What is a rooflight upstand?

The box the opening is turned into

Cut a hole in a flat roof deck and you have a rectangle of nothing, with the covering ending in mid-air at four raw edges. An upstand turns that rectangle into an object with sides. Four lengths of timber are framed around the trimmed opening, squared, levelled and fixed down into the structure, so the hole now has a wall around it.

Everything after that follows from having those four walls to work against. The roofer has something to dress the covering up. The rooflight has something to bear on and be fixed to. The insulation has something to butt against. And the point where waterproofing turns from horizontal to vertical, which is the weakest line in any flat roof, is lifted clear of the surface where water sits.

Upstand, kerb and builder’s kerb

Three words for very nearly the same thing, and the distinction is worth knowing because quotations use them differently.

• Upstand is the general term for any vertical element the covering is dressed up against, whether that is around a rooflight, a pipe or a parapet.
• Kerb is what most installers say when they mean the specific timber frame under a rooflight.
• Builder’s kerb means one made on site from timber, as opposed to a factory-made insulated kerb bought with the unit.

If a quotation says the price excludes the kerb, it means somebody else is building the box and you should find out who. That gap between trades is where flat roof installations go wrong more often than any product failure.

What it is built from

Structural timber, usually 47mm or 50mm thick softwood, in a depth that gives the height needed. On deeper kerbs the sides are framed rather than cut from a single section, and noggins are put in at the corners so the box cannot rack out of square while the covering is being dressed.

The outside face is normally sheathed in plywood or OSB to give the roofer a continuous surface to bond or torch onto. Timber alone is too knotty and too movement-prone for a membrane to stick to reliably. On larger units the top edge gets a capping so there is a defined line for the covering to stop at, rather than a rough sawn arris.

Where it takes its bearing

An upstand carries the weight of the glass, the frame, and anything that lands on the unit: wind, snow, and on a walk-on installation a person. That load has to reach the structure, not the deck board.

So the timber sits on the trimmers and trimming joists that were put in when the opening was formed, not on the 18mm ply or OSB decking. On a large unit the trimmers themselves get doubled up, because taking a metre or more out of the joist run means the remaining joists either side pick up what the cut ones were carrying. A kerb screwed through the deck into nothing in particular will sit fine for a year and then start to move.

What happens at the top edge

This is the detail that decides whether the installation is sound, and it is invisible once the unit is on.

The covering comes up the outside face, over the top corner, and stops on the top surface of the kerb, a short distance in from the outer arris. It is mechanically held there, usually under a batten or a termination bar, so it cannot slump or peel back in the heat. The rooflight frame then sits down on top of that, on a bed of sealant or a compressible tape, and its own upstand or flange covers the termination.

Water never reaches the joint between frame and kerb, because it has already been stopped by the covering below. Where that order is reversed, and sealant at the frame is the only thing holding water out, the installation has a few winters in it at best.

Insulation on the outside face

A bare timber kerb is a loop of solid material joining the warm room below to the cold air above, running all the way round the unit. In cold weather the inside face of it runs colder than the ceiling either side, and water forms along the reveal.

The answer is to treat the kerb as roof rather than as carpentry. Insulation board is fixed to its outer face as a continuation of the deck insulation, so there is no break in the thermal layer, and the vapour control layer is carried up the inner face and sealed to the frame. That adds thickness to the outside of the box, which is why the finished kerb is always wider than the opening it surrounds. Our page on whether flat rooflights cause condensation goes through what happens when this is left out.

Upstands supplied with the unit

Several manufacturers sell an insulated kerb designed for their own rooflight: a moulded or composite box, already the right size, already insulated, with a defined line for the covering to terminate on. Fakro and Keylite both offer them, and a number of the flat glass specialists do the same.

They take the guesswork out of the geometry and they suit a roof where the build-up is straightforward. They are not always the right choice. Their heights are fixed, and where the roof needs more height than the standard product gives, a site-built kerb is the only way to get it. On replacement work the existing opening rarely matches a proprietary size either.

What it looks like from the room below

From inside, the upstand appears as a reveal: a plastered or lined shaft between the ceiling and the glass, as deep as the kerb plus the roof build-up. On a warm roof with 150mm of insulation and a 150mm kerb, that reveal is roughly 300mm deep before the frame starts.

The reveal can be worked with rather than tolerated. Splaying it outwards, so the opening is wider at ceiling level than at the glass, throws noticeably more light into the room and softens the tunnel effect. That decision belongs at the survey, because a splayed reveal changes how the opening is trimmed.

Questions worth asking about yours

On a quotation, ask who builds the kerb, what height it will be, whether it is insulated on the outside face, and how the covering terminates on it. Four questions, and the answers tell you a great deal about who you are dealing with.

How the height itself is arrived at is set out on our page covering how high a rooflight kerb should be. We fit new units and full replacements across Oxford and up to twenty five miles by road, we have been installing rooflights for fifteen years, and the workmanship carries a ten-year guarantee.