What is an OSB board?
OSB (Oriented Strand Board) is a sheet material made from large wood strands that are pressed and bonded in cross-layered orientations. The strands are oriented: in the outer layers they run lengthwise, in the inner layer crosswise. This gives the board strength in all directions. OSB is recognisable by its coarse, visible strand pattern.
Types of OSB
| Type | Application | Moisture resistant |
|---|---|---|
| OSB/1 | Interior, decorative | No |
| OSB/2 | Structural, dry | No |
| OSB/3 | Structural, humid | Yes (limited) |
| OSB/4 | Heavy structural, humid | Yes |
OSB/3 is the standard for construction projects — suitable for structural use in a humid environment (but not for permanent outdoor use).
Where is OSB used?
- Timber frame construction — As wind bracing (stiffening panel) on the studs
- Roof decking — As a substrate for roofing material
- Floor panel — As subfloor on joists
- Formwork — Temporary use for concrete pours
- Wall and ceiling cladding — Industrial/rugged appearance
OSB in timber frame construction
In a timber frame wall, the OSB board sits on the outside of the studs:
- Studs and rails (the frame)
- OSB board on the outside — provides wall rigidity (wind bracing)
- Breathable membrane over the OSB
- Ventilation cavity
- External cladding (weatherboard)
On the inside:
- Insulation between the studs
- Vapour barrier
- Plasterboard or cladding
Dimensions
| Sheet size | Thicknesses (mm) |
|---|---|
| 122 x 244 cm | 9, 12, 15, 18, 22 mm |
| 125 x 250 cm | Same |
Common thicknesses:
- 9 mm — Wind bracing, light applications
- 12 mm — Standard structural
- 18 mm — Floor, roof decking, heavier structural
OSB vs. plywood
| OSB | Plywood | |
|---|---|---|
| Composition | Pressed strands | Cross-layered veneer |
| Appearance | Coarse, industrial | Smooth, more refined |
| Strength | Good | Better |
| Price | Cheaper | More expensive |
| Visible use | Only for industrial look | Yes |
| Moisture resistant | OSB/3: limited | WBP grade: good |
Related terms
- Timber frame construction
- Plywood
- Breathable membrane
- Battening