Quick answer: Subfloor and loft boards are tongue-and-groove timber or chipboard panels designed to span between joists, creating a solid walkable surface. Standard UK sizes are 2400 x 600mm with thicknesses of 18mm or 22mm. Use 18mm for loft storage, 22mm for habitable rooms, and moisture-resistant (P5) chipboard for any room exposed to occasional damp.
We stock UK standard subfloor and loft boards from Bridgend with next-day delivery across South Wales and the South West. Tongue-and-groove chipboard, P5 moisture-resistant boards, and structural plywood subfloor — all sized for standard joist centres.
Subfloor and loft board sizes
| Board type |
Standard size |
Best for |
| 18mm T&G chipboard |
2400 x 600mm |
Loft storage, light-duty boarding |
| 22mm T&G chipboard (P5) |
2400 x 600mm |
Habitable rooms, kitchens, bathrooms |
| 18mm T&G plywood |
2440 x 1220mm |
Structural subfloors, premium installs |
| 22mm T&G plywood |
2440 x 1220mm |
Heavy load areas, commercial subfloor |
All boards have tongue-and-groove edges on all four sides for a tight, squeak-free finish. P5 grade chipboard is moisture-resistant and the standard for UK domestic flooring.
How to choose: 18mm vs 22mm subfloor
18mm boards suit joists at 400mm centres and are sufficient for loft storage, light-traffic areas, and bedrooms. They're lighter and easier to manhandle through a loft hatch.
22mm boards are required for joists at 600mm centres and any habitable space — kitchens, living rooms, bathrooms, hallways. The extra thickness eliminates flex and supports point loads from furniture.
Subfloor vs loft boards: what's the difference?
Subfloor boards are the structural deck that sits on top of joists in a finished room, before the final floor covering (carpet, vinyl, engineered wood). They need to be flat, stiff, and moisture-resistant.
Loft boards are the same product, sold in a smaller pack-size for boarding out an attic where the area is smaller and the boards need to fit through a loft hatch. The 2400 x 600mm format suits standard loft hatches.
Both use the same T&G chipboard or plywood — the difference is in pack size and intended use.
Fitting subfloor and loft boards
- Lay boards perpendicular to the joists, staggering joints so adjacent rows don't end on the same joist.
- Apply PVA wood glue to the tongue-and-groove edges before fitting.
- Screw down with 50mm wood screws at 200mm centres along every joist.
- Leave a 10mm expansion gap around the perimeter of the room.
- For loft installs, run boards across at least three joists per board for stability.
FAQ
What thickness loft boards do I need?
For loft storage at standard 400mm joist centres, 18mm T&G chipboard is sufficient. For joists at 600mm centres, use 22mm.
Are loft boards waterproof?
P5 grade chipboard is moisture-resistant — it tolerates occasional damp but is not waterproof. For wet areas, use marine-grade plywood.
Can I walk on 18mm loft boards?
Yes, on joists at 400mm centres. They're rated for residential loft access and storage. For frequent foot traffic or habitable rooms, step up to 22mm.
What's P5 chipboard?
P5 is the EN 312 grade for moisture-resistant load-bearing boards used in humid conditions. Standard for UK domestic flooring.
For full structural sheet materials, see our plywood, MDF and sheet materials range. For load-bearing joists below the boards, browse our C24 timber range.
