OSB Insulated Flat Roof Skylight Upstand – 37 Sizes, 300×300mm to 1500×3000mm

Order a rooflight and you've solved half the problem — it still needs somewhere to sit. Mount a dome or flat rooflight straight onto a EPDM deck and you're relying entirely on the upstand detail built on site to keep the weather out and the heat in; get that upstand wrong and no amount of glazing quality will save the room underneath from cold bridging or a leak at the junction.

This is a pre-insulated OSB upstand — a rigid, ready-made kerb that lifts your rooflight clear of the roof surface and gives the installer a true, square, insulated base to bond the membrane and mount the rooflight onto. Rather than building an upstand from scratch on site out of timber and offcuts of insulation, you get a factory-made box with a 40mm XPS-insulated core between 15mm OSB faces, already close to your opening size, with a slight 6-degree fall built into the top on one side to help shed standing water away from the rooflight seal.

Where it's used

Fit it under any fixed or opening flat-roof rooflight, dome or lantern where the roof opening needs to be raised above the surrounding membrane — garden rooms, extensions, orangeries, warehouse roof lights and anywhere else a flat roof needs daylight. It suits both new-build openings and refurbishment projects where an old, poorly insulated upstand is being replaced along with the roof covering.

Sizes and specification

Available in 37 sizes from 300×300mm up to 1500×3000mm to match a wide range of standard and larger rooflight openings — order the size that matches your rooflight's kerb dimensions, checking the manufacturer's fitting instructions for any tolerance allowance. Every size shares the same build: 15mm OSB outer faces around a 40mm XPS-insulated core, giving a total heat loss value of 0.63 W/m²K, with a natural 6-degree incline across the short side to assist drainage. Suitable for both warm and cold roof constructions and compatible with any flat roof covering, including EPDM.

Installation guidance

Set the upstand on the roof deck at the prepared opening and fix down in line with the rooflight manufacturer's instructions. Dress and bond the EPDM membrane up and over the upstand faces in the usual way, finishing with a compatible trim or termination detail at the top edge before the rooflight is fitted. Always check your chosen rooflight's own fixing and kerb-size requirements before ordering, since tolerances vary between manufacturers.

Ordering

Measure the actual roof opening (or the rooflight manufacturer's specified kerb size) before choosing a size — the upstand should be ordered to match the kerb dimension the rooflight is designed to sit on, not the visible glazed area. Pair with our dome rooflight range, EPDM membrane and contact adhesive to complete the detail. Fitting a walk-on glass rooflight instead? Check its structural opening requirement first — trafficable glass units are load-bearing and need an engineered structural opening rather than this standard insulated kerb.