SIPs Roof Panels

How a panel roof stands up

A roof panel is the same sandwich as a wall panel, generally thicker and much longer. Its two OSB skins act like the flanges of a beam, so a panel can bridge several metres unsupported. Spans of about four metres between supports are common on domestic roofs; the engineer sets the real figure from the pitch, the roof covering and the snow load for your area.

On a single-pitch roof the panels run from a wall plate on the low side to a beam or the house wall on the high side. On a vaulted double-pitch roof they meet at a ridge beam, in glulam or steel, which rests on posts or gable panels. That beam is what stops the roof pushing the walls outward, so it cannot be left out to save money.

Edges are closed with timber, joints are made with insulated splines and sealed, and the panels are screwed down with long fixings into the beam and wall plate.

Reaching 0.15

Approved Document L limits a new roof in an extension to 0.15 W/m²K in England. A 172 mm urethane-cored panel falls a little short of that on its own. Suppliers close the gap in one of three ways: a thicker panel, a layer of rigid insulation over the top, or insulated plasterboard underneath.

Each has a consequence. Thicker panels are heavier to lift. Insulation over the top raises the roof line, which matters under a planning height limit. Insulated plasterboard lowers the ceiling slightly but also covers the joints, which helps airtightness. Ask to see the U-value calculation for the whole build-up, not the panel in isolation. Wales and Scotland publish their own figures.

Pitched roofs

Over the panels goes a breather membrane, then counter-battens running down the slope and tiling battens across. The counter-battens matter: they create a drained, ventilated gap so that any water getting past the tiles runs out at the eaves and the top OSB skin can dry.

Almost any covering works. Slate and plain tiles need a steeper pitch; interlocking tiles and standing-seam metal go lower. Give the supplier the covering before panels are designed, because its weight and minimum pitch feed the engineering.

Flat roofs

SIPs make a quick flat roof deck, but the build-up above them deserves care. A waterproof membrane laid straight on to the top skin traps any moisture in that OSB. The safer arrangements add a layer of rigid insulation above the panel so the OSB stays warm, or a ventilated void between the panel and a separate deck. A vapour control layer goes on the room side in either case.

A "flat" roof is designed with a fall of 1 in 40 so that it finishes no flatter than 1 in 80 once built. The fall comes from tapered insulation or timber firrings, or from setting the panels themselves at a slight slope.

Roof lights and openings

Openings are cut in the factory and lined with timber. A roof light narrower than one panel is simple. Wider openings interrupt the span, so the panels beside them are doubled up or a trimmer beam is added. Decide positions early and resist moving them later.

Roof lights count towards the glazing allowance in Approved Document L. If glazing across the whole extension exceeds roughly a quarter of its floor area, the designer has to show by calculation that the extension loses no more heat than a compliant one would.

Fire and ventilation

Inside, plasterboard fixed to the manufacturer's tested detail gives the roof its fire resistance. Outside, Approved Document B restricts which roof coverings can be used within six metres of a boundary; most tiles, slates and metal sheets meet the top classification, but ask about membranes and rooflight glazing.

A SIPs roof leaks very little air. That is the point, and it means the room relies on designed ventilation: trickle vents in the windows and an extractor wherever there is cooking or a shower.