Each issue, we take a look at a particular wood species or wood product. This time it’s the turn of OSB, as we look at the past, present and future of this environmentally friendly strand-based engineered board.
What exactly is OSB?
Oriented strand board is a composite engineered wood panel made of long strands (also called wafers or flakes) of wood, bonded together with synthetic resin adhesive. Its initial manifestation was known as waferboard or flakeboard and was credited to Californian Armin Elmendorf in 1963. After some years of evolution and refinement, the product came to be known as OSB. Since the mid-1970s it has expanded rapidly in Europe as an alternative for sheathing grades of plywood.
A strong and relatively cheap material, OSB is used mainly for structural purposes. But its characteristic aesthetic – a surface texture which shows the shape of its large and long wood strands – is being increasingly incorporated into architectural design.
One of OSB’s many benefits is that it can be made in the UK from small logs of fast-growing timber species, including routine forest thinnings that tend not to be used for any other wood products.
The timbers used in OSB vary depending on availability within economic range of the processing plant, but can include both softwood (spruce, pine) and hardwood (aspen).
How is OSB made?
Image © MEDITE SMARTPLY: The layered wood strands before being compressed into panels
The manufacturing process is very efficient, utilising virtually 100% of the wood with very little waste. Logs are debarked – with the bark going on to be used either for fuel or horticulture – then ‘stranded’ by rotating knives to create a ribbon of flakes, usually about 75mm wide. This breaks up to produce individual strands, approximately 75mm along the grain and 5-50mm across.
These strands are tumbled through a drier then mixed with resin and wax, before being layered on the bed. Often, fine strands are removed for reuse before drying and resin application, which reduces the amount of resin needed in comparison to other resin bonded panels.
A layer cake of timber strands
OSB is usually composed of three layers, with the strands oriented in alternate directions before forming the board. In the outer two layers, the strands follow the direction of the length of the plate; in the middle layer, they lie perpendicular. Image analysis techniques can be used to measure strand orientation on the production line, providing feedback to enhance the reliability and stability of engineered OSB.
After being compressed to the intended thickness and dried, the boards are cooled and cut to size, at which point they can receive modification treatments, surface coatings or tongue and groove cuts.
OSB’s properties
The cross orientation across layers, combined with the large size and length of the interwoven strands, creates an uninterrupted wood fibre that contains no knots, voids or points of weakness. This is what gives OSB its great strength and ability to carry loads.
Relative to their strength, OSB panels are light in weight and quite easy to handle and install. They can be cut by a hand or power saw, machined with normal woodworking machinery and fixed with nails, staples and screws.
The resin used to bind the flakes provides OSB with moisture resistance meaning it performs well in humid conditions.
OSB variants
Different sizes, thicknesses and grades of OSB are suitable for different levels of loading and environmental conditions. A variety of modified OSB products come with features such as enhanced moisture/vapour resistance, fire retardation or air tightness, making OSB increasingly suitable for building applications – and overcoming its limiting factors for external use.
Traditionally, resins containing formaldehyde were used to create OSB. However, there are now products containing no formaldehyde at all.
OSB in use
Image © David Jensen: The SHED Project by Studio Bark. Each self-build SHED takes only a day to construct
Because of its composition, OSB is primarily a construction panel. Its flat, even surface, strength and water resistance make it ideal for timber frame structures, and its common uses include flooring, roofing, wall sheathing and dry lining. However, its versatility lends itself to many alternative uses, including structural and interior applications.
MMC and low energy building
As panels can be easily produced to large scales, it is well suited to offsite and modular construction methods.
U-Build is an innovative modular building system designed by Studio Bark as a simple way to build durable, demountable structures. The SHED Project was an adaptation of this approach, using OSB to create environmentally friendly solutions for quality and affordable accommodation in vacant buildings.
Insulation filled modules were made from CNC cut sheets of MEDITE SMARTPLY OSB, which were then bolted together on a regular grid to create well-insulated and soundproof sheds. Used for both structure and finishing, the project used formaldehyde-free SMARTPLY OSB as an affordable, durable UK-made material, made from 100% sustainably sourced timber.
OSB is often used to create the external sandwich for high performance insulated SIP panels, as well as providing the web for James Jones JJI-joists, along with other brands too. The JJI-Joist combines softwood flanges with an OSB web, using the strengths of each where it is needed most to produce an innovative alternative to conventional construction timber with many advantages. Describing the application of OSB in their JJI-Joists, James Jones & Sons explain “as a complete manufactured product it allows us to offer an excellent strength to weight ratio, a composition that resists shrinking and warping, increases insulation capacity and has excellent sustainability credentials.”
Through modest yet highly effective applications such as these, OSB is now integral to many low energy and Passive House projects throughout the UK, helping to achieve low embodied carbon as well as high air tightness and thermal performance.
Image © James Jones & Sons: JJIJoists are the most widely specified engineered wood product in the UK
Read Designing Timber issue No 2 here