb type, its height above the finished roof, and the insulation inside it;
- how the covering will be joined to the kerb, by laminating, bonding or lapping;
- the lining shape, straight or splayed, and how it will be finished;
- who notifies Building Control and who handles any planning check.
The finish schedule that arrives with every Brightview quote covers the inside half of that list. We carry a 10-year workmanship guarantee on what we install and form. To start, ask for a quote and tell us what your roof is made of, or send a photo and we will identify it at the survey. For the full picture of the options, return to the flat roof rooflights page.