Flat roof rooflight upstands: height, insulation and finish

A flat roof is a shallow dish that is always trying to hold a little water. Raising the rooflight on an upstand keeps the glass and its seals out of that water, and gives the waterproofing somewhere to finish.

Think about what a flat roof does in a storm. It is laid to a gentle fall, but the membrane surface is never perfectly even, and water moves across it in a thin sheet that slows wherever something stands in its way. A rooflight frame set level with the covering would be that something. Every joint, corner and screw hole would sit at the lowest point of the wet zone.

The upstand changes the geometry. The frame now stands above the water line, the membrane turns up the outside face of the upstand like the side of a tray, and the rooflight is fixed to a dry, warm surface instead of a soaked one. The waterproofing and the glazing become two separate layers, each doing one job.

The upstand also carries the load. A glazed unit has real weight, and snow, maintenance access and wind all add to it. The upstand passes that load to the trimmed joists around the opening, evenly, rather than pressing a frame onto a membrane that was never meant to bear it. The trimmed joists beneath are a separate job from the upstand itself.

Then there is sound. A thin, hard edge around a large sheet of glass is a good way to amplify a downpour. A solid, insulated upstand, properly lined, gives the glass a stiff and damped bed to sit on. We come back to that below.

The upstand can be site-built timber, a pre-formed GRP moulding, a PVC or aluminium upstand sold with the rooflight, or a combination. Each suits a different roof, and the choice is made at the survey, not at the order desk.

Timber is the most forgiving. A carpenter can cut it to the opening on the day, which matters when a 1960s extension turns out to be a few millimetres out of square. A moulded upstand is faster to fit and arrives with a good insulating core, but it is made to a size, so the opening has to be made to match. Both are sound when they are fixed and waterproofed properly.

The supplier's own upstand is often the simplest answer for a frameless or dome unit, because the rooflight and its base are designed together. Our guide to frameless flat rooflights shows why those units pair so closely with their bases. For the plain question of whether a base is always needed, see does every flat rooflight need a kerb.

The working figure is 150 millimetres above the finished roof surface, measured to the point where the waterproofing ends. That is the height of the turn-up, not the height of the timber.

The distinction catches people out. A builder quotes a 200 millimetre upstand, and the roof then gets another layer of insulation and a new membrane, and the finished surface climbs. Suddenly the upstand that looked generous is barely above the roof. The number that matters is the clear height from the top of the final covering to the top of the waterproof turn-up, and it has to be checked after every layer is down, not before.

Several things push the height up:

1. A deep build-up on the roof, such as thick insulation above the deck, which raises the covering and leaves less of the upstand showing.
2. A roof with a slow fall or a known tendency to hold water, where more height buys more margin.
3. A gravel or green roof finish, which adds depth around the base.
4. A location where snow drifts against a parapet or a neighbouring wall.

Height also has a ceiling. A very tall upstand looks clumsy from the garden, makes the rooflight a target for wind and adds a deeper well inside. It also uses up headroom on stairs and landings where the glass is meant to give height, not remove it. Most domestic jobs settle in a band a little above the minimum, chosen so that the turn-up is comfortably clear and the outside still looks tidy.

For the short version of this question, see how high should a flat rooflight upstand be. The height is written on the quote as a finished dimension, so it can be checked on the day with a tape.

Glass is the coldest part of the roof, and a bare timber or concrete upstand is the next coldest. If the upstand is not insulated, it becomes a ring of cold that draws heat out of the room and pulls moisture onto the plaster.

Insulation goes in two places. The first is the body of the upstand: rigid board or foam filling the space between the outer skin and the inner lining, sized to give a continuous layer. The second is the junction where the upstand meets the roof insulation. That junction is where heat leaves most easily, because two layers of insulation meet at a right angle and any gap becomes a cold bridge.

Building Regulations set limits on heat loss through rooflights and on the performance of the roof around them, in Approved Document L. We install to the current version, and the glazing, the upstand and the roof around it are specified together so the opening does not spoil the thermal performance of the extension. Our page on energy efficient rooflights covers the glass side of that equation in more depth.

The roof type changes how this is done. In a warm roof, the insulation sits above the deck, and the upstand insulation overlaps it so the two layers are continuous. In a cold roof, the insulation sits between the joists under a ventilated void, and the opening has to be framed so the airflow is not blocked and the insulation is not compressed against the upstand. A warm roof is usually simpler to detail around an upstand. Our page on whether the roof needs to be insulated for a new rooflight explains the difference.

An upstand that is well insulated is also dry inside. Condensation forms on surfaces colder than the room air, so the warmer the inner lining of the opening, the less moisture it collects. A kitchen or bathroom, where the air carries a lot of steam, shows the difference fastest.

Warm air carries moisture, and moisture that reaches cold timber inside the roof condenses there. A vapour control layer stops the air getting in, and it has to be continuous around the opening, taped to the upstand's inner face.

This is the detail that gets skipped on rushed jobs, because it is invisible once the lining is up. The layer runs across the ceiling, turns up the inside face of the upstand and is sealed to it, so there is no path for humid air to enter the roof structure around the glass. Where it is missing, the first sign is often a dark mark on the ceiling beside the opening, months after the job looked finished.

In a warm roof, the vapour layer sits under the insulation, and the upstand's own vapour barrier has to join it without a gap. In a cold roof, it sits just above the ceiling lining, and the ventilation above has to stay clear of the upstand. Whichever build it is, we photograph the junction before the lining goes up, so the detail is on record.

The waterproofing climbs the outside face of the upstand, and a separate cover, called a cloak or cap, then hides its top edge. The membrane and the cloak are two layers, and the joint is overlapped so water runs over it, not into it.

Each roof covering has its own method. A single-ply sheet is bonded or mechanically fixed to the upstand and heat-welded or adhesive-jointed at the corners. A liquid-applied system is brushed up the face in coats with reinforcement at the corners. A felt roof is dressed up the face in layers. The correct treatment depends on what the roof already has, and it matters that the upstand is compatible with it. The covering specific guides show the detail in full: EPDM rubber roofs, GRP fibreglass roofs and felt roofs.

The corners get the most attention, because they are where three surfaces meet. A sheet that is folded neatly at a corner can open under the movement of a roof that warms by day and cools by night. A pre-formed corner, or a carefully cut and welded one, accepts that movement without opening.

Above the membrane goes the cloak, usually a thin metal or pre-finished plastic flashing that covers the top of the turn-up and overlaps the frame. It is fixed so that water runs over its outer face and drips clear of the upstand. The glazing frame is then set and sealed on top. Cut that sequence short and the joint depends on sealant alone, and sealant is the first part of any roof to age.

The upstand is a small dam across the roof. Water flowing towards an outlet meets it, and the roof has to be laid so the water divides and runs on rather than standing against it.

A gentle, continuous fall is what makes this work. Building regulations and roofing guidance aim for a fall that still drains after the roof has settled, often expressed as a ratio such as 1:80 at the finished surface. The upstand sits on the high side of the water where it can, and never in a dip. Where the rooflight has to go in the lowest part of the roof, a small shaped fall, called a cricket, can be built behind the upstand to divert water round it. Our guide to falls and drainage around a flat rooflight goes into it in full.

Sitting water often shows up as a dirty tide mark on the glass, and it is usually a sign that water is lingering against the base. It is explained in why water sits on a flat rooflight. Leaves and snow do the same job in winter, building up against the upstand and holding meltwater there. A modest extra height is cheap insurance against that.

The inside face of the upstand is part of the room, and it is visible every day. It is usually lined with plasterboard or a timber board, with an angled splay at the top to let more light in.

The lining has a practical job and a visual one. Practically, it protects the insulation and carries the vapour layer. Visually, it is the frame that the room sees. A lining that finishes flush with the ceiling looks as though the glass is part of the structure. One that leaves a thick, square reveal looks like a box in the roof.

The angle of the lining sets how much daylight reaches the room. A vertical reveal throws shadow across the glass edges, while a splayed one, opened out towards the ceiling, spreads light and cuts contrast. A white, matt paint finish bounces light best. A deep upstand makes a deep well, so the splay matters more as the upstand grows.

Corners are best finished with a tape-and-fill joint and a slim edge bead, so the plaster stays crisp. Movement cracks appear where a rigid plaster meets the window frame, and a small flexible bead at that junction prevents them. Our page on lining the reveal under a flat rooflight covers the finishes in detail.

A flat rooflight is exposed to rain in a way a pitched window is not. The upstand is one of the things that decides how loud a shower sounds in the room below.

Rain strikes glass and makes a sharp, high sound, and thin metal frames can ring with it. A dense, insulated upstand behind the frame damps that ringing and gives the glass a firm bed. Laminated glass, with its plastic interlayer, reduces the drumming further. Together they make a quieter room than a thin unit on a hollow base.

The seals matter as much. A compressible gasket between the glass and the frame, and a bead of sealant between the frame and the upstand, stop the vibration travelling into the structure. Where aircraft or traffic are the concern, the same build helps, because a rooflight is the thinnest part of a roof and sound finds it first.

That is why every quote we write comes with a written quiet spec, which records the upstand, the glass build and the seals we propose for the noise the room actually hears. Our guide to quieter glazing for a flat roof rooflight over a bedroom goes through the glass side.

Most upstand problems are decided in the first hour of the job and found two winters later. A short list, worked through in order, catches nearly all of them.

At the survey, we look at the roof covering and its age, the build-up depth, the direction of the fall and the position of the outlets. We note whether the roof is warm or cold, and whether a parapet or wall will collect snow beside the opening. From that we choose the upstand type and write down its finished height.

On the day, the order of work is fixed. The opening is formed and the joists trimmed. The upstand is fixed, then insulated and sealed to the vapour layer. The membrane is turned up and the cloak is fitted. The rooflight is set and sealed. Then the inside is lined and decorated. We check the finished height against the quote before the glass goes on, while it is still simple to change.

After the work, the upstand is covered by our 10-year workmanship guarantee, described on our guarantee page, and where the work is notifiable we handle the Building Control notification. To discuss an opening on your roof, get a quote and we will survey it.

These are the questions that come up once people have seen the quote.

Can the old upstand be reused when the glass is replaced?

Sometimes. If the timber is sound, the height is still above the roof surface and the insulation is intact, the existing upstand can carry a new unit. We check for rot, for a height that has been lost under new roofing and for missing vapour sealing. Where any of those fail, a new upstand costs little compared with the rooflight and is better done at the same time. See misted and failed rooflight replacement for how a replacement runs.

Does a taller upstand make the room colder?

Not if it is insulated. The extra height adds more insulated wall, not more cold glass. A taller upstand does add a deeper well inside, which affects light, so the lining is splayed to compensate.

Can a walk-on or fixed glass unit use the same upstand as a dome?

Often the footprint is similar, but the loads are not. A walk-on unit needs a stiffer, properly supported base, as explained under walk-on rooflight installation. A dome is light, and its supplier usually specifies the base. We match the upstand to the unit, not the other way round.

What if the roof is going to be recovered later?

Tell us at the survey. A roof that is about to be recovered should have the upstand set for the new finished surface, not the old one. Fitting the rooflight first and recovering later is fine, provided the height allows for the extra layer.