Rooflight Kerb and Upstand System

Forming an opening in a roof removes the deck across that area and, with it, every layer the deck was carrying. The kerb is what takes over those duties around the perimeter of the hole. It supports the unit that will sit on it, gives the waterproofing something to turn up and terminate against, and carries the insulation and vapour control around an aperture colder than the roof surrounding it.

The test against an envelope penetration sealing system is which came first. A kerb is formed as part of the roof, trimming the structure and creating an aperture that a unit is later set onto; a penetration seal restores continuity around something already passing through a completed surface. This entry stops at the outer face of the kerb, because everything above that face, the frame and the glazing and any opening mechanism, belongs to a manufactured unit governed by its own documentation.

A kerb does two unrelated jobs on the same piece of construction, and they fail in different ways. Structurally it is a continuation of the trimming, so anything that racks or settles moves the surface the unit is fixed to. Environmentally it is a ring of construction standing outside the roof's field build-up, and therefore colder than everything around it. A kerb that is structurally sound and thermally forgotten leaks nothing and still puts water on the ceiling.

That water comes from inside. A glazed unit is usually the coldest surface in the room below, so air reaching it deposits moisture on the glass and on the frame, and that moisture runs downward to whatever sits at the bottom of the aperture. Manufactured units generally deal with what forms on their own surfaces, but what forms on a cold uninsulated kerb behind the lining is not their concern and has nowhere to go except the reveal. The insulation return and the condensation channel answer the same problem from opposite ends.

The vapour control layer decides how large that problem is. Carried up the kerb and closed against the unit's frame, it keeps moist internal air largely away from the cold ring. Stopped at the deck, as often happens because the opening is formed after the field layers have been laid, it turns the aperture into a route by which internal air reaches cold construction from behind, where nobody is looking for it.

Water arriving from outside behaves differently again. A kerb standing in a roof plane obstructs the flow across it, so anything upslope has to be led around rather than allowed to pond against the turn-up. Kerb position and roof falls therefore cannot be decided independently: an aperture placed where water naturally converges hands the weathering a job the same detail elsewhere on that roof would never face.

The kerb also sets the terms for the rest of the unit's life. How high and where it stands governs whether the glass can be reached for cleaning, whether an opening light has room to swing, and whether the upstand can be worked on without disturbing the roof covering. These follow from the same decision that fixed the waterproofing termination, usually taken earlier and by somebody else.