The Upstand: The Most Important 150mm of Your Roof Light Project

Ask our technical line what goes wrong on site and the answer is rarely the glass. It is the upstand — the timber kerb the roof light sits on. Get those 150 millimetres right and the rest of the installation is a good afternoon's work; get them wrong and no amount of sealant will save the detail. This is the guide we wish every builder read before the joists go in.

What the upstand has to do

Three jobs: lift the glazing at least 150mm above the finished roof so standing water and wind-driven rain never reach the seal; carry the weight of the unit evenly on all four sides; and give the roof membrane something clean to terminate against. Height, support, termination — every rule below serves one of those three.

The rules that matter

• Minimum 150mm above the finished roof covering — measured after the insulation and membrane build-up, not from the deck.
• Level to a couple of millimetres and square across the diagonals. The glass is rigid; it cannot follow a twisted kerb.
• Solid timber, firmly fixed to the trimmed structural opening — the upstand transfers the glass load into the roof structure.
• Support the inner pane. On frameless units especially, the upstand must sit under the full glass build-up so the inner pane bears on timber — never cantilevered over the void with only the bond holding it. This is the single most important safety detail in the whole job.

Frameless, framed, walk-on: how the detail changes

Frameless units bond over the upstand, so the kerb's outer face is finished by the glass edge — accuracy is everything. Framed units land on top and fix through the frame, forgiving a little more variation and flashing conventionally. Walk-on units carry live loads, so the supporting structure is specified for the traffic above, and the finished paving level has to meet the glass — agree that interface before the screed goes down. Every order ships with the sizing and support guidance for its range.

Kerb first or glass first?

Either works — that is the luxury of made-to-measure. If the upstand exists, we manufacture to its exact internal opening. If it does not, pick the glass size that suits the room and build the kerb to match. What never works is guessing. Send your drawings or measurements through the bespoke enquiry page or put your builder on to us at 03330 902 592 — the technical call is free and it prevents the expensive kind of surprise.