Plywood boards or “sheets” are made up of three or more thin layers of real wood that are glued together.

The layers are known as “plies”, hence the name “plywood”. Generally, the thicker the plywood, the more layers it has.
It is a versatile material with a huge range of applications, from wall and floor covering to moulds for concrete structures, designer furniture to packaging.
Plywood is considerably stronger than some other wood-based sheet products, such as medium density fibreboard (MDF).
Plywood’s strength is due to the way the direction of each layer’s grain is alternated in relation to adjacent layers.
The rotation of the direction of each layer’s grain – called cross-graining – is often 90 degrees (a right-angle). This means that every other layer has its grain oriented in the same direction, with a layer oriented at 90 degrees in between. However, rotation can be as little as 30 degrees. In some thicker plywood, seven layers may be arranged successively at 0, 30, 60, 90, 120, 150 and 180 degrees).

Rotation of the grain has a number of benefits. It:

  • Reduces the likelihood of splitting when sheets are nailed at the edges
  • Reduces expansion and shrinkage, giving better dimensional stability
  • Gives the plywood consistency of strength in all directions across the board

 

 

A brief history of plywood

Ancient Egypt

Wooden articles made in Ancient Egypt around 3500 BC are the earliest known instances of the use of plywood. They were made from sawn veneers glued together crosswise in a very similar way to today’s plywood.

China, England and France

Around 1,000 years ago, the Chinese shaved wood and glued it together for use in furniture.

 

The English and French made panels on the general principle of plywood in the 17th and 18th centuries.

From household to construction

Early examples of plywood, usually made from decorative hardwoods, were most commonly used in the manufacture of household items such as cabinets, chests, desk tops and doors.

 

Plywood made from softwood for use in construction work made its appearance in the 20th century.
 

 

What is it used for?

Huge range of applications

The range of uses for plywood, both indoors and outdoors, seems endless. In construction, it can be used in walls, floors, roofs and stairs; as shuttering (a kind of mould) for retaining concrete while it sets; and in temporary framework to give a shape for the laying of bricks or stone when making archways.

 

Furniture

Plywood is also still used extensively in furniture-making.

Packaging, model-making and art surfaces

Other uses include secure packaging, sports and playground equipment, and even the bodywork of some vehicles and light aircraft.

Thinner plywood is often used in model-making, and some artists paint on it after first coating it with gesso – a sealing compound that provides a slightly rough surface, which is excellent for retaining paint.

Designed for particular purposes

Different types of plywood are designed for particular purposes. For instance, high-strength plywood, made from mahogany and/or birch was used in the construction of some World War II aircraft, whereas marine plywood, manufactured from durable face and core veneers with few defects, performs better in humid and wet conditions.
 

 

Characteristics

Strength

Plywood is strong, generally quite resistant to impact damage, comparatively light and relatively easy to cut and “work” with tools.

It excels as a sheet material to form or cover large, flat-sided, sloping or level shapes like walls, floors, some types of roof and large containers.

Useful for intricate work

Some types of plywood are useful for more intricate work such as model-making, wooden jig-saw puzzles and small boxes.

Large panels quickly cover big areas

Because plywood is available in large panels, big areas can be covered with a minimum of edge-joining and the wide choice of thicknesses makes it suitable for a large range of applications from thick shelving to thin cladding.
 

 

How is plywood made?
Plywood manufacture usually requires logs – referred to as “peelers” – that are greater in diameter and and straighter than the average log from which wood is cut.

The bark is removed before the peeler is heated and soaked for anything between 12 and 40 hours prior to cutting.
It is then placed in a large peeling lathe and turned on its long axis…
… while a long blade peels a continuous sheet, or ply, from the log.
The long sheet is cut into initial lengths and widths and surfaces are scanned for any defects.
The plies are then pressed and bonded together with glue, and the resultant boards cut to their final dimensions.

The final operation is usually the sanding – smoothing – of the boards. Some boards have coatings applied (such as melamine or acrylic) and their edges sealed.
 

 

What are the different types of plywood?
There is a vast range of plywood types. Below are some of the main types available. Speak to your builders’ merchant or take a look online if you’re looking for something very specific to fulfil a particular need.

Softwood plywood

This is a very common type of plywood used generally for construction and industrial purposes.

Hardwood plywood

This type has greater strength and stiffness. Its resistance to damage and wear makes it suitable for heavy-duty applications, including floors and walls.

Tropical plywood

Made of tropical wood from Asia, Africa and South America, this type is superior to softwood plywood because of its extra strength and evenness of plies. It is the preferred choice of many in the construction industry. Some examples feature very attractive grain and colouring, making it suitable for inclusion in some types of furniture.

Aircraft plywood

Made from mahogany or birch, and often both, this high-strength plywood has its layers glued together with adhesive that has extra resistance to heat and humidity. It was used for some aircraft in World War II, and today is used in various applications that demand similar strength and resistance.

Decorative plywood

This plywood has an attractive hardwood outer ply for used in furniture making, wall panels and other “high-end” applications. Other types of decorative outer ply include formica and resin-impregnated paper.

Flexible plywood

Sometimes referred to as “hatter’s ply” because of its use in “stovepipe” hats in Victorian times, flexible plywood or “bendy ply” is used to make curved shapes.

Marine plywood

Marine plywood, as its names suggests, is the choice for boats and many applications where humid and wet conditions are encountered. It’s resistant to fungal attack and delamination – when the plies begin to separate usually due to the effects of dampness. The disadvantage is that it is far more expensive than many other plywoods.

Fire-retardant plywood

This is plywood that has been treated with chemicals to improve its resistance to fire.

Phenol-faced  plywood

A hot laminate is fused to the surface of this plywood. The surface can then be left smooth for formwork – for instance, a mould for concrete structures or a temporary structure to hold brick arches and other shapes in place until the mortar has set – or patterns can be pressed into it for non-slip or decorative applications.
 

 

What sizes are available?
Maximum and minimum sheet sizes often depend on the particular type of plywood, but the most common standard size is 4ft X 8ft (1220 X 2440mm). Bigger and smaller sheets are often available, usually varying by 1ft (300mm) increments.
Thicknesses for plywood range from 1/16 inch (1.4mm) to 1 inch (25mm), although thicker sheets are available for some specialist applications.
 

 

How is plywood graded?
Different types of plywood are graded in different ways, depending on the type of wood they are made from or their country of origin. The grading relates to the quality of the wood used and whether one or both outer plies, or faces, have very few or many defects, and if any of the defects have been repaired as part of the manufacturing process.
For instance, the grades for birch plywood are:

  • S Grade (highest) – only minor knots and characteristics
  • BB Grade (medium) – spliced-in oval patches replace any major knots and defects
  • WG Grade (lower) – open defects on smaller knots with some repaired larger knots
  • C Grade (lowest) – open defects allowable

There are also Brazilian, Chilean, Finnish, Russian, Swedish and several other grades. Check the grade before buying to ensure the plywood meets the requirement for the particular job.
 

 

What standards are there for plywood?
There are various standards for plywood used in a wide range of applications.

For instance, in the construction sector, the European standard for wood-based panels, EN 13986, requires that plywood used in the construction sector must comply with one of the three performance classes within EN 636, and suppliers must provide evidence to substantiate this.
The performance classes are based on the moisture resistance of plywood used in various parts of buildings such as roofs, partition walls, floors and timber frame external walls.
Some types combine excellent weather resistance and strength properties for outdoor use, complying to specific standards such as BS 1088 (plywood in marine use), while the structural standard code BS 5268-2:2002 applies to the strength of plywood used in construction work. It’s advisable to check that any plywood you buy meets the appropriate standard for the intended use.