The short answer
An upstand, or kerb, is the short wall that lifts a flat rooflight clear of the roof surface. Its height decides whether ponding water, snow and splashback can reach the glass joint. A site-built upstand also has to meet a maximum U-value of 0.35 W/m²K under Approved Document L, so it needs insulation as well as height.
What an upstand is and what it holds back
A flat roof is never perfectly flat. It has a slight fall towards the outlets, and shallow pools linger after rain. A rooflight set straight onto the deck would sit in them. The upstand, also called a kerb, lifts it clear: a timber box or manufactured section around the opening, wrapped in the same waterproof layer as the roof.
Height does three jobs. It keeps standing water away from the rooflight's seal, keeps splashback and snow off the frame, and gives the membrane a vertical face to climb so the waterproofing finishes under the rooflight cap. A membrane that stops at deck level leans on sealant, and sealant is the first thing to give out.
Each maker states a minimum upstand height for its units, and it differs between products. It is on the installation sheet, and we build to that figure, not a rule of thumb. A kerb lower than the maker asks for can affect the unit's own guarantee.
Site-built upstands and the 0.35 limit
A kerb is either factory-made and supplied with the rooflight, or built on site by the roofer from timber, insulated and covered in roof membrane. Approved Document L (2021) treats the two differently.
In Table 4.2 the rooflight is held to 2.2 W/(m²·K), and a unit sold with its kerb as one assembly is judged on that same figure. When the roofer builds the kerb on site, the table caps its U-value at 0.35 W/(m²·K), against 0.15 for the roof around it.
The 0.35 limit rules out a bare timber box. An uninsulated kerb is a gap in the insulation, losing heat in winter and giving room moisture a cold face to condense on. Insulation goes inside, outside or both, and the membrane wraps the lot. Thickness depends on the product and the calculation, which your building control body can confirm before work starts.
What the box is made of
A site-built kerb is normally formed from treated timber and plywood, fixed down through the deck into the joists so it cannot rock. The corners are tightly framed because the membrane has to turn them, and the top is left level so the rooflight frame beds evenly. A kerb built on a deck that moves, or on joists that have rotted, will move too and pull the waterproofing with it. We check the timbers beneath before building on them.
How the membrane is taken up and over
A flat roof is weakest where the waterproof layer changes direction. At a kerb the membrane runs along the deck, climbs the face and finishes where the rooflight frame sits over it, and every fold at a corner can let water in.
Detailing depends on the covering. EPDM, a single-ply rubber membrane, is stuck down and shaped into corners with preformed pieces. GRP, glass fibre laid in resin, runs up the kerb in one piece and cures into a continuous skin. Felt is lapped and finished with a cap sheet and top flashing. All share one principle: no cut edges facing the weather, laps that shed water, and a top edge covered by the rooflight cap.
Corners and upstand height together
A taller kerb gives the membrane room to be formed properly and protects the top edge from splashing. Where it is too low, the termination gets crammed under the cap and water finds it. Our guide to flat rooflights, fixed, opening and walk-on describes the products that sit on top of the kerb.
Falls, ponding and where the kerb sits
Where the kerb sits matters as much as how tall it is. IKO and Rockwool, citing BS 6229, describe flat roofs designed with a 1:40 fall to give a finished 1:80. That slope is gentle, and a kerb across it acts as a dam, with water banking up on its high side.
The usual remedy is a small tapered area on the high side, called a cricket or saddle, that steers water round the kerb. Without one, silt and leaves settle behind it and the membrane stays wet. On the survey we find the fall with a level, so any ponding risk is flagged before a price is given.
A tired covering deserves attention before the rooflight does. Still deciding what suits your roof? See roof windows against skylights.
Checking an existing kerb and what a fix involves
Outside, look for lifted membrane at the foot of the kerb, hardened sealant, stains on its face, debris behind it and water that stays for days. Inside, look for brown rings on the ceiling around the rooflight and condensation running down the kerb lining; a cold, damp upstand often means missing insulation rather than a leak.
Resealing a corner or patching a split is quick if the kerb is sound. One that is too low, rotten or uninsulated has to be rebuilt: lift the unit, form a new upstand to height with insulation, wrap it in membrane and reset the rooflight. Renewing the covering soon anyway? Do both together. MyBuilder, 2026, gives typical national flat roof costs of £70 to £100 per m² for bitumen felt, £90 to £130 for EPDM and £95 to £150 for GRP; they are not our prices.
Our survey photographs your kerb and roof, and the quote that follows covers the work. Choosing a position for a new opening? Start with where to put a roof window. Our skylight work is covered by a written 10-year workmanship guarantee; ring 01453 933613 to arrange a look.