OSB Boards

What OSB board is

Oriented strand board is made from rectangular strands of timber, not chips and not fibres. The strands are laid down in layers with the grain of each layer running across the one below it, sprayed with resin, and pressed under heat until the whole thing is a single panel. That cross-laying is where the strength comes from, and it is the same idea that makes plywood strong, done with strands instead of veneers.

You can see it on the face. The strands are large enough to read as a pattern, which is why OSB looks the way it does and why some people leave it exposed on purpose. For most jobs though it is a working board that ends up behind plasterboard, under a floor covering or on the outside of a frame with a breather membrane over it.

The grade we carry, and why

OSB comes in four grades under EN 300. OSB/1 is for interior fitments in dry conditions. OSB/2 is load bearing but only in dry conditions. OSB/3 is load bearing and rated for humid conditions. OSB/4 is heavy duty load bearing in humid conditions and costs a good deal more.

We carry OSB/3, because it is the grade that actual building work calls for. A wall that is sheathed in September is going to get rained on before the roof goes on, and a floor deck will be walked on wet more than once before the house is closed in. OSB/2 does not tolerate that. Both of our sheets are also listed on the OSB/3 boards page.

OSB sheet sizes and thicknesses

Every sheet is 2440×1220 mm, the standard 8×4 foot size, and we hold two thicknesses that between them cover almost everything OSB is asked to do.

11 mm is the sheathing and roof decking board, at 19.6 kg a sheet. It goes on the outside of a timber frame to brace it, and over rafters to take the felt and battens. At that thickness one person can carry a sheet up a ladder and hold it in place while they nail it, which matters more on a roof than anything else about the board.

18 mm is the flooring board, at 32.1 kg a sheet. It goes down over joists, gets walked on for the rest of the build, and usually stays as the subfloor under whatever finish arrives later. It is also the thickness for hoarding, site huts, shelving in a garage and anything that has to take a knock.

If the panel is carrying load, the joist or stud spacing on your drawing decides the thickness rather than your preference, so check it before you order.

Working with it

OSB blunts blades. The resin content is high and the strands are hard, so a blade that cuts softwood all day will start to drag after a few sheets. Use plenty of teeth and accept that you will be changing blades more often than you expect.

The cut edge tears rather than chips, which leaves a furry line. That is fine where the edge is hidden and worth a light sand where it is not. Cutting face up with a circular saw and face down with a track saw keeps the better face clean, the same as any sheet material.

The dust is coarse and there is a lot of it. Cut outside if you can, use extraction if you cannot, and wear a mask either way. This is more of a nuisance with OSB than with plywood simply because of the volume.

For fixings, ring shank nails or screws both hold well in the face. Keep them back from the edge, because a fixing too close to a corner will push the strands apart and lift the surface. On a floor deck, glue the joists as well as nailing and the finished floor will be quieter for it.

Moisture, and what “humid conditions” really means

OSB/3 is rated for humid conditions, and that is a specific thing. It means the board keeps its strength through the wetting and drying that happens on a building site, and in service in a room like a utility or an unheated garage. It does not mean the board is waterproof and it does not mean it can be left out.

The edges are the weak point. Water gets into the exposed strands at a cut edge and swells them, and that swelling does not go back down when it dries. A board that has sat in a puddle will have a thickened, soft edge for the rest of its life. Stack sheets flat and off the ground, keep them covered, and get the building closed in.

Anything genuinely outdoors, or anywhere it will be repeatedly soaked, wants a different board. Marine plywood is the usual answer, and the range is on the marine plywood page.

Leaving it on show

The strand pattern has become a finish in its own right, and it turns up on shop fit outs, studio walls, workbenches and furniture. If that is the plan, buy a sheet or two more than you need and pick your faces, because the pattern varies and a run of panels looks better when they have been chosen rather than taken in order.

Seal it. Bare OSB marks easily and the resin can bleed through a thin coat. A clear water based lacquer or a hardwax oil both work, and both will darken the board a little, so try it on an offcut before you commit to a wall of it.

How much does OSB board cost?

OSB costs less than plywood for the same structural job, which is why it is the standard board for sheathing, floor decks and hoarding. The price of a sheet rises with thickness, so the fair way to compare is by the area you need to cover rather than by the board: an 18 mm floor deck costs more per square metre than 11 mm sheathing, but it is the thickness a floor needs. The calculator on each product page turns your floor, wall or roof area into a sheet count, and the current price for each thickness shows on its product page and in the cart.

OSB or plywood?

Choose OSB when the panel is structural and hidden and the budget matters: sheathing, decking, hoarding, formwork. It is consistent, it has no voids, and it costs less for the same structural job.

Choose plywood when you need a clean face for painting or veneering, when the edge will be seen, when the panel needs to hold a screw in that edge, or when it will get wet more than occasionally. For those jobs, see our plywood range, or softwood plywood for an everyday structural sheet with a cleaner face.

OSB board in Dublin and across Ireland

Both thicknesses of OSB board are stocked in our North Dublin warehouse for collection, and we deliver across Ireland. Work out the thickness from your drawing or from the job, sheathing and roofs in 11 mm and floors in 18 mm, then use the calculator on the product page for the number of sheets. If the board will be on show, add a sheet or two so you can pick your faces, and seal it with a clear lacquer or hardwax oil once it is fixed.

For MDF, plywood, cladding boards and every other panel we stock, see our full range of sheet materials.