Guide · Flat Roof Rooflight Installation
Upstands, kerbs and warm roof build-ups
A flat rooflight does not sit on a flat roof. It sits on a kerb, a raised box built off the roof structure, and that box is finished before the glass is ordered. Get it right and the unit is a straightforward day’s work. Get it wrong and no glass specification, no brand and no quantity of sealant will rescue the detail.
This page is about what is under the glass: how a kerb is built, how tall it needs to be, how it meets a warm roof build-up, and where heat escapes around it. It sits under flat roof rooflight installation, which covers the wider job from survey to finished reveal.
What a kerb actually is, timber by timber
A kerb is a box. Four lengths of treated timber, fixed to the deck or to trimmers set in the joists, forming a raised rectangle that the rooflight lands on. The internal dimension of that box is the number the unit is ordered to, and it is what a supplier means when they ask for the structural opening size. Confusing it with the external size of the glass is how units arrive that will not fit.
The box does three jobs at once. It lifts the glass above the level water can reach. It gives the roof covering something vertical to turn up against, so the covering terminates on a face rather than dying into a flat plane. And it provides a true, flat bearing for the frame to be fixed down onto. All three have to be right simultaneously, which is why the kerb is worth more attention than it usually gets.
Where the insulation sits, and what that does to the kerb
A warm roof puts the insulation above the deck, so the joists and the deck sit inside the heated envelope. That is the default build-up on every new flat roof going up across Essex, and it changes the kerb in one specific way: the insulation and its overlay board raise the finished surface by anything from 100 to 175mm, so a kerb has to clear that thickness before it has gained any height at all.
The sequence on site is fixed. Vapour control layer, kerb up, insulation cut tight around the outside of the kerb, then the covering dressed up the face and over the top. A kerb added after the insulation has been boarded out means cutting back into a finished layer and hunting for something structural to fix into. Where the build-up is a cold roof, the arithmetic and the ventilation path are different again.
Measuring 150mm from the correct surface
The industry figure is 150mm from the finished roof surface to the top of the kerb. The phrase carrying all the weight is finished surface. Not the deck, not the vapour barrier, not the top of the insulation board. The datum is the membrane, the cap sheet or the GRP topcoat, whatever the water will actually be sitting on.
Work an example. A warm roof with 120mm of PIR and an 18mm overlay board raises the surface 138mm above the deck. A kerb standing 150mm proud of the deck therefore stands 12mm proud of the roof, which is no upstand at all. That single arithmetic slip is the most common measuring error on flat rooflight jobs, and it tends to surface on the day the glass arrives.
The 150mm is measured from what the water touches, not from the timber you are standing on.
Insulating the kerb itself
An uninsulated kerb is a strip of bare timber standing in the cold with a heated room directly beneath it. On a 1200 x 800mm rooflight that is four metres of perimeter, 150mm tall, doing nothing to hold heat in. It is a small area, but it runs continuously around the warmest part of the ceiling, which is where warm moist air collects.
The fix is straightforward and costs very little at the time. Rigid insulation, usually 25 to 50mm, is fixed to the outside face of the kerb from the deck up to the underside of the frame, then the covering is dressed over it. The insulation on the roof plane butts tight to that vertical layer so there is no gap at the junction. Done properly the perimeter is continuous and the timber box is inside the warm side.
The cold bridge at the perimeter, and how it shows
When the kerb is left bare, the effect appears on the ceiling below within a year or two. A grey line traces the reveal where it meets the plaster, sometimes with spotted mould in the corners, and it is at its clearest in February when the temperature difference is greatest.
The mechanism is not a water problem. The plaster at that line is measurably colder than the surrounding ceiling, so relative humidity at the surface climbs, dust binds to the damp film and mould finds the conditions it needs. Cleaning it removes the mark and not the cause. Insulating the kerb at the time of installation removes the cause, and it is one of the four invisible details that the ten year workmanship guarantee rests on.
Building off the deck or off the joists
There are two honest ways to stand a kerb up. Off the deck, where the kerb is screwed down through the deck board into the joists and trimmers beneath. Or off the joists, where the trimmers are set to form the opening and the kerb legs land directly on the structural timber, with the deck butted up to the outside face of the box.
The second is stiffer and is the usual approach on larger openings, on anything intended to take foot traffic, and on any unit above about 1500mm in either direction. The first is quicker and is entirely adequate for a modest unit in a well supported deck. What does not work is a kerb sitting on a deck board with no structural timber under the line of the box, because the fixings then rely on 18mm of ply to carry wind uplift.
Timber sizes, treatment and how the box goes together
Treated softwood, typically 47 x 100 or 47 x 150 depending on the height required, or two thicknesses laminated where the kerb needs to stand taller than stock sizes allow. Corners are screwed rather than nailed, and on a long side the box gets an intermediate noggin so the face cannot bow when the covering is stretched over it.
Treatment is not optional. The kerb occupies the wettest few hundred millimetres of the entire roof, sitting in the run-off path with a covering wrapped tight around it. Untreated timber there will soften from the inside long before anything is visible from the room below, and by then the fixings holding the frame down are gripping into something that has lost its strength.
The facing the covering has to stick to
Every flat roof covering wants a continuous, smooth, dry substrate at the upstand, and the kerb has to provide it. An 18mm WBP plywood facing does that. Rough sawn timber does not, and neither does bare insulation board where the covering needs a mechanical fixing along its top edge.
With GRP the plywood is not just a backing, it is part of the laminate: the resin bonds to the board and the board becomes the shape the covering takes. With single ply the plywood carries the termination bar and the fixings. With a torch applied felt system it takes the nails or screws holding the first layer while the cap sheet is bonded over. The covering decides the detail, and the trades around the rooflight are covered in more depth on single ply, felt and GRP roofs around a rooflight.
Tapered kerbs, and the version that sheds water
A kerb does not have to be level on top. Where the unit is a sloped flush rooflight, the kerb is built with a fall across it, commonly around 5 degrees, so the glass sheds water and the dirt that travels with it. Flatter than about 3 degrees and rain sits on the pane long enough to dry out and leave a mineral tide mark that will not wash off.
The fall is built in one of two ways: the tops of two opposing sides are ripped to the angle before assembly, or the box is built level and firrings are fixed across the top. What must not happen is a sloped unit shimmed onto a level kerb. The frame then bears on two points, the fixings pull it into a twist and the seal along the low edge is doing work it was never designed for.
The top of the kerb, and how the unit lands on it
The frame sits on the kerb top, bedded on a compressible tape or a low modulus sealant, and is fixed down through the frame at the spacings the manufacturer sets. The bearing surface wants to be flat to within two or three millimetres over its length, because a rooflight frame is stiff and will not follow a hollow.
The covering terminates over the outer arris of the kerb top and is trapped under the frame. That order is what makes the detail work: water running off the glass lands on the covering, on the outside of the box, and has no route that takes it under the unit. Where the covering stops on the vertical face and the unit is bedded straight onto bare timber, the joint is relying on sealant alone, which is a maintenance item pretending to be a detail.
Getting the kerb square, level and out of wind
Three checks, all made before the unit is ordered rather than after it arrives. Diagonals measured corner to corner and equal, so the box is square. Each side level along its own length. And the four top corners sitting in one plane, which the trade calls out of wind and which is checked with winding sticks or a long straightedge across the diagonals.
Five millimetres of twist across a 1200mm box is enough to leave a visible gap under one corner of the frame. Pulling that out with fixings loads the frame permanently and, on a large unit, can be enough to stress the sealed edge of the glass. It is far quicker to plane the kerb than to argue with the glass.
Factory insulated kerbs against a site built box
Both are legitimate. The choice usually comes down to who is on site when the roof is being formed and how unusual the size is.
| Site built timber kerb | Factory insulated kerb | |
|---|---|---|
| Sizing | Any dimension, cut on the day | Standard sizes, specials on lead time |
| Thermal performance | Depends entirely on the insulation added | Insulation built in, consistent |
| Tolerance | As good as the carpentry | Square and true out of the wrapper |
| Programme | Formed as the roof is built | Ordered ahead, dropped in |
| Suits | Retrofit into an existing roof | New extension roofs, repeat sizes |
Kerb height on a terrace or an accessible roof
Where the flat roof is a walked surface, the finished level is not the membrane. It is the paving on pedestals, the timber decking or the ballast sitting above it, and the 150mm is measured from that. In practice a terrace build-up adds 60 to 120mm, so the kerb has to grow by the same amount or the upstand disappears under the deck boards.
That extra height is also useful for a different reason. A raised kerb on an accessible roof forms a visible edge that people can see and step round, and it keeps the glass out of the line of dragged furniture. Where the glass is meant to be walked on rather than walked around, the whole detail changes and is covered on walk-on rooflights.
Setting the kerb against the ceiling line below
Every millimetre of kerb is also a millimetre of reveal depth inside the room. Add it up on a typical rear extension: 220mm of joist, 140mm of insulation and overlay, 150mm of kerb, and the opening in the ceiling is looking up a shaft over half a metre deep before the glass starts.
That depth is not a fault, and splaying the reveal turns it into an advantage by spreading light across the ceiling instead of dropping a hard rectangle onto the floor. But it has to be decided early, because the splay angle sets how much wider the ceiling opening is than the kerb, and that in turn affects where a joist or a downlight can sit. Drawing a section through the opening before the roof goes on takes ten minutes and settles all of it.
What a survey finds wrong with existing kerbs
The list repeats itself across Essex. Kerbs built at 75mm or 100mm because the timber in the van was 100mm. Coverings terminated on the vertical face with a bead of mastic at the top rather than dressed over the kerb. No insulation anywhere on the perimeter. Corners where the timber has gone soft and a screw turns without biting. And kerbs built after the covering was laid, sitting on top of the membrane with fixings driven straight through it.
None of that is visible from the room below, which is exactly the problem. It is visible in five minutes on the roof, and it is the reason a survey happens before a fixed price is given rather than after.
What to ask a builder to leave you
On a new extension the roof is usually somebody else’s trade, so the useful thing is a short written list handed over at the design stage. Internal kerb size to the millimetre, agreed against the unit being ordered. Height measured from the finished roof surface, not the deck. Square, level and out of wind. Faced externally in 18mm WBP plywood. Insulated on the outside face, continuous with the roof insulation. Covering dressed up the face, over the top and turned in.
Six lines. They cost nothing to specify while the roof is still on paper, and they remove almost every argument that happens on the day the glass turns up. Where Sunspire forms the opening as well, the same list is simply how the job is done, and the arithmetic between kerb, insulation and finished surface is checked twice before the unit is ordered.
Keep reading
More on flat roof rooflight installation
Walk-on rooflights and where they make sense
Read the guide →Single-ply, felt and GRP roofs around a rooflight
Read the guide →Flat rooflights on a cold roof build-up
Read the guide →How many rooflights a flat roof extension needs
Read the guide →Flat rooflight standard sizes and made to measure
Read the guide →
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