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Plywood is an engineered panel, typically composed of an odd number of thin layers of wood veneers, called plies, bonded together with a rigid adhesive and with the grain direction of adjacent layers perpendicular to each other.

History

Plywood has it origins in laminating veneers around 3,500 years ago in Egypt during the days of the Pharoahs. The early Greeks and Romans also used veneers and plywood mainly for furniture. From the mid 1800's 'modern' plywoods were utilised in pianos, furniture and tea chests. Plywood came of age as a versatile construction material in the 1930's when water resistant resins were used as glues giving plywood longevity and integrity.

Why We Make Plywood..

Timber is around thirty times weaker across the grain than along its grain direction. This is obvious when we split wood with an axe. Thus by bonding adjacent timber veneers at right angles using a rigid adhesive as happens in the cross laminated construction of plywood, we utilise this superior strength and stiffness along the grain to overcome this weakness. Put another way, plywood has no natural line of cleavage. Plywood maintains the integrity of the timber and finds a huge range of applications which utilise the enhanced properties mentioned below.

Dimensional Stability..

Timber expands and contracts across its grain as it takes in and loses moisture to the atmosphere. The change in length along the grain due to changes in moisture is 1/100 of that across the grain. This factor, combined with timber's much greater strength along the grain, in the cross laminated structure of plywood locks up this movement. This gives plywood excellent two way dimensional stability which is advantageous in applications such as large areas of flooring or concrete formwork.

Split Resistance..

The cross lamination controls any tendency for splitting along the grain and thus enables nailing or screwing very close to all panel edges. Additionally, plywood is highly resistant to edge damage when compared to other panels. These two properties combine to make plywood a rugged building panel.

Panel Shear or Shear Through Thickness Capacity..

Panel shear capacity is the ability to resist loads in the plane of the panel. As plywood's cross laminated structure prevents the tendency to split along the grain it has double the panel shear capacity of timber. This makes plywood an excellent material for bracing for residential wall framing, floor, wall and roof diaphragms, the webs in box, C or I-beams, and for gusset plates in timber portal frames.

Resistance To Concentrated Loads

Plywood's cross lamination spreads loads sideways and gives plywood its excellent ability to carry high concentrated and impact loads that would break or shatter many other materials.

Resilience, Impact And Fatigue Resistance

As the structure of the timber is maintained and the fibres not smashed during plywood manufacture these closely related properties are all derived from the parent wood. Timber has high short term load capacity, and is able to elastically spring back or recover its original shape after shock or impact loads. Plywood's structure further enhances this high resilience and impact resistance.

Plywood, being an organic cellulose material is not subject to the fatigue failure of crystalline materials, e.g.metals and plastics. Plywood thus can endure cyclic stresses much longer than these crystalline materials which means it is able to maintain its strength under repeated loading.

Thermal Insulation With Low Thermal Mass

Plywood, like its parent wood is a good thermal insulator plus it has a relatively low specific heat when compared to other building materials. These two properties can be utilised for thermally efficient floor, wall and roof construction.

Chemical Resistance..

Plywood reacts to chemical exposure much the same as wood, in that it has reasonable resistance to acid (pH 2) and alkaline (pH 10) conditions. Thus it can safely be used in most areas exposed to chemicals, for example in heated indoor swimming pool enclosures. Plywood performs very well in seaside applications under exposure to salt mist, thus preservative treated plywood makes an excellent external cladding for beachside housing. The preservation being to protect the wood from fungal attack. In fact, plywood cladding has a low vapour permeance thus it is also a good vapour barrier.

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