OSB or Plywood for a Flat Roof Deck?

What each board is

Plywood is made from thin layers of wood veneer glued together with the grain of each layer at right angles to the one below. That cross-grain construction gives it dimensional stability and a smooth surface. OSB, oriented strand board, is made from long wood strands pressed and glued in layers, also at right angles. It is dense and strong, with a rougher, chunkier face. Both come in several grades, and only the grades rated for humid or exterior conditions belong on a roof.

For OSB, that means OSB3, which is built for load-bearing use in humid conditions. Standard OSB2 is for dry interiors and swells badly if it gets wet. For plywood, the answer is an exterior or WBP-type grade with a waterproof glue line, not interior sheet. Mixing up grades is a frequent source of failure, and it is invisible once the covering has been laid.

How they behave when wet

All timber boards take up water, and the question is how they cope. Plywood tends to dry out and return close to its original shape. OSB tends to swell at the edges when wet, and it may not fully recover, leaving a raised lip at joints that shows through a thin membrane. OSB3 is much better than lesser grades, but a roof deck left uncovered through a wet spell may still swell at the cut edges.

This matters during the build. A deck that has been rained on before the covering goes down has to dry out before sealing, or the moisture is trapped. We cover that in our page on fitting a flat roof in winter. A deck left damp then sealed is a common start to rot and blistering, which our guide to flat roof blisters describes.

Which suits which covering

GRP lays resin and glass directly onto the deck, so the board face shows through. A smooth, stable surface makes for a neater laminate, and the movement of the board at joints can crack a rigid covering. Plywood is therefore the usual preference under GRP, with joints supported and tongue-and-groove edges where possible. EPDM is bonded with adhesive over a sound, smooth deck and can tolerate OSB3, though a rough face needs a clean, even finish for the bond. Torch-on felt generally goes over a fixed layer of underlay or insulation, so the board face matters less.

Our comparison of EPDM, GRP and felt shows what each covering demands from the surface beneath it.

Thickness, support and fixing

Board grade is half the picture. The other half is how it is held. A deck needs to be thick enough for the joist spacing, commonly 18 mm for typical domestic spans, and the board edges need to land on a joist or on noggins between joists. A joint left floating moves under foot and splits the covering above it. Boards are laid in a staggered pattern with a small expansion gap, and fixed with screws or ring-shank nails at the right spacing.

Chipboard decks, found on many older extensions, are a different matter. Chipboard swells and crumbles when wet, and a roof that leaked for a while may have a deck that looks sound but has lost its strength. Walking on such a roof is risky, as our page on walking on a flat roof explains.

Keeping the deck dry on site

Even the best board is only as good as its handling. Sheets should arrive off the ground and covered, be fitted in dry weather and be sealed with the covering as soon as they are down. If rain catches a laid deck, it needs to dry before the membrane goes on, and a moisture meter or a simple wait is better than hope. Cut edges and screw holes are the thirstiest places, so some installers seal them. A deck that was soaked, sealed and then slowly dried from below tends to produce the staining and blisters seen on roofs less than a few years old.

What we use and why

Cost follows from all this, though the board itself is a small part of a roof bill. The cheaper sheet is not always the cheaper roof if it fails early. We choose the deck to suit the covering and your roof, and say what we are fitting before work starts. Where the existing deck is sound, we keep it, and where it is soft we replace only what needs replacing. The free survey shows you the state of the boards in photographs, and the fixed written quote includes any decking needed. Take a look at flat roofing in Basingstoke for the other parts of the job.