Rooflight upstands
What is a rooflight upstand or kerb?
The short answer
A rooflight upstand, also called a kerb, is the low box-shaped frame that lifts a rooflight clear of the roof surface. It keeps the glass above water running across a flat or gently sloping roof and gives the waterproof covering a face to be turned up against. Roof windows in a pitched slope are different: they bed into the covering and need no upstand.
How a kerb holds a rooflight above the water, what the regulations expect of it, and what decides the work.
Where the kerb sits in the roof build-up
A flat roof is built as a deck, then insulation, then a waterproof membrane. A rooflight cannot rest straight on that surface and stay dry, so a frame is formed around the opening first. That frame is the upstand. The membrane is taken up its outer face and across the top edge, and the glazed unit is fixed to the top of the frame.
Building Regulations use the kerb to tell two products apart. Approved Document L describes a rooflight as glazing that stands out of the roof plane on a kerb or upstand, while a roof window lies in the slope, facing the same way as the roof around it. A skylight on a flat roof is therefore a rooflight with a kerb. A window set into a pitched slope, held by a flashing kit instead of a frame, counts as a roof window. Our answer on rooflights and roof windows sets one against the other, and the skylights and roof windows page covers the whole range.
Why standing water makes the height matter
No flat roof is truly flat. A slight fall carries rain towards an edge or an outlet. BS 6229:2025, which replaced the 2018 edition in December 2025, keeps a minimum finished fall of 1:80, and roofers usually design for around 1:40 so the water still finds its way off. Even then, water can collect on the uphill side of an opening, and the kerb stands in its way.
So the height of the upstand, and the care taken turning the membrane up its face, decide whether the skylight stays dry. A frame set too low, or a covering finished badly at the corners, lets water creep through the joint between roof and frame. When a flat-roof skylight drips, that joint is a likelier culprit than the glass, and it is the first place we look before blaming the unit.
The flat roof skylights page covers the glazing that sits on a kerb.
What Part L asks of the frame and the glass
For a rooflight going into a house that already stands, whether new or as a replacement, Approved Document L 2021, Table 4.2, caps the rooflight at a U-value of 2.2 and adds a separate cap of 0.35 for an upstand built on site. That second figure is heat loss through the frame itself, so the kerb needs insulation of its own and cannot be a bare timber box.
From 24 March 2027 the 2026 edition of the document takes over, and its Table 3.2 repeats those values. A roof window in a slope has a different target: a U-value of 1.4, or a Band B window energy rating. That is why the two products are not interchangeable on paper, and why nobody should hold a rooflight to the roof window figure.
Your building control body, whether a registered building control approver or the council's team, confirms how the rules apply to a given job. For the glazing numbers in plain words, read about roof window U-values.
Ready-made kerb or one built on site
Upstands come in two forms. Some arrive factory-made with the rooflight, insulated and cut to suit the unit. Others are put together on the roof by a roofer or carpenter from timber, insulated, then wrapped in the roof membrane. A built-up kerb suits an unusual opening, or a new flat roof where the upstand is formed as part of the same covering job.
Neither wins every time. The factory frame is predictable and the site-made one is adaptable. What counts is that the frame is square and level, pinned firmly to the deck, insulated to meet the limit above, and finished in the same covering as the rest of the roof so that no joint is left to fail. On a long run of roof, the kerb also has to sit where the fall can carry water past it, not leave a pond behind it.
Large glazed lanterns rest on kerbs in just the same way. Our guide to roof lanterns explains how they differ from a single rooflight.
What we look at before a rooflight goes on a kerb
We start on the roof itself. Photographs record the deck, the covering and the fall, and show whether an old opening or kerb is worth keeping. A kerb on a tired roof is a poor investment, so if the covering beneath is close to worn out we tell you that plainly and explain whether the rooflight should wait for a new covering.
Next comes the opening. A wider one may need the roof structure trimmed once someone has checked it, and a new rooflight in a flat roof can raise planning questions that turn on the building. Flats, listed buildings and homes in a conservation area need a word with the council before anything is ordered, so check with your local planning team.
The figure then arrives on paper as a single total covering opening, kerb, covering and unit. Four things push it up or down: the opening's size, the kind of kerb, the roof covering and how we reach the roof. The debris leaves with us at the end.
Related questions
Kerb and rooflight questions
Can a leaking flat-roof skylight be mended without a new unit?
Frequently the glass is sound and the fault lies in the kerb: a low upstand, a split corner in the membrane or a failed seal at the joint. We photograph the kerb, the covering around it and the glazing seals, then tell you whether work on the upstand will hold or the unit has to be replaced.
Does a kerb make the skylight more visible from the street?
It raises the glass above the roof surface, so a rooflight shows more than a window lying in the slope. Whether that needs planning permission depends on the building and its setting, so put the question to the council's planning officers first, particularly for a flat, a listed building or a conservation area.