Roof light upstands explained

A roof light upstand is the raised kerb formed around the opening in a flat or low-pitch roof. It is not decorative structure — it is the interface between the roof build-up, the weathering detail and the glazed unit. Get the upstand wrong and even a well-made roof light will be difficult to seal, impossible to dress cleanly or prone to long-term water ingress.

What the upstand does

On a typical flat roof installation, the upstand performs four jobs:

1. Raises the glazed unit above the finished roof surface so rainwater drains away from the perimeter rather than ponding against the glass edge.
2. Provides a vertical face for the roofing membrane and insulation to turn up and terminate — the critical weather line on a warm or cold roof.
3. Supports the roof light on its bearing surface, with edge cover and fixings as defined by the manufacturer.
4. Sets the internal finish — on a frameless unit, the plasterboard or ceiling lining returns to the inside face of the upstand, so its position defines the visible aperture at ceiling level.

The upstand is therefore part of the roof design, not an afterthought around the glass.

How upstands are built

Most domestic and light-commercial upstands are formed in timber, lined with plywood or OSB, insulated and then dressed by the roofer. Metal kerbs are used on some commercial decks. The exact build depends on whether the roof is warm-deck, cold-deck or inverted, but the principle is the same: a continuous, structurally sound kerb around the full perimeter of the opening.

Before the roof covering is completed, confirm:

• Opening size — width and length to the manufacturer’s order dimensions, including any frame overlap.
• Upstand height — sufficient for membrane turn-up, insulation and the supplier’s minimum edge cover.
• Squareness and twist — diagonal measurements equal; bearing plane level within tolerance.
• Structural support — trimmers and headers sized for the span and any additional load from the glazed unit.

At Vant Glass frameless roof lights, units are made to your exact order size. The upstand must be prepared to match the fixing detail supplied with your order — do not assume a generic kerb size.

Upstand height and roof pitch

On flat roofs, drainage falls run towards rainwater outlets, not necessarily away from every side of the opening. The upstand must still present a continuous weather barrier. Where falls create unequal heights around the kerb, the roofer and installer coordinate so membrane terminations and throat sealant details remain continuous.

On pitched roofs, the upstand (or curb) is shaped to follow the roof angle. The glazed unit may be horizontal while the roof pitches — the manufacturer’s curb detail accommodates that geometry. See our guide on roof pitch and roof light installation for sequencing on sloped decks.

Interfaces to get right

Roof membrane to upstand — Turn the membrane up the external face and terminate below the roof light weathering line. Insulation must be continuous around the kerb to limit cold bridging. Internal finish — stop linings short of the glass, leaving the manufacturer’s ventilation gap. Coordinate the structural opening with the ceiling aperture early in design.

Common mistakes

• Oversized or undersized opening — Forces packers, stresses glass or leaves insufficient edge cover.
• Out-of-square kerb — Unit will not sit evenly; seals compress on one side and open on another.
• Membrane terminated too low — Water tracks behind the turn-up in wind-driven rain.
• Installing before the roof is weathertight — Water in the build-up soaks insulation and delays the project.
• No record of dimensions — Changes on site without telling the glass supplier lead to remakes or awkward site trims — toughened glass cannot be cut down after manufacture.

Checklist before ordering glass

1. Confirm roof type (warm, cold, inverted) and total build-up depth.
2. Obtain the roof light manufacturer’s upstand and edge-cover requirements.
3. Set out the structural opening and kerb; verify diagonals and level.
4. Complete membrane dressings and allow the build-up to dry.
5. Re-measure immediately before the unit is delivered.

A correctly formed upstand is the foundation of a reliable roof light installation. Specify the glazed unit from a manufacturer who publishes clear fixing and weathering details, and build the kerb to those dimensions — not the other way around.

Every Vant Glass roof light is made to order in Britain, backed by a 20-year guarantee and free UK mainland delivery. Configure frameless or framed sizes in the online calculators or call 03330 902 592.