What is OSB?
The Wood Guide · The wood register
Engineered woodWhat is OSB?
Strong, light and cost-effective. Boards made of oriented wood strands that are increasingly replacing traditional plywood — and for good reasons.
- Oriented Strand Board
- Four classes under NEN-EN 300
- 80–90% yield from the log
What is OSB?
Strong, light and cost-effective. Boards made of oriented wood strands that are increasingly replacing traditional plywood — and for good reasons.
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Oriented strands, and that word matters
OSB stands for Oriented Strand Board: a board made up of layers of wood strands of a predetermined shape and thickness, bonded together with a binder.
What gives OSB its characteristic structure is the orientation of those strands. In the outer layers they run parallel to the length of the board; in the core layer they lie crosswise or are randomly scattered. It is precisely this oriented build-up that sets OSB apart from its predecessor, waferboard, in which the flakes were simply jumbled together.
The result is a board that is strong in the direction you load it — and that you recognise straight away by its strand pattern.
OSB belongs to the category of engineered wood, along with plywood, chipboard and MDF. More background in processed wood.
The four quality classes
Under the NEN-EN 300 standard there are four classes. The class tells you straight away whether a board is intended for dry or humid conditions, and whether it may be used for load-bearing work.
| Class | Suitable for |
|---|---|
| OSB/1 | General use in dry conditions, mainly decorative |
| OSB/2 | Load-bearing structures in dry conditions |
| OSB/3 | Load-bearing structures in humid conditions |
| OSB/4 | Heavy-duty load-bearing structures, dry or humid |
On the Belgian market, mainly OSB/2 and OSB/3 are sold. OSB/3 is more resistant to moisture and therefore suitable for moderately humid conditions and sheltered outdoor use. No OSB board belongs fully exposed outdoors.
Why OSB is gaining ground on plywood
There are concrete reasons why OSB is increasingly replacing traditional plywood.
Raw material
Thick logs suitable for peeling — which plywood needs — are becoming scarcer and more expensive. OSB can make do with thinnings.
Energy
Manufacturing plywood takes roughly twice as much energy as manufacturing OSB.
Yield
From a given quantity of wood you get 80 to 90% OSB, compared with 30 to 35% for plywood. More board from the same tree is also better for the environment.
Use
OSB is increasingly used as a subfloor for engineered wood flooring, where plywood used to be the standard.
OSB versus underlayment
Two structural boards with a different purpose: OSB is structural, underlayment is often used for visible work.
| Characteristic | OSB | Underlayment |
|---|---|---|
| Composition | Layered, oriented wood strands | Various types, often more attractive |
| Use | Structural: roof, floor and wall | Visible work and various constructions |
| Moisture resistance | OSB/3 suitable for moderately humid conditions | Depends on the type, usually lower |
| Strength | Very strong, suitable for construction | Comparable to OSB |
Where OSB is used
Roof structures
Strong and stable, ideal as roof sheathing under the covering.
Floor structures
Widely used as a subfloor under laminate or parquet.
Wall structures
Gives structural rigidity to interior and exterior walls in timber-frame construction.
Furniture making
Less common, but usable for robust furniture where the strand pattern is precisely what defines the look.
How OSB performs
OSB scores well on several points. The boards are made from thinnings — wood released when forests are thinned out — and production generates hardly any waste. What waste there is goes to fuel or horticulture.
Whereas chipboard contains 7 to 9% glue, OSB uses less binder thanks to its larger strands. Most OSB boards meet the E1 standard for formaldehyde emissions, with values between 6.5 and 8 mg per 100 grams according to EN 120.
As with other sheet materials, sawing releases fine dust that contains resins. Ventilation, dust extraction and a dust mask are no unnecessary luxury.
Frequently asked questions about OSB
What does OSB stand for?
Oriented Strand Board — a board made of oriented wood strands. The strands in the outer layers run parallel to the length of the board, those in the core crosswise or at random.
Which OSB class do I need?
OSB/2 for load-bearing work in dry conditions, OSB/3 for load-bearing work in humid conditions. On the Belgian market, those are the two you will come across most.
Can OSB be used outdoors?
OSB/3 can withstand moderately humid conditions and sheltered outdoor use. No OSB board belongs fully exposed outdoors — there you choose solid wood or a material made for the purpose.
Why is OSB increasingly replacing plywood?
Two reasons: thick logs suitable for peeling are becoming scarcer and more expensive, and plywood takes roughly twice as much energy to make. On top of that, you get much more OSB from the same quantity of wood.
Can I paint OSB?
Yes, but the rough strand surface soaks up a lot of paint. Use a primer and expect to apply several coats. If you want a smooth surface, you are better off choosing MDF.
The whole board family
OSB, plywood, chipboard, MDF and HDF each do something different. The register also includes solid wood.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: OSB up close: large, flat wood strands pressed in layers — Lionel Allorge, CC BY-SA 3.0 · OSB production: a mat of wood strands heading for the press — Matthias Kabel, CC BY-SA 3.0 · Walls sheathed with OSB, ready to be insulated — Okram, CC BY-SA 3.0