How to install a flat roof rooflight, step by step

The installation is decided at the survey, not on the day. By the time a crew arrives, the position, the upstand, the glass and the lining have all been written down and agreed.

That written record matters because a flat roof skylight installation has very little room for improvisation. The opening size fixes the joist work. The joist work fixes the ceiling edge. The upstand height fixes how the roof covering is finished. If any one of those is still open on the morning, the whole sequence stalls. So the survey notes travel with the crew: measured drawings of the roof, the joist direction, the covering type, the position of the nearest outlet and the quiet spec for the room.

The quiet spec is part of that paperwork. It records what we heard at the survey, whether that was rain on the existing felt, traffic on the road outside or aircraft overhead, and names the glazing build we chose in response. A laminated acoustic inner pane, for instance, is ordered weeks before the roof is opened. It cannot be swapped in on the day. You can read more about how we set it out on the quiet spec page.

The last decision before the day is weather. Opening a flat roof in steady rain is poor practice. We watch the forecast for the day of the opening and the day after it, and we would rather move a start than leave a deck exposed overnight.

The first hour is spent indoors. The opening is set out from underneath, because the ceiling is the part you will look at every day.

Furniture is moved or covered, floors are laid with protection, and dust sheets are taped up to make a tent under the opening. Cutting a ceiling and a deck creates fine dust, and plaster dust travels further than people expect. A sealed tent keeps it in one place.

With the room protected, the centre lines go on. We mark the finished ceiling opening first, then measure upwards to transfer the corners to the roof through small pilot holes. On the roof, those four holes become the corners of the structural opening. The structural opening is then worked out from the upstand: it is the finished hole plus the thickness of the upstand walls, less whatever the lining will add inside. Because the ceiling and roof sizes are tied together this way, the light well stays square and the rooflight sits where the drawing said it would.

One check goes in at this stage that saves awkward conversations later. Before any cutting starts, we confirm from below that nothing runs through the marked area: electric cables, a water pipe, a ventilation duct or a flue. Anything found is diverted before the roof is opened.

The roof is opened in a controlled order: covering first, then insulation, then the deck. Only as much is cut as can be made weathertight by the end of the day.

On a flat roof the covering might be felt, EPDM rubber, GRP fibreglass or a single ply membrane. Each one is cut back differently, and each one decides how the new upstand will be dressed later. A felt roof is cut back to clean layers so the new covering can be lapped in. An EPDM sheet is cut cleanly and kept flat, because the new sheet will be bonded to it. A GRP surface is cut with a diamond blade and the edges ground back so that fresh resin can bond. Our page on adding a flat rooflight to a felt roof shows the lapping in more detail.

Below the covering sits the insulation and then the deck, usually ply or OSB boards on timber joists. The insulation is lifted out and set aside dry. The deck boards are cut along the marked lines, from the top, with the ceiling still in place below so that nothing falls into the room. The tent earns its keep here.

The opening is now a hole in the roof, and the clock starts. A weathering sheet is kept on the roof, ready to be pulled over the opening if the sky changes. Good practice is to have the next stage already laid out before the deck is cut, so the gap between cutting and covering is as short as it can be.

If the opening lands between two joists and is narrower than the gap, no joist needs cutting. Many rooflights are wider than that, and then the structure has to be rebuilt around the hole.

A joist that is cut no longer carries its share of the roof. So before it is cut, the load has to be given somewhere else to go. The method is the standard one in timber construction. Two trimmers are fixed across the opening, at the ends of the cut joists, and the cut joist ends are hung from them in joist hangers. The trimmers themselves are doubled, which means two pieces of timber side by side, and bolted or nailed to a schedule. The joists either side of the opening, called trimming joists, are doubled as well, because they now carry the extra weight that used to be shared.

The sizes depend on the span, the joist depth, the roof loading and whether a large opening removes more than one joist. On a bigger opening a structural calculation is worth having, and a steel or engineered timber beam may replace the doubled trimmer. It is one reason the survey opens up the ceiling edge to look at the real joist run. There is a separate page on why joists need trimming for a flat rooflight.

Once the frame is fixed, it is checked with a long straight edge and a level. The top edges of the trimmers need to sit flush with the existing joists, because the deck returns over them. Any rise or dip here shows up later as a twist in the upstand.

The upstand is the raised frame that the glass unit sits on. It lifts the rooflight clear of the water running across the roof, and it is where the weatherproofing is won or lost.

We build it in one of two ways. Some rooflights come with a factory-made kerb, insulated and ready to set. Others are installed on a site-built upstand, formed from treated timber and insulated on the job. Either way, it is fixed down through the deck into the trimmers, not just nailed to the ply. Fixing into the structure stops it from moving as the roof expands and contracts with the seasons.

Height is the next question. The finished upstand has to stand high enough above the roof surface that water cannot be pushed or blown over the top, and that the covering can be dressed up it with a proper lap. As a working guide, we build to at least 150 millimetres above the finished surface of the roof, and more where the fall is deep or standing water is likely. The full detail is on the upstand page.

The insulation goes on the inside and the outside of the upstand. This keeps the cold timber away from the warm room air, which is what stops condensation from forming on it. A vapour barrier from the roof build is lapped up to the inside of the upstand and taped, so that damp room air cannot get into the roof through the joint.

The fall needs a look at this stage too. A flat roof is laid to drain, with a designed fall of around 1:40 that still delivers 1:80 once built and settled. The upstand is a dam across that fall. If the roof is sloping towards one edge, the high side of the upstand needs water to divide around it, and the low side needs a clear run to the outlet. Where the existing fall is weak, a small tapered firring piece can be added on the high side to send water round. The reasoning is set out on falls, drainage and why flat rooflights are never perfectly level.

With the upstand fixed, the roof covering is taken up its outside face. This is the stage that turns a timber box into a watertight detail.

The method depends on the roof. On felt, new layers are lapped into the existing ones and then dressed up the upstand, with a torch-on capping sheet or a cold-applied system. On EPDM, a single sheet is bonded to the deck and upstand with adhesive, with corners formed from pre-moulded pieces rather than folded, which avoids pleats that can open up. On GRP, the laminate is carried up the upstand in resin and glass mat, then topped with a finishing coat, so the roof and the upstand become one continuous skin.

• Corners are formed, not folded, wherever the system allows.
• The covering runs up past the top edge of the upstand, to meet the underside of the rooflight frame.
• Laps face away from the prevailing water flow, not into it.
• The top edge is held by a clamp or a retaining bar and sealed, never left to sit free.

When the covering is complete, the roof is water tested in the area around the upstand. A hose runs over the new detail for a set time while someone watches underneath. It costs little, and it shows a fault while the crew is still on site. The roof is also walked for any missed laps, pinholes or cuts.

That is the moment the roof is watertight again, even before the glass goes on. On larger jobs, it is where a day usually ends.

The rooflight is one of the heaviest items on the job. It is lifted by hand or by light crane, bedded and fixed.

A sealed double glazed flat rooflight unit can weigh a good deal once a frame, a glass build and a laminated inner pane are combined. Larger sizes need several people, suction lifters or a hoist. It travels up in its packaging and is unwrapped on the roof, which keeps the glass clean and the edges protected.

Before it is set, the seating face on the upstand is cleaned and dry. A continuous bed of sealant or a compressible gasket goes down, depending on the product. The unit is lowered straight, not dragged, and then checked for square with a diagonal measurement across the frame. If the two diagonals differ, it is not square and the fixings are left slack until it is.

Fixings go through the frame into the upstand at the spacing the manufacturer specifies, and are tightened evenly so that the gasket compresses all the way round. Over-tightening one corner is the classic way to bend a frame, and it shows later as a seal that does not close.

For frameless designs, where the glass sits directly on the upstand with no visible frame, the bedding and fixing follow a different order, and the flashing is part of the glass system. We cover that on the frameless flat rooflights page.

The glass is in, and the joint between the frame and the upstand is now the most important line on the roof.

A metal or rubber flashing closes that joint from the outside. It overlaps the upstand covering and tucks under or over the frame, depending on the product. The aim is a joint that sheds water in layers: what gets past the first line meets a second. Sealant alone is never the plan. It is a back-up to the overlap, not a substitute for it.

Seals matter for noise as much as for water. A compression gasket that is correctly seated blocks the small gaps that let sound across, which is why a tired seal on an old unit can be the source of the whistle or hiss people describe in a wind. On a new installation, the seals are checked and the frame is closed fully, so the unit is as quiet as its specification allows. The wider question of glass is on quieter glazing over a bedroom.

Any vents, openers or electric actuators are fitted and tested at this point. On a fixed unit there is nothing to adjust. If the job includes an opening rooflight, the mechanism is run through its full travel and its rain sensor, where fitted, is tested.

The protective film stays on the glass until everything above is finished, then comes off. Debris, offcuts and dust are cleared from the roof and the outlets, so the first rain has a clear run.

Inside, the work changes from building to finishing. The opening in the ceiling is turned into a light well, and this is what you will actually see.

The sides of the opening, from the ceiling up to the underside of the rooflight, are called the reveal. They are framed with timber battens, insulated, and then covered in plasterboard. The insulation is what prevents cold bridging, where the cold from the outside finds a short path to the inner surface and produces a damp patch or a line of condensation. A well-insulated reveal stays warm to the touch.

The angle of the reveal changes how the light spreads. A splayed reveal, which widens towards the room, throws daylight further across the ceiling. A square reveal is plainer and gives a tighter pool of light. Light colours reflect more, so white or a pale tone helps. The choices are set out on lining the reveal under a flat rooflight.

Plasterboard is fixed, taped and skimmed, or finished with a dry joint compound, and sanded. Corners get a bead so they stay sharp. The junction between the reveal and the existing ceiling is the place where a visible crack could appear if it is rushed, so it is scrimmed and given time to dry.

Then it is painted. A mist coat and two topcoats is standard. A matt finish cuts reflected glare from the glass, which is worth considering in a bedroom.

The number of days depends on the size of the opening and the structure. The table sets out how the same steps spread across jobs of different sizes.

Treat the table as a pattern rather than a promise. A bigger opening, a beam, a difficult covering or a wet spell can add a day. The written quote states the expected programme for your roof, and we tell you early if it changes.

Adding a rooflight to a flat roof is usually notifiable work, because it changes the structure and the thermal performance of the roof. We handle the Building Control notification where the work requires it.

The rooflight must meet current Building Regulations. Part L governs thermal performance, which affects the glass specification and the insulation around the upstand. Part K applies where anyone might walk near or fall against the glass. Part B matters where the roof is close to a boundary, because some plastic rooflights and domes are restricted there. Part F covers ventilation, which is relevant to opening units and to any room where the new light changes how air moves. The general picture is in our page on putting a rooflight in an existing flat roof, and official guidance is on the Planning Portal.

At the end you should receive:

• the Building Control sign-off, where the work was notifiable;
• the product details and the glazing specification, so the build on the quote matches the glass on the roof;
• care notes for cleaning the glass and clearing leaves from the upstand;
• our 10-year workmanship guarantee, described on our guarantee page. It is a workmanship guarantee and is not insurance backed.

These are the practical questions that come up once a date is in the diary.

Will the roof be open to the weather overnight?

It should not be. The opening is planned so that the deck is cut and the new upstand is covered the same day, or at the latest weathered with a temporary sheet that is fixed down and checked before the crew leaves. If rain is forecast, the start moves.

Do I need to be at home?

Someone needs to be there to let the crew in and to approve the opening position from below before the first cut. After that, you can go about your day, though the room underneath is out of use while the work is carried out.

Can the job be done in a single day?

A small opening between existing joists on a simple roof can be weathered in a day, with the lining finished afterwards. Anything that needs trimmed joists, a beam or a bigger covering takes longer, and the programme on your quote says how long.

What if we find something unexpected in the roof?

Sometimes the deck or joists are in a worse state than the survey could show. We stop, show you, and agree the change in writing before continuing. Nothing is added to the job without your say.

If you want this sequence written out for your own roof, with the glass and the upstand named, get a quote or message us on WhatsApp.