How to buy good quality plywood

We might all think we know what plywood is, but the facts may surprise you. Gavin Fidler clears up some common misunderstandings about what plywood actually is before outlining some tips for buying quality plywood that can be used safely structural applications.

What is plywood? 

The term ‘plywood’ is occasionally used as a ‘catch-all’ term for all wood-based panel products. However, just like other wood-based panel products such as particleboards, oriented strand board (OSB) and fibre building boards, plywood is a distinct product with specific features. 

Plywood consists of an assembly of layers (plies) glued together, with the direction of grain in adjacent layers usually at right angles - a technique referred to as cross-graining.

There are two types of plywood: 

• Core plywood (or plyboard), such as blockboard and laminboard, have a central core of wood strips or other materials, with two or more outer wood veneers. 
• Veneer plywood plies are all made of veneers orientated with their plane parallel to the surface of the panel. 

Find out more about plywood and other wood-based panel products in our Wood Information Sheet: WIS 2/3-23 Introduction to wood-based panel products.

What to look for when buying plywood 

At the design phase, specifiers can determine performance standards by limiting the acceptable levels of knots, core gaps or voids, splits or surface flaws in the material. Plywood performance also relies heavily on the quality of glue used and, as market forces drive prices downwards, manufacturers make cost savings by using cheaper glues. Purchasers should be aware that it is visually impossible to tell a good plywood glue bond from a bad one as glue dyes can be used to disguise a poor-quality glue.

How to specify plywood for quality 

When buying plywood, the details provided by a third-party certification stamp can be key to preventing the potential misunderstandings around quality that a simple visual inspection can raise. The stamp will include information on the grade or class of that product.

There are currently a number of European Standards and British Standards which deal with plywood grades

General European Standards for plywood:

• BS EN 635 which deals with the surface appearance.
• BS EN 636 which deals with the specifications.

British Standard for marine plywood

• BS 1088.

Most plywood used in construction work will be more reliant on glue bond quality than aesthetics. In the UK market, reference is frequently made to the class of the plywood, which relates primarily to the bond quality achieved rather than the durability of the timber veneers. 

What plywood to use and where 

• Class 1 plywoods are non-structural and will typically be used for interior furnishings only. 
• Class 2 plywood applications are typically where the boards will be protected from wetting but may be exposed to some degree of moisture during or following the construction phase. This can include floor decking, roof sarking and wall sheathing.
• Class 3 plywoods are able to withstand full exposure to external weather conditions, but may not be when in ground contact. These boards are useful for external cladding, site hoarding, signage and playground equipment.
• Class 2 and Class 3 structural grades are capable of carrying structural loads and will be marked with an ‘S’, eg BS EN 636-2S.
• Marine plywood contains Class 3 glue bonds but is also fabricated using naturally durable timber species. This means the boards can resist fungal decay, making them useful in environments where they are permanently exposed to wetting, including external environments that are in contact with the ground. Material sold as marine plywood without reference to BS 1088 may not be of the same quality.

How to I know my plywood is strong enough?

Design to Eurocode 5 requires characteristic design values for the product based on testing. For structural wood-based panels, these values are declared in the manufacturer’s Declaration of Performance (DoP) that is prepared before the product is placed on the market. There is an obligation for all parties in the supply chain to ensure this technical information is passed along the chain, encompassed within the UKCA/CE documentation. 

Where a specific strength property is required, purchasers should include this requirement within the purchase specification and check the UKCA/CE documentation and the associated DoP for the goods received at delivery to ensure that specification is met.

Is the plywood I bought and used going to fail on site? 

Where plywood fails in service, laboratory testing can be undertaken to confirm whether the plywood was of the correct grade and of an appropriate quality. At BM TRADA, we have seen many examples of the wrong type or quality of plywood being used on site and subsequently failing.

BS EN 314 (parts 1 and 2) set out the methodology for plywood glue bond testing using the ‘shear test method’, where plywood test specimens are cut, exposed to moisture via a pre-treatment process which varies according to glue bond class and then clamped in a testing machine where loading is applied in tension until failure. Once the specimen fails, both the failure load (in Newtons per square millimetre) and failure mode (wood fibre failure or glue failure) are calculated and rated against minimum performance requirements.

By retaining a small sample of any plywood you use in your construction, you will be able to have that product tested if the worst happens.

What to do if your plywood fails in service 

As part of a plywood manufacturer’s factory production control system, they should be undertaking glue bond testing on batches of plywood to monitor and confirm performance. This testing is normally carried out by the manufacturer in their own laboratory but may be audited as part of any certification they hold. 

Some UK plywood importers and distributors choose to undertake additional plywood testing, and TDUK requires its members to conduct third-party testing to verify the quality and performance of certain types of plywood that they place on the UK construction market.

The majority of plywood testing which BM TRADA undertakes relates to either failure of plywood in service or additional conformity testing for UK plywood importers.

Plywood testing

BM TRADA has a timber testing laboratory and is UKAS accredited for plywood testing. Whether you are a plywood importer or distributor looking for additional conformity testing or have a project where plywood has failed prematurely in service, we can offer comprehensive testing and, if necessary, expert reports to suit your needs.

Call our technical services helpline on 01494 569 601 or see more about our plywood testing services.