The short answer
A rooflight upstand, also called a kerb, lifts the glazing clear of the waterproof surface of a flat roof. It must stand high enough above the finished covering, once the insulation and any falls are in place, to meet the manufacturer's minimum, because a low upstand lets standing water reach the joint. The exact height comes from the product's own instructions, not a rule of thumb.
What an upstand is and what it does
A flat rooflight does not sit straight on the roof deck. The glass rests on a kerb, otherwise known as an upstand, built up around the opening. The waterproof covering is turned up the outside of the frame and finished beneath the rooflight's own upstand cover or cap, so the joint between roof and glass is lifted away from the water on the surface.
That small raise matters because a flat roof is never perfectly dry. After rain, water lies in shallow pools and drains slowly towards the outlets, and in winter it may freeze. An upstand keeps the weak line, where glass or frame meets the waterproofing, well above that level.
Upstands come in several forms. Some are made of timber and clad on site. Others are factory-made from GRP or insulated plastic and supplied with the rooflight. A built-up upstand needs insulation in the frame as well as outside it, or cold bridging will cause condensation on the inside face.
Why the height is not a free choice
The correct height is the one the rooflight manufacturer specifies for that product and roof covering, taken from the top of the finished waterproofing, never from the deck below. By the time the insulation board, the falls and the final membrane or felt are on, the surface sits higher than the deck, and an upstand cut to deck height ends up too short.
An upstand that is too low causes the common failures. Water splashes or ponds against the joint, the turn-up of the covering has too little height to be sealed and clamped properly, and snow or leaves pile against the glass. The result is a damp stain on the ceiling that seems to come from the rooflight but is really a detail failure.
Flat roofs also need falls. Guidance from IKO, citing BS 6229:2018, gives a design fall of 1:40 to achieve a finished minimum of 1:80. A roof that slopes towards a rooflight, not away from it, can load the upstand with water. Our page on flat skylights that leak shows how that happens.
Insulating the upstand, and what Part L says
Part L of the Building Regulations, Approved Document L Volume 1, gives 2.2 W/m²K for a rooflight fitted new or as a replacement in an existing home. Note 12 says the figure covers the rooflight and kerb together as one unit, and a kerb constructed on site is held to 0.35 W/m²K.
In practice that means the kerb is not a bare timber box. It needs insulation to a good standard, continuous with the insulation on the roof, with no gap that lets cold air meet the warm ceiling. A cold upstand collects condensation on its inner face, which can run down the lining and look like a leak.
When we replace a flat roof covering, the rooflight upstands are checked against the same rules. A new covering can trigger a thermal upgrade in some cases, so read the notes on roof window Building Regulations and ask your building control body about your own case.
Upstands and condensation
Condensation on the glass is normal on cold mornings and clears as the room warms. Water gathering on the upstand lining, on the plasterboard splay or in a corner is not, and it often points to a thermal gap in the kerb.
Look for black spots along the splay or a damp band beneath the glass. These suggest that warm moist air in the room is reaching a cold surface, usually the kerb or the junction with the ceiling. Better insulation in the upstand, a continuous air seal between the lining and the ceiling, and good ventilation in the room all help.
Our page on roof window condensation explains the causes in more detail. If the condensation is inside a sealed glass unit, between the panes, that is a failed unit, not an upstand fault, and the glass needs replacing.
Retrofitting and replacing around an old upstand
Many older flat roofs have a rooflight on a low, thin upstand that was acceptable when the felt was laid and has not been touched since. When the covering is renewed, the new waterproofing builds up the surface, and the old upstand ends up lower relative to it. Reusing it can leave you with the problem described above.
The options are to raise the upstand, to replace the frame, or to swap the whole assembly for a factory-made rooflight with its own kerb. A broken pane in an otherwise sound frame is a smaller job; our page on broken roof window glass explains the repair route.
Where the roof structure has to be altered to fit a larger opening, rafters or joists may need trimming. We cover that in trimming rafters, because a bigger opening changes how the load is carried.
What we check on survey
When we look at a flat rooflight, we start with the upstand. We measure its height above the finished roof surface, check how the covering is turned up and sealed, look at the falls around it, and inspect the insulation inside the kerb if we can see it from the loft or the room.
The quote arrives in writing with pictures of the kerb itself, so you can see what we are looking at. Sometimes a repair to the turn-up is enough. Sometimes the kerb or the whole rooflight has to go. Either way the reasoning comes with the price.
If you want to send us something first, a few photographs of the rooflight from the roof edge, from the room and from the loft, plus the make if you know it, are enough. Our skylights hub lists the rest of the topics.