Guide
Plywood or OSB?
OSB3 is usually the cheaper choice for structural decking and sheathing in humid conditions, while structural plywood is stiffer, holds fixings near its edges better and copes better if it gets wet.
Written by the SK Trade team. Last reviewed 1 November 2026.
Where each one fits
| Use | OSB3 | Structural plywood |
|---|---|---|
| Floor decking | Good (T&G grade) | Good |
| Flat roof deck | Used, check system | Often preferred |
| Wall sheathing | Common | Common |
| Exposure to wet | Short-term only | Better tolerance |
Thickness
18 mm is common for floors and roof decks on joists at 400 mm centres. Wider spacing needs thicker boards; follow the board maker's span tables and your design.
Fixing
Fix boards with the specified screws or ring-shank nails at the spacings the maker gives, and leave expansion gaps at edges.
Related products
Related questions
What plywood should I use outside?
Use plywood with an exterior bond, class 3 under BS EN 636, or marine ply for very wet conditions.
What is OSB3?
OSB3 is oriented strand board graded for load-bearing use in humid conditions under BS EN 300. It is the common grade for floors, roofs and sheathing.
Sources
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