What is P5 flooring chipboard and where does it go?

Almost every suspended floor in a house built in the last few decades is P5 chipboard, and it is a good product used correctly and a disaster used carelessly. The grade system is worth ten minutes because the difference between P2 and P5 looks like nothing in the rack and is everything in a floor.

Step by step

1. Learn what the P-grades cover.P1 and P2 are general-purpose interior boards for dry conditions — P2 is the furniture and shelving grade. P3 is non-load-bearing for humid conditions. P4 is load-bearing for dry conditions. P5 is load-bearing for humid conditions. P6 and P7 are heavy duty versions of the last two.
2. Use P5 for a domestic floor deck.It is load bearing and moisture resistant, and it is what a floor deck specification calls for. It normally comes as 2400 by 600mm boards, 18 or 22mm thick, tongued and grooved on all four edges, and usually dyed green.
3. Match the thickness to the joist spacing.Thickness is a structural decision tied to the joist centres and the load. 18mm is common at 400mm centres and 22mm at 600mm, but take the actual figure from the manufacturer's table for the board you are buying rather than from a rule of thumb.
4. Glue every tongue and groove joint.The joints are glued with a PVA — usually a D3 or D4 — as the boards are laid. This is not optional: an unglued T&G joint in a chipboard floor is the classic source of creaking, and the glue is what makes the deck act as one sheet.
5. Lay the long edges across the joists and stagger the ends.Boards run with their length across the joists so each board is supported by several, and end joints are staggered so no joint line runs continuously. Every end joint must land on a joist or on a noggin.
6. Leave an expansion gap at the perimeter.A gap round the edge of the room, normally around 10 to 15mm, which the skirting then covers. A deck laid tight to the walls has nowhere to go when it takes up moisture, and it lifts in the middle.
7. Keep it dry during construction and after.It is moisture resistant, not waterproof. It tolerates a wet construction period; it does not tolerate a leaking washing machine. Swollen chipboard flooring does not recover and the affected boards have to be cut out and replaced.
8. Do not tile straight onto it.Chipboard flooring moves and flexes more than tiles will tolerate, and the surface is not a good key. Tiling on a chipboard floor needs an overlay board or a decoupling system — there is a dedicated guide on tiling over a timber floor.

Tips

• Some P5 boards have a factory-applied moisture-resistant coating on the face as well as a moisture-resistant resin. That coating is a construction-period protection and it wears off; it is not a permanent waterproofing.
• If you are lifting boards to run cables or pipes, cut over a joist and screw a batten alongside to support the cut edge — chipboard has no strength at an unsupported cut edge at all.
• Screw rather than nail. Chipboard flooring screws with a coarse thread hold far better and can be undone, which matters the first time you need to get under the floor.

Common mistakes

• Using P2 furniture chipboard as a floor — It is neither load bearing nor moisture resistant. It deflects between joists, breaks up at the edges and swells at the first sign of damp. It is not a cost saving, it is a floor that fails.
• Not gluing the tongue and groove joints — The joints work against each other under foot traffic and the floor creaks. Retrofitting glue into a laid floor is essentially impossible, so it has to be done as it goes down.
• Laying it tight to the walls — The deck expands slightly with humidity and, with nowhere to go, it buckles upwards along a joint. The perimeter gap is what prevents it and the skirting hides it.
• Letting a leak sit on it — Swelling is permanent and it destroys the tongue and groove as well as the surface. A slow leak under a bath or behind a washing machine can write off several boards before it is noticed.

Questions people ask