The short answer
Yes, in practice every flat rooflight sits on a kerb, also called an upstand. It lifts the glass and the waterproof joint above any water lying on the roof. Some units look flush from inside or outside, but a low upstand is still hidden beneath the frame. Without one, the joint would sit at the lowest point of the roof.
What a kerb actually is
A kerb is a short raised frame around the roof opening, and the rooflight is fixed to its top edge.
Think of it as the foundation for the glass. Joists are trimmed to make the opening, a box of timber, GRP, or an insulated engineered upstand is built or bonded around it, and the flat roof covering is carried up its outside face. The rooflight then sits on the top. The roof membrane, whether felt, EPDM or GRP, is welded, glued or laminated to the kerb's outer face, which gives a continuous waterproof skin from the roof surface to the top of the upstand.
A builder's upstand is the same idea in masonry or timber form, often built as part of an extension. Our short page on what a builder's upstand is explains what that means for the rooflight supplier, and flat roof rooflight upstands covers height, insulation and finish.
Three jobs the kerb does
It is easy to think of it as a box, but it carries out three separate jobs, and dropping any of them causes trouble.
- It keeps the joint dry. A flat roof is never perfectly flat. The fall built into it means water moves towards the outlet, but after rain it can linger, and it may freeze. A joint placed flush with the roof surface would sit under that water. The kerb lifts the joint clear, so the critical seal between glass and roof is above the wet line. Good practice for flat roofs expects an upstand of at least 150 mm above the finished roof level for this reason, and we confirm the exact height for each roof at the survey.
- It carries the weight. Glass is heavy. The kerb spreads the load of the rooflight back to the trimmed joists, rather than leaving it to rest on the roof covering, which is not built to carry it.
- It stops the cold getting through. A bare timber edge around an opening is a cold bridge. An insulated kerb, with the lining carried up to the glass, keeps the edge of the ceiling warm and stops condensation forming at the reveal. That is part of meeting the thermal rules for new work under Part L of the Building Regulations.
The kerb does a fourth, quieter job too. A dense, lined upstand gives the frame something solid to sit on, and keeps rain from booming through a hollow box, which is a detail we write into the quiet spec for rooms where sound matters.
The flush-looking ones are not the exception
Some rooflights appear to sit level with the roof. Look closer and there is still a kerb below the glass.
Frameless and low-profile rooflights are designed so that the glass looks like a pane laid on the roof, with no visible frame. The look is real, but the build is not simpler. The glazing unit still sits on an upstand, and the upstand is tucked beneath the glass edge so you never see it from inside. Often the visible change is that the internal reveal is lined flush with the ceiling, so the eye follows a clean line from plaster to glass. What changes is where the kerb is, not whether it exists. Our page on frameless flat rooflights shows how that detail works.
A walk-on rooflight is another case. It sits on a kerb or supporting frame that is stronger than a standard one, since it carries foot traffic. The walk-on rooflight hub sets out the requirements.
| Kerb type | Typical use | Points to check |
|---|---|---|
| Timber upstand | Most extensions | Insulation, lining, felt dressing |
| GRP moulded | GRP roofs | Bond to the roof skin |
| Insulated factory kerb | Refits, tight reveals | Fit to the opening |
| Masonry upstand | Built into walls | Height above the roof |
When the kerb already exists, and when it has to be made
The amount of work depends on what the roof has today.
New roofs get the kerb built as part of the structure, with the roofer and the rooflight supplier agreeing the size before the covering goes on. On an existing roof, a kerb may have to be made in the same visit as the rooflight. That means trimming the opening, framing the upstand, insulating it and dressing the roof covering up its sides. The steps are in our page on how a flat roof rooflight is installed, and whether you can put a rooflight in an existing flat roof covers the difference.
Whichever route, the kerb has to be measured to suit the rooflight, not the other way round. A unit ordered to the wrong upstand size leaves gaps, and we avoid that by measuring at the survey and confirming the dimensions in writing before an order goes in.
Kerb questions owners ask
Most of these come up when the quote arrives and someone spots a line for the upstand.
Can the kerb be lower than 150 mm?
Sometimes a lower height is used with particular products and details, but it is the exception, and it calls for a good reason. The standard height keeps the joint clear of standing water, and we would not go lower without a clear, written design for that roof.
Why is the kerb insulated if the glass is already double glazed?
The glass is one part of the opening. The kerb is the rest of its perimeter, and uninsulated timber around the edge is where heat escapes and condensation starts. Insulating and lining it finishes the thermal job.
Does a taller kerb look worse from the garden?
A taller upstand shows more, but a slim, well finished one is barely noticeable. The finish, whether a coloured trim or a matching roof dressing, makes a bigger difference than a few centimetres. We talk through the finish at the survey.
To have the upstand specified for your own flat roof, get a quote, or go back to flat roof rooflight installation.