There are a lot of choices out there when it comes to timber sheet material. When you walk into the builder’s merchant are you confident you are picking the correct one for the job? Dismissing the warped sheets of shuttering plywood, for the nice fair-faced ply and what’s OSB?

Sheet material on shelves

What’s the difference between OSB and ply?

OSB (Oriented strand board) tends to be cheaper, it is formed from small bits of wood compressed into layers and bonded with resin, with no knots or voids. On the other hand, Plywood is made by laminating wood veneer together in different directions. Both have different specifications available for various applications. The small strands of wood used in OSB mean that smaller trees can be used and therefore more sustainable than larger trees required to make ply, especially the hardwood variety.

How do you choose the correct board?

First of all make sure your timber is marked FCS or PEFC, indicating it has come from a legal and sustainable source. The larger builders merchants can provide full procurement history.

Then choosing the correct board comes down to where the board will be used: internally or exposed to the elements and will it’s final form be structural? When you know this key information you can use the following information to purchase the correct sheet.

The glues are toxic – Formaldehyde Emission

Pure formaldehyde is a colourless, flammable gas with a strong pungent odour. It is extremely irritating to the mucous membranes and is associated with certain types of cancer in humans and other animals. Formaldehyde is classified as a human carcinogen (cancer-causing substance).

Encyclopedia Britannica

Formaldehyde is used to make the resin in sheet material. Materials containing Formaldehyde will release formaldehyde gas into the air.

Formaldehyde in sheet material is declared as either E1 (less than 0.1ppm), E2 (greater than 0.1ppm, less than 9.3ppm) or not to be used internally.


Used for generations, plywood is the sheet material of choice, used for making furniture, floors, walls, roofs, temporary shuttering, hoarding and even as the finish on walls.

The board should be marked by either a sticker or printed with BS EN 13986 to show the board is suitable for use in construction, and then look for BS EN 636, it specifies the requirements for ply and references other standards. It is based on the bending properties of the sheet. There are three classifications:

Plywood for use in dry conditions (EN 636-1)
Bonding quality has to meet class 1 of EN 314
Use class 1 of EN 335-3: Inside a construction and not exposed to weather or wetting.

Plywood for use in humid conditions (EN 636-2)
The bonding quality has to meet class 2 of EN 314 and
Use class 2 of EN 335-3: Undercover and not exposed to weather, wetting can occur.

Plywood for use in exterior conditions (EN 636-3)
The bonding quality has to meet class 3 of EN 314
Use class 3 of EN 335-3: Above ground and exposed to the weather.

It’s worth noting the durability will be affected if the panel is cut and will need treatment to the cut edges.

You may of at one time asked for WBP, because you want a ply for exterior use to form a roof. If you ask for WBP now, the retailer should be offering you at least a sheet marked EN636 – 2, which is fine if you’re planning on covering it over with a waterproofing product.

Structural Uses

If you intend to use plywood for a roof, floor or a wall you will need a structural grade material. One of EN 636 supplementary properties will be shown on the label: “S” for structural application and “NS” for non-structural/general application. ie. EN 636 1 NS: Class I – Non structural

Surface Grade

The quality of the front and back sides of the ply is available too but varies from different sources, the grades I found were as below but it wouldn’t hurt to check with your stockist if you want high-grade plywood to work with.

AB Grade – The best, small filled knots and nearly same colour grain
B Grade – Smooth knots and grain
BB Grade – Bigger knots and grain
C Grade – Knots and splits
CC Grade – Know and splits used on the reverse side of a board with a higher quality


Image 1 (below) is a ply sheet from B&Q. The clear information is its size, it is made of a hardwood, from a well-managed forest. Not until you look at the smaller print within the CE Declaration of Performance that you will find EN 13986, EN 636-2 S and E1, to specify the plywood is suitable for structural purposes but not to be used externally unless covered, and a low Formaldehyde content so is safe to use indoors.

B&Q label on ply wood
Image 1

Image 2 (below) is plywood from Jewson. You can just about make out the FSC accreditation, EN 13986, bond class 3, E1 and EN 636-2 NS. Bond class 3 or EN 314 class 3, means that the glue used is capable of being exposed to elements, but the overall sheet is only graded to EN 636-2, and therefore should not be left exposed to the elements.

Image 2

Marine Ply

Then there’s Marine Ply, with increased resistance to the elements and comes with its own standard BS 1088, which I’m not familiar with (maybe a future article).


OSB boards are not as difficult to understand. You still have to pick the correct use class and watch the Formaldehyde content. However, most builders merchants tend to only stock OSB/3, which is good for use class 2.

Use class 1 – Inside a construction and not exposed to weather or wetting.
Non structural

Use class I – Inside a construction and not exposed to weather or wetting.

Use class II – Undercover and not exposed to weather, wetting can occur.

Use class II – Undercover and not exposed to weather, wetting can occur.
Heavy-duty structural

OSB sheets can also be purchased with zero added Formaldehyde and flame retardant added at the manufacturing stage, to improve performance under fire conditions. These sheets will come at a premium price.

More Infomation

If you want to know more about sheet materials, the British Standards documents are available for free at libraries around the country. If you found this article useful, please share it.

And check out the Concrete Guide.