Oriented strand board (OSB)
OSB: classes OSB/1–OSB/4 to EN 300, strength, common thicknesses and formats, and what to watch out for as bracing sheathing.
German: OSB-Platte
A board pressed from thin, elongated strands, with the direction of the strands changing from layer to layer. Like plywood, its strength therefore depends on direction, and it is two to three times stronger than conventional particleboard.
How is it made?
The elongated strands are laid in three layers with alternating directions. They are pressed into a board with PF, MUF or PMDI adhesive. Because of its high adhesive content, OSB is relatively vapour-tight – with carefully sealed joints, it can also act as an airtight, vapour-retarding layer.
EN 300 distinguishes four classes: OSB/1 is non-load-bearing, for dry interior use; OSB/2 is load-bearing in dry conditions; OSB/3 is load-bearing in humid conditions too; and OSB/4 is made for heavy-duty use.
Tongue-and-groove OSB and moisture control
For floors and floor and roof sheathing, there are also tongue-and-groove boards, usually 2500 × 675 or 6250 × 675 mm.
In terms of vapour, OSB is a vapour retarder, but not a vapour barrier. According to the standard design value (EN ISO 10456), its water vapour resistance factor is μ = 50 (dry) / 30 (wet), so the equivalent air layer thickness (sd) of an 18 mm board is only about 0.5–0.9 m. Manufacturers’ values may differ – the exact value is given in the product data sheet, and the moisture performance of the build-up must be verified by calculation.
The board can also be used on the inside as an airtight layer if every joint and connection is carefully sealed (for example with adhesive tape). A tongue-and-groove joint on its own is not airtight, and not every OSB product is suitable as an airtight surface – this must be confirmed by the manufacturer.
Technical data
- Build-up
- thin, elongated strands in three layers with alternating directions
- Raw material
- pine, Douglas fir, poplar and other species
- Strength
- bending strength 14–30 N/mm² in the main direction, 7–16 N/mm² in the cross direction
- Density
- 550–650 kg/m³
- Adhesive
- PF, MUF or PMDI
- Common sizes
- thickness 12, 15, 18, 22, 25 mm; board 2500 × 1250 or 2800 × 1250 mm, tongue and groove 2500 × 675 mm
- Standard
- EN 300 (OSB/1–OSB/4), EN 13986
- What is it for?
- bracing sheathing, load-bearing walls, the webs of I-joists
| Type | Format (mm) | Thickness (mm) |
|---|---|---|
| square edge | 2500 × 1250, 2650 × 1250, 2800 × 1250, 3000 × 1250 | 12, 15, 18, 22, 25, 30 |
| tongue and groove | 2500 × 675, 6250 × 675 | according to the manufacturer’s range |
Based on retailer ranges; exact sizes are given by the manufacturer.
Applications
- load-bearing and bracing sheathing on walls, floors, roofs and in box elements
- the webs of I-joists
- subfloor and flooring boards, formwork, interior design
What to watch out for
- The class must be chosen for the location: humid conditions require at least OSB/3.
- Outdoors it can only be used with weather protection.
- Strength depends on direction: in the main direction it is twice as high as across – the board must be laid with its main direction perpendicular to the supports.