Plywood

Availability

Structural plywood is made in thicknesses from 7 mm (3 ply) to 25 mm (7 ply).

Veneers are typically 2.4 mm or 3.0 mm thick (thicker or thinner veneers are occasional used).

A 7.2 mm panel might be 3 equal veneers 2.4 mm thick, and a 12 mm panel might be 5 ply with 2.4 mm veneers or 4 ply with 3 mm veneers, in which case the two 12 mm panels will have different properties.

Normal sheet sizes are 1.2 m x 2.4 m or 1.2 m x 2.7 m, as sheet length is limited to the length of a log that can be peeled in a veneer lathe. 

Some products are still made to the old imperial measure of 1.22 m x 2.44 m (4 feet x 8 feet). Consult manufacturers’ literature to check available sizes. Some will manufacture non-standard sizes.

Grades for structural plywood

There are two grade specification requirements for plywood in AS/NZS 2269: Plywood – Structural. These are surface grade (visual quality) and stress grade (strength and stiffness).

Plywood visual quality is designated according to the surface grade of the face and back veneers.

For example, A-C plywood has a quality A (clear) face veneer and quality C back veneers.

High quality clear veneer (A grade) is used for the outer veneers of the best plywood.

Lower quality D grade veneer can be used for inner veneers, or the lowest structural grade panels. B & C grade veneers of intermediate quality have fewer defects than D grade veneer.

Visual quality is not well correlated with strength or stiffness because other characteristics like density and grain angle are not easily seen. 

Strength properties of plywood are determined through manufacturing, with controls of log sources, sonic and resonance grading of logs and veneers, and in-grade testing and verification.

The stress grade is usually specified as F8 or F11 or other grade with a corresponding suite of characteristic properties.

Suppliers Database

Applications

Plywood has useful qualities which make it suitable for a wide range of applications. These applications include:

• Floors, roofs and decking.

• Diaphragms and shear walls.

• Beams and frames.

• Plywood web breams.

• Plywood for structural connections.

• Stressed skin panels.

• Folded plates.

• Formwork.

Non structural applications include:

• Furniture.

• Joinery.

• Cladding.

• Internal linings.

• Shelving.

More information on Applications of Plywood - Printable PDF

Manufacture

Structural plywood is manufactured in accordance with joint Australian and New Zealand standard AS/NZS 2269.

The thermosetting adhesives used to bond the veneers together, derived from synthetic resins, are set and cured by heating.

The properties of the adhesive can determine the durability of the plywood. In Australasia, structural plywood to AS/NZS 2269 must be bonded with phenolic (marine) adhesive.

Selected logs are rotary peeled in a lathe to form a ribbon of veneer which is dried, clipped to sheet widths, and graded. Sheets of plywood are made by applying phenol formaldehyde resin to veneers of predetermined grade in a specified arrangement.

These veneers are hot pressed to cure the adhesive and form the plywood panels. Plywood sheets are usually sanded on both sides after gluing. For some grades, small defects are filled prior to sanding and final panel grading. Plywood can also be preservative treated, or overlaid with resin impregnated papers.

Formaldehyde

Formaldehyde is a natural part of the environment and human metabolism, but in high concentration, it can be an irritant to a small percentage of sensitised people.

In recent tests, formaldehyde in very high concentrations has been identified as a carcinogen.

Carpets and other interior fittings are sources of formaldehyde, and there are background levels in nature which vary according to the proximity of different plant species that emit different amounts.

Emissions of small quantities of formaldehyde from the adhesives in boards have not been a problem in most normal, adequately ventilated environments in New Zealand buildings.

Phenolic bonded boards are low emitters because the adhesive is moisture and heat stable. New manufacturing standards now exist with set limits for classification of panels as low emitters or higher emitters. For example, refer AS/NZS 2098.11 Methods of Test for Veneer and Plywood Method 11: Determination of Formaldehyde Emissions for Plywood, and new clauses in the standards for plywood manufacture.

Quality control

As a manufactured item, plywood is subject to rigorous quality control procedures. Appearance control is by grading, patching or processing in the factory.

Bond and structural requirements are monitored in New Zealand by qualified and audited third party organizations like the EWPAA to ensure the plywood has durable and reliable strength properties.

Performance

Features of plywood include:

• Workability.

• Impact resistance.

• Flexibility.

• Dimensional stability.

• Two-way strength and nail holding.

• High panel shear strength and stiffness.

Features of Plywood - Printable PDF

Specification includes:

• Visual quality and finishes.

• Flooring and decking.

• Ventilation.

• Specification.

Specification - Printable PDF

Stress grades

The stress grade of a plywood sheet is determined in accordance with AS/NZS 2269 from a process specification that may include sonic and resonance or other grading of the constituent veneers, followed by in-grade testing and continuous monitoring of plywood sample properties.

Each sheet is labelled with an F grade or stress grade according to its claimed properties. Each F grade has an assigned characteristic bending stress, bending modulus of elasticity, compression stress, tension stress etc, given in Table 1.

Section properties - Printable PDF

Design procedures include:

• Modification factors.

• Fixings.

Design Procedures - Printable PDF

Plywood web beam design includes:

• Deflections.

• Bending deflection.

• Shear deflection.

• Deflection due to nail slip.

• Total deflection.

• Web stiffeners.

• Lateral torsional stability.

Plywood web beam design - Printable PDF

Joints include:

• Plywood for structural connections.

Joints - Printable PDF

Stresses in Plywood components includes:

• Basic equations.

• Axial stress and bending in the plane of the sheet.

• Bending stress for plywood loaded normal to the plane of the sheet.

• Shear stresses.

• Panel shear.

• Rolling shear.

Stresses in Plywood components - Printable PDF

Specific design of engineered I-joists includes:

• Serviceability.

• Strength.

Specific design of engineered I-joists - Printable PDF

Durability

Plywood manufactured with exterior quality waterproof adhesives is suitable for exposed uses, provided it is also correctly treated and appropriately protected for these conditions.

Plywood can be treated after gluing in accordance with AS/NZS 1604.3 to a level of H3 “treated against decay for exterior cladding, not in-ground contact”.

It is difficult and not usual to treat panels to higher levels of exposure because successive glue lines restrict flow of the treatment salts.

This means that only envelope treatment is practicable for thicker panels, but this is sufficient for most uses of plywood that need H3 treatment. Consult the treatment standards for more information.

Face veneers will check with small splits if they are left unprotected in adverse conditions.

This is normal and can be minimised with good quality acrylic paint systems.

Long term exterior durability for applications like claddings requires a well maintained acrylic paint system on the treated plywood.

Plywood is not recommended for fully exposed horizontal applications like decking because severe checking will occur, but it is a good substrate for membranes in this application.