HOW TO CHOOSE THE CORRECT PLYWOOD

Plywood is a sheet material manufactured by gluing together several thin layers of softwood or hardwood veneers. Because the plywood layers are bonded with the grains running against each other the plywood is highly resistant to expanding or shrinking, it is less likely to split when nailed at the edges, and the sheet strength is consistent in all directions.

Compared to other materials plywood’s many benefits include price, lightness, and strength. Thinner sheets can also be bent into curved components.

Choosing the correct plywood can be a confusing job so we have prepared this guide to explain the different types available and their relevant standards.

Plywood Standards

Plywood consists of only two materials, wood and glue. The interaction between the two determines the performance of the plywood. The following two standards classify this performance:

• EN 314-2: 1993 – Plywood - Bonding Quality, Requirements

This defines 3 classes of bond dependent on the intended end use of the plywood. Bonding quality is determined by the adhesive type and core veneer quality (physical defects such as knot holes and splits):

• (Bond) Class I: suitable for dry interior use only
• (Bond) Class II: suitable for use in humid areas or exposure to occasional wetting
• (Bond) Class III: suitable for unprotected exterior use or exposure to frequent wetting

Plywood is tested to destruction in a laboratory by exposing it to simulated hostile weather. An assessment is then made of how well the bond has survived the weathering process. Having established to EN314 the bonding quality, EN636 can then be assessed.

• EN 636: 2003 – Plywood - Specifications

This classifies plywood by considering both the bond quality and the biological durability of the species of wood used in the plywood. Where biological durability means the natural capacity of the wood to resist fungal decay and detrimental effects due to beetles.

• (Specification) Class I: suitable for dry interior use only
• (Specification) Class II: suitable for use in humid areas or exposure to occasional wetting
• (Specification) Class III: suitable for unprotected exterior use or exposure to frequent wetting

As EN314 and EN636 are harmonised, this means that it is not possible, to achieve an EN636 Class higher than the assessed EN314 class. For example, to achieve EN636 Class II (frequently labelled EN636-2) the minimum bonding quality must be EN314 Class II. Some of the plywood sold in the UK will achieve EN314 Bond Class III, however when assessed to EN636, it will only achieve EN636-2, because of the limited biological durability of the wood. Exceptions to this might include tropical hardwood throughout plywood and marine grade plywood.

It is worth bearing in mind that, provided the EN314 bonding is Class III to start with, an otherwise EN636-2 plywood can be upgraded to EN636-3 by preservative treatment, to treatment class T3 (DD CEN/TS 1099:2007).

There are two types of Constructional Plywood:

Structural Plywood: In addition to other product information Structural Plywood has proof of strength performance provided by the Supplier. Such plywood should confirm to BS5268 (and may also be referred to as CE2+) indicating that the panel satisfies Building Regulation 7 for structural use in buildings.

General Plywood: this does not have strength performance data provided by the supplier and so should not be used in a structural application.

Both types of constructional plywood can be used in the three end use classes for plywood defined in the European Standard EN 636, as below:

Exterior Use - Permanently outside exposed to the weather:

• Structural EN 636-3 S
• General EN 636-3 G

Humid Use - Most construction uses fall into this category as they are covered or rarely exposed to weather, but still at risk of wetting:

• Structural EN 636-2 S
• General EN 636-2 G

Dry Use - Interior with no risk of wetting:

• Structural EN 636-1 S
• General EN 636-1 G

The most common types of plywood available for use in UK construction are structural plywood for humid uses (EN 636-2S) and general plywood for humid uses (EN 636-2G). These types of plywood are made from durable resins which are suitable for most construction uses. Plywood for Exterior use is also available but may be less cost effective as both the resins and the wood veneers within the product must be durable in the long term. There is also a special type of exterior Plywood called Marine Plywood which has its own British standard BS 1088 2003, and as the name suggests must be durable enough to endure regular exposure to extreme weather conditions. However, Marine Plywood rarely has proof of strength performance provided by the supplier and so should not be used load bearing construction uses, unless this strength data is provided.

Types of Plywood

Plywood has many different variations, the most basic types are:

• Softwood plywood this is usually made either of cedar, Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF) or redwood and is typically used for general building, roofing or concrete formwork where its mechanical function is more important than its appearance.
• Hardwood plywood: this is made of wood from angiosperm trees and used for demanding end uses. Hardwood plywood is characterized by its excellent strength, stiffness and resistance to creep. It has a high planar shear strength and impact resistance, which make it especially suitable for heavy-duty floor and wall structures. Hardwood plywood has excellent surface hardness, and damage- and wear-resistance.
• Marine plywood: this is manufactured from high quality face and core veneers, in biologically durable hardwoods, incorporating advanced resin technology to give long life in both humid and wet conditions. It is intended for use in most applications where a good resistance to wood-destroying organisms is of the highest importance.

Other commonly used terms when describing plywood include:

• Shuttering plywood: Normally the cheapest type of plywood this is used for boarding up or making formwork for poured concrete. It will have poor quality face veneers with knots and defects and may have some voids within the core.
• WBP plywood: much of the plywood sold in builders' merchant is made to an exterior grade referred to as ‘WBP’ (weather and boil-proof). This means that the glue line will not break down when subjected to adverse weather conditions and can also withstand immersion in boiling water.

Using Plywood

• Plywood should be cut using powered saws to avoid splintering. Before making the cut score a line with a knife to sever the surface fibres.
• Because of the alternating grains plywood is can be joined with screws even close to its edge.
• Better quality plywood boards may be painted or treated with a preservative.