When building or replacing a ground-floor or upper-floor deck in a UK house, three wood-based panel products compete for the same role: moisture-resistant chipboard (grade P5), oriented strand board (OSB3), and structural plywood. All three are acceptable under Building Regulations and span between joists at standard centres, but they differ in stiffness, moisture tolerance, surface quality, and price. Getting the choice wrong costs money at fitting or causes problems later — a swollen chipboard floor is common in new-build defects claims, while over-specifying plywood everywhere adds unnecessary cost.
What Each Panel Is
P5 chipboard (also called flooring-grade chipboard or moisture-resistant chipboard) is made from wood chips and resin pressed into a homogeneous panel. Grade P5 is specified in BS EN 312 and indicates load-bearing use in humid conditions — the standard requires at least V313 bond classification (cyclic boiling test). It comes with tongue-and-groove (T&G) edges on all four sides in flooring grades, which controls differential deflection between adjacent panels and eliminates the need for noggings at all joints.
OSB3 (oriented strand board, grade 3) is made from large, flat wood strands laid in cross-oriented layers and bonded with phenolic or PMDI resin. Grade 3 per BS EN 300 denotes load-bearing in humid conditions. Unlike chipboard, the layered strand structure gives OSB directional stiffness — the face-grain direction (along the length of the board) has higher bending stiffness, so panels must be laid with their long axis perpendicular to the joists. OSB3 flooring panels are available with T&G edges.
Structural plywood is a cross-laminated veneer product graded to BS EN 636 or BS 5268-2. C24-face grades with WBP (weather and boil proof) glue lines — also sold as “exterior grade” — offer the highest stiffness, strength, and dimensional stability of the three. It can be used in any orientation and resists point loads better than chipboard or OSB of equivalent thickness.
Thickness and Joist Spacing
Panel thickness is selected to achieve adequate stiffness for the joist spacing, limiting deflection to span/360 (or 14 mm maximum) under a characteristic imposed load of 1.5 kN/m² (domestic) per BS EN 1995-1-1 (Eurocode 5).
| Panel type | 400 mm joist centres | 600 mm joist centres |
|---|---|---|
| P5 chipboard | 18 mm | 22 mm |
| OSB3 | 18 mm | 22 mm |
| Structural plywood (C24 WBP) | 15 mm | 18 mm |
Plywood’s higher stiffness allows a thinner panel at 600 mm centres, which partly offsets its higher material cost. For joist centres wider than 600 mm (non-standard or engineered joist systems), consult the panel manufacturer’s span tables.
Moisture Resistance in Practice
All three products claim “moisture resistance” for humid conditions, but behaviour under real construction moisture varies considerably.
P5 chipboard is the most sensitive. It will swell at edges if left exposed to rain or if a wet screed or adhesive is applied before the panel has fully dried following a damp pour. Edge-swelling of 3–5 mm is common on sites where chipboard has been laid before the roof is watertight. Once swollen, the edges do not fully recover and create a visible ridge through carpet or vinyl. If you are working on a site with an incomplete weathertight envelope, do not use P5 chipboard as the floor deck until the building is dried in.
OSB3 performs better under brief wetting. The strand orientation and resin system mean it swells less at edges and recovers more completely. However, prolonged saturation will still cause delamination and surface blistering, particularly at cut edges. Seal all cut edges with a compatible primer before laying.
Structural plywood is the most dimensionally stable under moisture cycles. WBP (phenolic) glue lines are unaffected by wetting and drying. Surface veneers may raise slightly after wetting, which can be sanded back. Plywood is the best choice for bathrooms, utility rooms, and any floor deck where the finished surface is wet-applied (tile adhesive, resin, etc.).
T&G vs Square Edge
Tongue-and-groove edges eliminate the need for solid nogging at every panel edge junction — a significant time saving. T&G panels are standard in flooring chipboard and are available in OSB3 and plywood flooring grades. The T&G joint also transfers point loads across the junction, reducing differential deflection under furniture legs. For all domestic floor decks, specify T&G on all four edges. Square-edge panels require a nogging or solid support at every edge, which adds material and labour cost.
Cost Per m² (2026, UK)
Prices below are for materials only, supply only (not installed), standard sheet sizes (2400 × 600 mm or 2400 × 1200 mm T&G).
| Panel type | Thickness | Approx £/m² (supply) |
|---|---|---|
| P5 chipboard T&G | 18 mm | £8–£12 |
| P5 chipboard T&G | 22 mm | £10–£14 |
| OSB3 T&G | 18 mm | £9–£14 |
| OSB3 T&G | 22 mm | £11–£16 |
| Structural plywood (WBP) | 18 mm | £14–£20 |
| Structural plywood (WBP) | 22 mm | £18–£26 |
Installation costs (fixing panels, adhesive on T&G joints, screwing at perimeter) typically add £6–£12/m² for labour in 2026.
Where Each Panel Is Best Used
| Situation | Recommended panel |
|---|---|
| Domestic first floor, building fully watertight | P5 chipboard 18–22 mm T&G |
| First floor, building not yet weathertight | OSB3 22 mm T&G |
| Bathroom or wet room substrate | Structural plywood 18 mm WBP |
| Heavy point loads (plant room, plant on wheels) | Structural plywood 22 mm WBP |
| Commercial / heavy domestic use | OSB3 or plywood, check span tables |
| Ground floor over void or insulation (floating) | P5 chipboard 22 mm or OSB3 22 mm |
Fixing and Adhesive
All three panels should be:
- Conditioned on site for 48 hours before laying (unstacked, in the room where they will be used)
- Laid with all joints staggered — no cross-joints (the pattern should look like brickwork)
- T&G joints glued with a waterproof PVA adhesive (e.g. per BS EN 204 D4 class)
- Fixed to joists with ring-shank nails (50 mm for 18 mm panel; 65 mm for 22 mm) or screws at 200 mm centres on the perimeter and 300 mm in the field
- Boards floating in the field — do not glue or fix the centre of large panels to prevent buckling with moisture movement
A 10–12 mm expansion gap must be left at all perimeter walls to allow seasonal movement.
[!warning] Adhesive compatibility Certain solvent-based tile adhesives and some polyurethane floor adhesives can attack OSB3 surface resin. Always check the adhesive manufacturer’s substrate compatibility list before laying ceramic or LVT directly onto OSB.
UK Standards Summary
- BS EN 312 — particleboard specifications, including P5 moisture-resistant load-bearing
- BS EN 300 — OSB — definitions, classification and specifications (OSB3 = load-bearing, humid)
- BS EN 636 — plywood specifications
- BS EN 1995-1-1 (Eurocode 5) — design of timber structures (panel spans, deflection limits)
- Building Regs Part A — structural stability (floor decks must meet span table requirements)
