What is a rooflight upstand?
An upstand is the raised frame built off the roof structure that a flat rooflight sits on top of. It lifts the glass clear of the roof surface, gives the waterproof covering something vertical to be dressed up against, and provides a level, square platform for the unit to be fixed down onto. It is the least visible part of the job and the part that decides whether the installation is still dry in a decade.
Almost every water problem around a flat rooflight traces back to this component: its height, how it was insulated, or how the covering was terminated against it. The wider job is set out on flat roof rooflight installation.
Upstand, kerb and builder’s kerb: the words people use
Three words describe roughly the same thing and suppliers use them inconsistently, which causes real confusion at ordering.
Upstand is the general term for the raised perimeter. Kerb is used interchangeably with it, more often by roofers than by glazing suppliers. Builder’s kerb means specifically one constructed on site out of timber, as distinct from a preformed insulated kerb bought as a product. When a supplier quotes a unit “to suit a 150mm builder’s kerb”, they mean the glass is sized to land on a timber frame you are building yourself.
Timber upstands, and how they are built
The common site-built version is a box of treated softwood, usually 150mm deep, fixed down onto the deck and through into the trimmers around the opening. The corners are properly jointed and screwed rather than butted and nailed, because the whole assembly has to stay square while a heavy unit is landed on it.
It is then decked over on its outer faces with plywood or OSB so the covering has a continuous surface to bond to. A timber fillet is run at the base of each face, so the covering turns from horizontal to vertical over a forty five degree angle rather than a sharp arris. Membranes crack at sharp angles. That fillet costs almost nothing and prevents a specific, predictable failure.
Insulated preformed kerbs
The alternative is a factory made kerb, typically a rigid insulation core faced in GRP or a similar laminate, delivered as four sides or as a single moulding. It arrives square, it is already insulated, and it removes the cold bridge that a bare timber box creates.
They come in fixed heights and standard opening sizes, and they suit new build and new extension work where the opening can be set out to the product rather than the product cut to the opening. On an existing roof with an awkward opening, a site-built timber kerb is often the more practical route.
Why the covering is dressed up the sides
This is the whole point of the component. The waterproof layer on a flat roof is continuous, and at a rooflight it has to stop somewhere. If it stopped at the edge of the hole, the joint between roof and glass would be the only thing holding water out.
Instead the covering runs up the outer face of the upstand and terminates near the top, well above any water that could sit on the roof. The unit then lands over that termination and covers it. Water arriving on the roof has to climb the full height of the upstand before it can reach the joint, which under normal conditions it cannot do.
How the unit fixes down onto it
Most flat rooflights land on the top surface of the kerb with an outer flange oversailing the covering termination. A compressible sealing tape or a bed of sealant goes between the frame and the top of the kerb, and mechanical fixings pass through the flange into the timber.
Two things matter here. The fixings must sit above the level water could reach, since every fixing is a hole through a weatherproof line. And the top of the kerb must be genuinely level and genuinely flat, because a frame bedded onto a twisted kerb is stressed from the day it is fitted and the seal is asked to bridge a gap that keeps moving.
Thermal bridging at the upstand
A timber kerb is a solid path from the warm inside of the roof build-up to the cold outside air. Left bare, its inner face runs several degrees below the surrounding ceiling in winter, and that is exactly where condensation forms and where mould turns up on the plasterboard reveal.
The detail that solves it is insulating the outer face of the kerb, carrying the roof insulation tight up to it and continuing it vertically, so the thermal line is unbroken from the roof plane to the underside of the glazing frame. The vapour control layer is carried up the inner face and sealed to the frame. Neither step is visible afterwards, and neither can be added later.
Ordering: the three sizes and which one to give
Sizing errors at this stage are expensive because the kerb is already built by the time the glass arrives.
| Dimension | What it measures | Where it is used |
|---|---|---|
| Structural opening | The hole in the deck between trimmers | Carpentry and structural design |
| Kerb external size | Outside face to outside face of the upstand | The size most units are ordered to |
| Daylight opening | Visible glass inside the frame | What you actually see from the room |
The daylight opening is always the smallest of the three, and the difference between it and the structural opening on a deep build-up is considerable. That gap is why a splayed internal reveal matters so much on a flat roof.
Where upstands go wrong on Essex flat roofs
Three patterns recur. A kerb built too low so the unit sits flush with the covering, usually done for the look of it from the garden, which leaves nothing between standing water and the frame. A kerb built square to the room rather than square to itself, so one corner is out and the frame never beds properly. And a kerb built off the deck boards alone, with no fixing into the trimmers, which moves as the deck deflects.
The height it should actually be is a question in its own right, and it is answered on how high a rooflight kerb should be. Sunspire builds the upstand as part of the installation, which is why the workmanship carries a ten-year guarantee rather than the guarantee resting on the brand of glass.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements