The short answer
For most flat roofs, exterior-grade plywood or OSB3 laid to the board maker's instructions is the sensible choice. Chipboard swells and crumbles once it gets wet, so it is a poor choice outdoors. The right thickness depends on joist spacing and the covering maker's specification, which is why the deck is checked before any price is fixed.
How the common deck boards compare, what thickness and fixings depend on, and why the state of the deck often decides the whole flat roof job.
Three boards, three different behaviours
The deck is the flat surface of boards that the waterproof covering sits on. Whatever goes on top, GRP, EPDM or felt, the deck has to stay flat, dry and firm for the covering to work. Three boards turn up in practice.
Exterior-grade plywood is made from thin layers glued crosswise, which gives it good stiffness and resistance to water getting in at the edges. It is a long-standing choice for flat roofs and tends to hold its shape. OSB3 is oriented strand board graded for damp conditions, made from compressed wood strands. It is widely used for flat roof decks and is usually cheaper than plywood, but its edges can swell if left exposed to rain before they are covered. Chipboard is made of fine particles and glue. It softens and breaks down when wet, so it should not be used where rain could reach it, even though old flat roofs sometimes still have it.
None of these is a magic answer. Both plywood and OSB3 are fine when installed properly and kept dry until the covering goes on. Check the board's grade stamp or data sheet, and make sure your roofer names the grade they plan to use.
Thickness, joists and fixings
There is no one thickness that suits every roof. Thickness depends on how far apart the joists are, because a board spanning a wide gap flexes. A deck that bounces under foot will flex the covering too, and repeated movement is what cracks GRP and splits seams. The board maker's data sheet and the covering manufacturer's specification both set the minimum, so ask for those rather than trusting a figure from memory.
How boards are laid matters as much as which board it is. Sheets should be staggered, supported on every edge, and fixed with the right screws or nails at the spacing the maker gives. Small gaps between boards allow for movement as they take up moisture and dry out. Boards butted tight can buckle when they swell. Edges and joints are best kept dry through the build, so work is often sequenced to lay the deck and cover it quickly.
If you need a roofer to explain their choice, ask three things: what grade of board, what thickness, and how it will be fixed. A plain answer to all three tells you a good deal about the rest of the job.
Falls are built into the deck
Nothing about a flat roof is perfectly level. It needs a slope so water runs to an outlet instead of lying in puddles. BS 6229:2025, which covers flat roofs finished in continuously supported flexible coverings, retains a 1:80 minimum finished fall for general areas of roof and internal gutters. IKO, citing the 2018 edition, advises designing for a fall of 1:40 so that the finished roof still reaches its target once the build is complete. Our answer on the minimum fall for a flat roof goes into the numbers.
Falls are usually created by fixing tapered timber battens, called firrings, on top of the joists before the boards go on, or by tapering the insulation. If the old deck already dips, laying new boards onto it just copies the dip, and ponding water speeds up the ageing of whatever covering sits over it.
A deck that has sagged because of rotted joists or wet boards needs more than a new covering. The sagging flat roof answer explains how to tell a dip from a failing deck.
Why the deck often decides the job
When an old covering comes off, the boards underneath are seen properly for the first time. Soft, black or crumbling areas mean water has been getting in for some time, and a new covering laid over a wet or rotten deck will fail early however good it is. That is why a flat roof survey looks for ceiling stains, a springy feel underfoot and signs of moisture around outlets and upstands, and why photos of the deck go into the discussion.
It also affects the price. The cost of a flat roof often depends less on the covering than on what is under it: how much deck needs replacing, whether joists are sound, whether the falls need rebuilding and how access is arranged. A fair written price states what is included. Ours is a single total price once the survey is done, with deck photos shared with you as soon as the old covering is stripped.
If the roof is in a state where the covering has failed, putting GRP over old felt is rarely the answer, because the deck has to be seen first. See our flat roofing page for the wider service.
More on this in Flat Roofing in Crawley
Two deck questions
How can I tell if my flat roof deck is rotten?
Signs include a spongy or bouncy feel underfoot, a visible dip, brown stains on the ceiling below, and soft or flaking boards at the edges or around outlets. You cannot always see it from above, since a covering hides the boards. A survey, usually with a look from the room below, is how a roofer confirms it.
Can a new covering go over a damp deck?
It should not. Trapped moisture can blister the covering, rot the boards further and cause early failure. The deck needs to be dry and sound before anything is laid. If the boards are only damp at the surface, they can be allowed to dry first, but rotten sections must be cut out and replaced.