For many UK flat-roof details, the rooflight upstand is designed to finish at least 150mm above the completed roof covering at its lowest point. This helps protect the opening from standing water, snow, splashback and blocked drainage.
Measure from the finished roof
The height is measured from the completed waterproofing or roof finish—not from the structural deck. Insulation, screed, falls, paving and green-roof build-ups can all change the final level.
A nominal 150mm timber kerb installed before the roof build-up may therefore be too low once insulation and waterproofing are added.
Falls and thresholds
On a tapered or sloping roof, check the lowest exposed point around the complete perimeter. Terraces, paving and green roofs need careful coordination because their finished surfaces may sit above the membrane.
Any reduced-height or flush detail needs project-specific design and agreement from the relevant designer, waterproofing provider and building-control team.
Project checklist
This is general UK detailing guidance, not approval for a particular roof. Always check the current specification, waterproofing warranty and project requirements.
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