A flat rooflight on a GRP roof, timed with the roofer

GRP stands for glass reinforced plastic. It is a polyester or similar resin, reinforced with glass fibre matting, laid wet on the roof and left to cure into a single hard layer with no joins.

The build-up starts with a deck. This is usually exterior grade plywood or OSB3 board fixed to the joists, with the joints tight and every board screwed down flat. The roofer lays a layer of chopped strand mat on the deck, wets it out with catalysed resin and rolls the air out with a ribbed roller. A pigmented topcoat, often called a gelcoat or finishing coat, goes over the top. Edge trims, drip details and upstands are laminated in as part of the same operation, so the whole roof becomes one continuous membrane.

That continuity is the appeal. There are no welded laps, no bonded seams and no glued joints to fail. It is also the reason a rooflight needs thought. A felt roof or an EPDM roof can be cut and lapped later without much fuss. A cured GRP roof is rigid, and it only bonds well to fresh laminate in particular conditions. You can read how the other membranes compare in adding a flat rooflight to a felt roof and fitting a flat rooflight into an EPDM rubber roof. Fibreglass asks for more planning than either.

The membrane also talks back to the room below. A rigid skin over a thin deck carries rain sound very directly, and a large area of flat glass added to it can make a kitchen ring in a downpour. We come back to that near the end.

The roofer needs something to laminate onto, and that something is the upstand. On a GRP roof it is almost always a timber frame, built up from treated softwood and skinned with the same board as the deck.

The height of the finished upstand is measured from the finished roof surface, not from the bare deck. The usual working target is at least 150 millimetres of clear upstand above the surface of the roof, which is the figure the roofing standards point towards, and the height needs to grow when the roof is deep in insulation or when water is likely to pond nearby. We set out the reasoning in flat roof rooflight upstands: height, insulation and finish and in the shorter answer on how high a flat rooflight upstand should be.

Fibreglass is unforgiving about shape. Glass matting will not bend tightly around a square timber corner. It lifts, bridges the angle and leaves an air void that later shows as a blister. So the upstand is prepared before any resin is opened:

• The vertical corners of the kerb are planed or sanded to a radius, so the mat follows them.
• The internal angle where the upstand meets the deck gets a small timber or resin fillet, so the laminate sweeps round instead of folding.
• The top edge is left square and clean, with a clear plan for where the laminate will stop and what will sit on top of it.
• The timber is dry, because resin over damp wood sheds its bond.

If you are not sure what an upstand is or why it is built this way, what a builder's upstand is for a rooflight covers the basic idea in a couple of minutes.

The question of timing is the centre of this page. There is more than one workable order, and each suits a different roof. The table sets them out.

On a new extension the first order is the cleanest. The carpenter frames the opening and builds the kerb. The roofer then laminates the whole roof, including the faces of the upstand, and finishes the top edge. Only when the laminate has cured does the rooflight installer come back, bed the glass unit on the top of the kerb and fix and seal it. The glass stays in its packaging, stored flat and away from the work, until the resin work is done. Nothing on the roof is splashed, sanded or dusted.

The second order, where the glass goes on first and the roofer laminates up to it, is sometimes chosen to get the building weathertight quickly. It works, but the glass has to be masked and protected, and the roofer has to cut the laminate in carefully around the frame. Resin drips on an acrylic dome or on the outer face of a glass unit are difficult to remove without scratching.

A pre-formed GRP upstand is a moulded kerb made in a workshop and lowered onto the deck, with the roofer laminating it into the roof. It is useful on small openings where building a timber kerb would take longer than making the moulding. The lead time is the catch, and the size has to be final before the order goes in.

Whichever order is chosen, one person should hold the sequence. We agree it at the survey and write it on the quote, including who supplies the kerb, who lays the membrane and who fixes the glass. Our steps for installing a flat roof rooflight show where each trade fits in the wider job.

The resin work on the day follows a rhythm, and each part of it has a reason that shows later in the finished joint.

First, the deck and the upstand are cleaned, dried and primed if the resin system needs it. The roofer cuts the matting into pieces sized to the kerb, leaving extra at every corner. On a rooflight this means the laminate is not laid in one sheet across the roof and up the upstand, but in overlapping pieces, each one wetted out and rolled before the next goes on.

The matting is run from the roof surface up the face of the upstand and finished on the top edge. A second run, across the top and partly down the inside face, locks the edge. The laminate should overlap at the corners by a hand's width or more, so there is no straight seam where two pieces butt together. A butt joint in fibreglass becomes a line of weakness, and thermal movement finds it.

Then the roofer rolls out every bubble. Air under the matting is the most common early fault in GRP work, and it is almost always caused by rushing or by damp in the timber. A well-rolled laminate looks matt and slightly textured, with no white patches where fibre stands proud of the resin.

Finally the topcoat goes on. Some roofers add a second layer of finishing coat on the vertical faces, because these surfaces see more sun than the flat deck and because the top edge is where your eye goes when you look at the finished roof.

GRP is a chemical reaction, and chemistry has preferences. The resin needs a certain warmth to cure, and it needs a dry surface to bond to. That makes the weather window tighter than on most other flat roof work.

Cold slows the cure. At low temperatures the resin stays tacky and soft, which is unhelpful on a vertical upstand where a soft laminate can sag. Damp is worse. Moisture on the deck or in the timber, even as a film of dew in the early morning, interferes with the bond and can show later as lifting or whitish staining. Rain on uncured resin damages the surface. Sun is not a friend either: hot timber can make the resin cure too quickly, before the air has been rolled out.

The working answer is to plan the roof for a dry spell, to start after surfaces have dried out, and to cover or postpone if the forecast turns. A sensible roofer will tell you the same. When the two trades are coordinated, the rooflight installer keeps the kerb covered and the opening protected while waiting for the laminate, so the weather is not the thing that decides the order.

In a Crawley spring, with dew lying on the lawns until mid-morning and the odd cold snap, a one-day laminate on the upstand faces is more realistic than trying to force the whole job into one early start. Give the roof the time the resin asks for.

A common starting point is a GRP roof already in place. The extension is a few years old, the kitchen is dark and someone has suggested a skylight in the fibreglass flat roof. It can be done, and the detail takes more care than on a new build.

The opening is cut from above through the GRP, the deck and any insulation, then framed in the joists. Two checks come first. One is structural: whether the joists need trimming, which we explain in why the joists need cutting for a flat rooflight. The other is the roof itself: how old it is, what its fall is like, and whether there is a warm or cold build. Our answer to whether you can put a rooflight in an existing flat roof goes through those checks, and whether the roof needs insulating for a new rooflight covers the insulation side.

The new kerb is then built on the deck around the opening, and the laminate is tied into the old roof. Here the chemistry matters most. Fresh resin bonds to cured resin chemically for only a short while. After that, the old surface has to be ground back to bare fibre with a coarse disc and cleaned so the new laminate can key mechanically. The roofer tapers the old edge, overlaps the new matting well onto it and blends the topcoat. A sound, clean, dry and carefully abraded edge is the difference between a join that disappears in five years and a line that opens along the overlap.

Colour is another reality. Old topcoat fades, so new topcoat rarely matches exactly. Often the sensible approach is to recoat the whole roof, or at least a full section from edge to edge, rather than leave a fresh island of colour around the glass.

A GRP roof is built to a designed fall, and a rooflight upstand interrupts it. The upstand is a dam across the route the water wants to take. If it sits too close to the high side of the roof, water banks up against it and finds every pinhole in the laminate.

So the kerb position is checked against the falls at the survey, not after the deck is boarded. A flat roof is typically designed to a fall of 1:40, so that once built it still drains at 1:80 or better. On a roof that falls toward a single gutter, the rooflight is usually set where water can divide and run around it, with the long sides running with the fall rather than across it. A small timber cricket, or a tapered insulation piece, can be added on the high side to send water round the corners. Falls, drainage and why flat rooflights are never perfectly level goes into the geometry, and why water sits on a flat rooflight explains the glass side of the same problem.

GRP is better than most membranes at coping with some standing water, because it has no seams to lift. That is not a reason to design the roof to hold it. Standing water shows sooner as staining and later as a tired topcoat.

Once the laminate is cured, the rooflight goes on. The rooflight installer checks the kerb first. The top should be flat, dry and free of resin nibs. The glass unit sits on a compressible seal or a bead of the sealant recommended by the manufacturer, then is fixed through the frame with stainless fixings, avoiding any fastener that would puncture the laminate below the flange line.

An important detail is how the frame lands relative to the laminate. The frame flange or overhang should lap over the top edge of the GRP so water sheds clear of it. The exposed laminate on the vertical faces is then the roofer's responsibility to finish, and the glass fitter's to keep clean.

Seals are where the quiet specification starts to show. A poorly compressed seal or a hardened gasket can whistle in wind and pass sound into the room. Good seals are chosen for the glass and frame, are fitted to an even line, and are checked when the job is done. Our quiet spec records which seals and which upstand finish we have written for the room.

A GRP roof is a hard, thin shell. Rain on it is loud when the deck is thin and the ceiling below has little mass. Rain on a large flat glass panel added to that is louder again, because glass rings where a soft membrane would absorb.

The answer is a specification, not a slogan. The glass can be a laminated acoustic inner pane with a thicker outer pane. The cavity is chosen for the job. The seals are compressed evenly. The upstand is insulated and lined so the sound does not travel through the timber. And at the survey we listen to the room, because a bedroom on an approach path and a kitchen in a quiet cul-de-sac need different things. Quieter glazing for a flat roof rooflight over a bedroom goes through the build, and the wider principle is on the flat roof rooflight installation hub.

Around Gatwick, and alongside the M23 and the A23, the sound that comes down through a roof can be aircraft and traffic as much as rain. A laminated inner pane works on all three.

A GRP roof usually comes with its own guarantee from the roofer, covering the membrane. Our work carries a 10-year workmanship guarantee, which is not insurance-backed, and covers the installation of the rooflight and its upstand. A well-run job makes the boundary clear on paper.

So before anything starts, ask for three things in writing. Who builds the kerb? Who laminates it and who signs off the top edge? And who decides, on the day, whether the weather allows the laminate to go ahead? On our quotes these are written into the job sheet. The guarantee page sets out what ours covers.

Where the rooflight work is notifiable, we handle the Building Control notification, and we install to current Building Regulations, including the thermal requirements of Approved Document L and the structural and safety points that apply to the opening.

Take a single storey kitchen extension on a 1960s house, flat roof laid in GRP about five years ago. The homeowner wants a 1,000 by 1,500 millimetre fixed rooflight over the dining end.

At the survey we would check the deck and fall, looking for where the roof drains and whether the kerb position leaves room to dress the laminate into all four corners. We would measure the ceiling, check the joists and note how far the opening sits from the wall. We would ask about noise: the kitchen may sit under a flight path or beside a busy road, and we would listen to decide on the glass.

The steps then run in a set order. The carpenter cuts and trims the opening. A timber kerb is built and covered. The roofer grinds the old laminate around the kerb, lays fresh matting up the upstand faces, overlaps the old roof by a generous margin and finishes the edge. The rooflight installer returns after the cure time, fits and seals the glass and lines the reveal inside. Lining the reveal under a flat rooflight explains the last part. The roof is recoated edge to edge if the colours disagree.

The job is unremarkable when it goes well. No resin on the glass, no air under the matting, water running clear around the kerb and a kitchen that does not ring when it rains.

Can I fit a rooflight myself if the roofer has already finished?

The opening involves structural work, a new kerb and a detail in the roof surface. Cutting through the deck, trimming the joists and tying new laminate into old are all linked, so it is a job for a planned sequence rather than a casual afternoon. A survey first will tell you what the roof needs.

How long should the laminate cure before the glass goes on?

That depends on the resin system, the temperature and the thickness of the laminate. Follow the roofer's guidance and the glass manufacturer's instructions. The rooflight should go on when the laminate is hard, dry and free from tack, not when the calendar says so.

Will the GRP guarantee be affected?

Ask the roofer who laid the roof. Put the question before any work starts, and ask for the answer on paper. Coordinating the two trades, as described above, keeps the membrane work in the hands of the membrane specialist.

Is fibreglass a good roof for a rooflight?

Yes, when the upstand is planned with it. The jointless skin climbs a kerb neatly, and the result is one of the tidiest joints available on a flat roof. The care goes into the sequence, the timber preparation and the weather. If you would like a written plan for your own roof, get a quote and we will set out the sequence and the quiet spec for the room.