A flat rooflight on an EPDM roof, dressed into the rubber

EPDM is a synthetic rubber that stays flexible in cold weather and is sold in wide rolls. Those two habits are exactly what a rooflight opening asks for.

The wide roll matters most. A rooflight breaks a flat roof with four corners and four short walls of upstand, and every seam near them is a place water can test. On a small extension or a garage roof the membrane is often a single sheet with no join at all, and the installer can cut an opening in it, bond the sheet up the upstand and finish the corners with purpose made fittings. There are very few joints to get right because the material was chosen to avoid them.

The flexibility matters second. Rubber stretches around a corner and follows a slight movement between the timber upstand and the deck without cracking. It does not need a blowtorch, which is one reason EPDM suits roofs where the ceiling below has been freshly plastered and the owner would rather not have open flame overhead.

The other side of the coin is that rubber does not forgive dirt. It bonds with adhesive rather than heat, and adhesive only grips a clean, dry, compatible surface. Most of what follows is about getting that surface right.

The first visit is about working out what the existing membrane is and how it is held down. The answer decides the whole method.

A rubber roof is fixed in one of three ways. It may be fully bonded to a timber or insulation deck, loose laid and held down with ballast, or mechanically fixed under battens or at the edges. Each one changes how an opening is made. A fully bonded sheet can be cut and lifted in a controlled area. A loose laid sheet has to be secured around the opening before the ballast is moved, or the membrane will creep. A mechanically fixed roof needs the fixings traced so that none are cut through.

We also look at age and condition. A membrane that has been in place for years carries a weathered surface of dust, pollen and algae. New rubber will not bond to that. It has to be cleaned with a proper splice wash and primed, and the sheet must be sound enough for the new work to hang from it. Where the old sheet is tired, cracked at the edges or has lifted from the deck, the better answer is to renew the membrane across the whole roof, or at least across the zone around the opening, rather than dress new rubber onto old.

The underside tells us as much as the top. Before we commit to a position, we check:

• Where the joists run, so the opening can be framed with trimmers rather than forced across a span.
• What insulation sits in the roof, because some foam boards react with solvent adhesives.
• Where the falls run and where the water leaves the roof.
• Whether a wall, parapet or party wall is close enough to squeeze the working space.

Our page on putting a rooflight in an existing flat roof covers the wider question of whether a given roof can take one at all.

On an EPDM roof the glass never sits on the membrane. It sits on an upstand, a low wall that raises the frame clear of the water, and the rubber is dressed up and over that wall.

The height is set by two things. The membrane has to turn up far enough above the finished roof surface to be sealed properly, and the frame has to stand clear of water that collects around it, along with snow and leaf litter. The accepted minimum for the waterproof turn-up is 150 millimetres above the finished surface, and we measure from the surface of the finished roof, never from the bare deck. If there is insulation and a roofing build-up on the deck, the upstand has to be tall enough to clear all of it and still reach the minimum. Our guide to upstand height, insulation and finish walks through that sum, and the answer on how high a flat rooflight upstand should be gives the short form.

There are two common ways to get one. A site built timber upstand is framed on the deck, clad in exterior grade ply and insulated on both faces. A manufactured upstand arrives ready made, often in insulated GRP or a composite, and is fixed to the deck and lapped with the roofing build-up. The second route is quicker and more consistent. The first gives more freedom on an odd size. Either way the face that receives the rubber has to be flat, dry and free of splinters, sharp screw heads and loose paint. If you want the background, what a builder's upstand is explains the idea in plain terms.

The quiet side of the upstand is easy to miss. A hollow, uninsulated timber box acts a little like a drum skin under rain. A properly insulated upstand, lined on the inside and fixed without gaps, is part of the same specification as the glass. We write the upstand build into the quiet spec for that reason.

The membrane goes on in a set order, and the order is what makes the joint part of the roof rather than something stuck on afterwards. This is the sequence we follow on a typical bonded roof.

1. Prepare the faces. The deck, the upstand faces and the edge of the existing sheet are cleaned, dried and primed where the adhesive maker asks for it.
2. Lay the field sheet. The main membrane is folded back from the opening and bonded to the deck, so it runs right up to the foot of the upstand with no gap.
3. Turn it up the wall. The sheet is carried up the face of the upstand in one piece where it can be, stretched into the corner without wrinkles and bonded across its full height.
4. Finish the corners. Each of the four corners gets a preformed corner or a layer of uncured flashing, so nothing depends on a folded, cut and taped corner.
5. Take it over the top. The rubber is wrapped across the top of the upstand to its inner edge, so the top surface under the frame is also covered.
6. Fix the frame. The rooflight frame is fixed down through the membrane into the upstand and sealed so that the top edge of the rubber sits under the frame, never exposed to the weather.

The temperature on the day matters. Rubber dressed on a cold morning is stiff and wants to spring back from the corners, and adhesives cure more slowly. A dry day with the deck warmed by some sun gives a much better bond. We plan the membrane work for a dry window and do not cut the existing sheet in the morning and leave it open for rain.

The corners decide whether the job lasts. A straight run of rubber up a wall is easy. A corner has to take the same sheet in two directions, and that is where poor work shows itself.

A preformed corner is a moulded piece of rubber that already has the shape, so it only has to be bonded and taped at its edges. Uncured flashing is a thin, soft rubber that is cut to size, pressed into the corner and left to vulcanise in place, which suits awkward returns. Both are made to go with the membrane. What we avoid is the folded corner, where the sheet is cut and pleated and the pleat is sealed with tape, because a pleat is a path for water.

Where sheets overlap, the lap is cleaned, primed and closed with seam tape rolled down hard across the full width. The lap is laid so that the water flows over it rather than into it, which means working from the low side of the roof towards the high side. Seams that run up the falls toward an upstand are kept well away from the corners, so a joint and a corner never share a spot.

The same principle runs through every material. The page on adding a flat rooflight to a felt roof and the one on rooflights in GRP flat roofs show how the same opening is handled in a different skin.

EPDM is stable, but it is not indifferent to its neighbours. A few materials are on the wrong side of it, and a good detail keeps them apart.

Bitumen is the classic one. Bitumen based products, including old felt, bituminous paint and some sealants, attack rubber over time, so there should be a separation layer where an old roof is being adapted and the two meet. Some polystyrene insulation is softened by solvent adhesive and has to be covered with a board or a fleece before the rubber goes over it. Certain oils and hydrocarbons do the same damage. Hot work anywhere near the membrane is avoided, because rubber will scorch and lose its shape.

This is also why a rubber roof is not treated as an afterthought when the extension is built. If the rooflight is planned before the roof goes on, the upstand is built in and the membrane is laid once, in one visit, round the finished kerb. The opening then has no cuts in an aged sheet and no mixed materials. Adding a rooflight to a finished roof can still be done well, it just asks for more preparation, and we say so at the survey.

Rubber will hold water without harm, but a rooflight is cleaner and quieter when water does not sit against it. The falls around the opening do that job.

A flat roof is laid to a fall so that water runs to a gutter or an outlet. A rooflight upstand is a small dam across that fall, and the water has to divide around it. On the high side, the membrane behind the upstand should drain away with a slight fall, rather than trap a pool against the wall. On a longer roof, a small tapered insulation piece called a cricket is often set behind the upstand to push the water off to both sides. On a short, wide roof, the frame may just be moved a little toward the centre so the water has room.

The standing water answer on why water sits on a flat rooflight explains the glass side of this, and the guide to falls, drainage and why flat rooflights are never perfectly level covers the roof side in full. A flat roof should drain at a fall of 1:80 at the very least once built, and a careful installer checks the real fall with a level before choosing where the kerb goes.

With the membrane done, the glass goes on, and this is where the quiet side of the brand shows up. Rain on a rooflight over a bedroom or kitchen can drum, and a rubber roof around it does nothing to soften that.

A rubber membrane is soft, so it does not ring the way a thin metal flashing can. What makes noise is the glazing build. A single thin pane reacts to every drop. A laminated inner pane, a good glass thickness and a sensible cavity damp that reaction, and the seals at the frame keep the pane from chattering in the wind. We write those choices down in the quiet spec. Our guide on quieter glazing for a flat roof rooflight over a bedroom goes into the options, and the answer on what glass a flat rooflight should have covers the basic choices.

Because Crawley sits beside Gatwick, customers ask about aircraft as well. Glazing built for rain helps here too, and the survey is when we listen to which sound the room really hears. Daylight is still the reason for the rooflight. Quiet is the part that is easy to lose if nobody plans for it.

A new rooflight is a change to the roof, and it falls under the Building Regulations in the same way as the roof around it.

Part L sets the thermal performance of the new glazing and of the upstand and roof around it, so a rooflight in a heated room has to be specified with the right U-value and an insulated upstand that avoids a cold bridge. Part F covers ventilation, and a rooflight that opens can help meet the need for rapid ventilation in a room. Part K applies where the roof can be reached and where a person could fall, which matters for a rooflight set low enough to be stepped on, and is one reason to specify glass that is safe to walk or lean on in some positions. Part B matters when the roof is near a boundary, because the fire rating of the glazing and the membrane can limit where an opening can sit. The page on how close to a party wall a flat rooflight can go explains the boundary side in detail.

We install to current Building Regulations and we handle the Building Control notification where the work is notifiable. You can read the official guidance on the Planning Portal, which also says when a roof change needs planning permission. Permitted development can cover a flat roof rooflight, but a conservation area or a listed building changes that, and we check the position for your address.

Not every rubber roof takes every rooflight. The membrane and the glass have to be chosen together.

Our guides on fixed flat rooflights for extensions, frameless flat rooflights and dome rooflights set out the differences. For a glass roof that people will actually stand on, see walk-on rooflight installation.

A rooflight in a rubber roof is a matter of a day or two on site, depending on the size of the opening and whether the roof is already finished. The sequence is simple and we keep it calm.

We start inside, marking the opening on the ceiling and checking for pipes, cables and joists. The roof is then opened in a controlled area, the joists are trimmed and the upstand is built or fixed. The membrane is dressed up and around it as described above, and the frame and glass are fitted. The room is made good inside, the roof is cleared, and the quiet spec is checked against what was built. A fuller walk through is in how a flat roof rooflight is installed, and the joist answer explains why the ceiling is cut the way it is.

Work is carried out on a dry window, and the roof is never left open overnight. The installation comes with our 10-year workmanship guarantee. It is a workmanship guarantee and it is not insurance-backed, and the terms are on the guarantee page.

For the price of the work, see rooflight and skylight costs, and for the wider service, return to flat roof rooflight installation in Crawley.

Can a rooflight go into an EPDM roof that is already finished?

Yes, in most cases. The sheet is cut in a controlled area, the upstand is built or fixed, and new rubber is bonded to the cleaned existing membrane. The age of the roof decides how much of it is renewed.

Does the rubber have to be replaced around the rooflight?

Not always. If the existing sheet is sound, clean and bonded, it can be kept and the new rubber joined to it. If it is aged, cracked or lifting, we would renew the membrane around the opening or across the whole roof.

Will a rubber roof make the rooflight quieter?

The membrane is soft, so it adds no ringing of its own. The quiet comes from the glazing build, the seals and the insulated upstand, which we set out in the quiet spec.

How high does the rubber go up the upstand?

It runs up the full face and over the top edge to sit under the frame. The turn-up must reach at least 150 millimetres above the finished roof surface.

Can I get a quote for a skylight in my rubber flat roof?

Yes. Get a quote or message us on WhatsApp with a photo of the roof and the room below, and we will arrange a survey.