OSB/3 Boards

What the 3 in OSB/3 means

EN 300 sorts oriented strand board into four grades, and the number tells you two separate things: whether the board can carry load, and whether it can cope with damp. OSB/1 does neither and is for interior fitments. OSB/2 carries load but only in dry conditions. OSB/3 carries load and is rated for humid conditions. OSB/4 does the same as OSB/3 with a higher load rating, for jobs that specifically call for it.

OSB/3 is the one that almost every drawing means when it just says OSB. It is what a timber frame gets sheathed in, what a domestic floor deck is laid in, and what goes over rafters before the felt. If you are working from a specification and it names a grade, match it. If it does not name one, OSB/3 is the safe assumption and the one to price.

What “humid conditions” covers, and what it does not

This is the part worth being precise about, because the phrase sounds more generous than it is.

Humid conditions in EN 300 means the board keeps its structural properties through the damp a building sees during construction and in normal service. A wall sheathed in October and not roofed until December is exactly the case the grade exists for. So is a floor deck that gets rained on through open joists, or a board fixed in an unheated garage or a utility room where the air is damp half the year.

What it does not mean is waterproof. OSB/3 left standing in water will take it up, and the strands at a cut edge will swell and stay swollen. It is not an exterior board, it is not a shower substrate, and it is not something to leave out over a wet winter under a tarpaulin that fills with water. If the panel is going to be soaked repeatedly or permanently, you want marine plywood instead and the extra cost is the point of it.

Where it goes

Sheathing is the most common use. The board is nailed to the outside of a timber frame and it does two jobs at once: it stops the frame twisting out of square, and it gives the breather membrane and the outer finish something to fix to. The bracing job is the reason the grade matters, because a board that has softened in the rain is not bracing anything.

Flooring is the other big one. An 18 mm deck over joists takes the traffic for the whole build and usually stays as the subfloor afterwards. Glue the joists as well as nailing and you get a floor that does not creak in a year. Leave a small expansion gap at the walls, because the board does move a little with humidity and a deck fitted hard against the blockwork will lift at a joint.

Roof decking is where the 11 mm board earns its place. One person can carry a sheet up a ladder and hold it while they fix it, which is worth more on a roof than any other property of the board.

Beyond the building itself it is the default for site hoarding, temporary huts, garage shelving and workbench tops, for the plain reason that it is strong, consistent and cheap enough to cut up without thinking about it.

The two thicknesses we carry

11 mm for sheathing and roof decking, 18 mm for flooring and anything that takes a load or a knock. Both in the standard 2440×1220 mm sheet.

Thickness on a structural job is set by the spacing of whatever is underneath, not by preference. Standard domestic joist centres suit 18 mm. Wider centres need more board, and that is worth settling before the delivery rather than after the joists are up.

Fixing it

Ring shank nails or screws both hold well in the face. Keep fixings back from the edge, because one driven too close to a corner pushes the strands apart and lifts the surface around the head. Pre-drilling is not usually necessary in the face, but it helps near an end.

The board does not hold a screw in its edge the way a solid timber or a plywood edge does, so do not design a joint that relies on it. Where an edge fixing is unavoidable, add a batten behind and fix through the face into that.

Cutting and dust

The resin blunts blades quickly, so use plenty of teeth and expect to change them more often than you would cutting softwood. The cut edge tears rather than chips and leaves a furry line, which is fine where it is hidden and worth a light sand where it is not.

The dust is coarse and there is a lot of it. Cut outside where you can, run extraction where you cannot, and wear a mask either way.

Leaving it visible

The strand pattern is a finish in its own right now, and OSB/3 turns up on studio walls, shop fit outs and furniture as often as it does behind plasterboard. If that is the plan, order a couple of sheets more than the job needs and choose your faces, because the pattern varies from board to board.

Seal it before it gets handled. 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 darken the board a shade, so try it on an offcut first.

Ordering

Both thicknesses are held in North Dublin for collection or delivery, and we deliver nationwide. If you are working to a drawing that calls out a grade, match it, and check the joist centres on the drawing to confirm the thickness before you order. Enter your area in the calculator on the product page and it works out how many sheets you need. The wider range is on the OSB page and on sheet materials.