Last edited 18 Jan 2021

Oriented strand board


Oriented strand board (OSB), also known as flakeboard or sterling board, is a type of engineered sheet timber product that is typically used as sheathing in wall panels, flooring and roof decking. OSB has similar properties to plywood, another engineered sheet timber product, and is suitable for load-bearing applications. It is less commonly used for exterior applications.

It was first developed in America in the early-1960s, and is made of layers of compressed timber strands bonded with adhesive. The individual strands – or flakes – are usually about 2.5 x 15 cm, and lie unevenly across each other, resulting in OSB’s rough and variegated surface.

OSB is typically manufactured to a ratio of 95% timber strands to 5% wax and synthetic resin adhesives (typically, moisture-resistant binders such as phenol formaldehyde (PF), urea formaldehyde (UF), isocyanate (PMDI) or a combination).

Timber strands are sifted and lain into a mat located on a forming line before being coated with the resin. The required thickness of the panel determines the number of layers that are placed. A thermal press is used to compress the mat which bonds the strands by heat activation and curing of the resin. The mat is then cut into individual sizes as required.

Impermeability to water can be achieved through the use of additional membranes.

OSB can be more cost-effective than plywood. In terms of strength, unlike natural timber there is not a continuous grain, however, it does have an axis – found with the alignment of the surface strands – along which its strength is greatest.

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