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Plywood – the original engineered wood product

Plywood is used for a wide range of structural, interior and exterior applications – from house construction and internal panelling through to concrete formwork. Benefits include superior dimensional stability and an excellent strength-to-weight ratio as well as its ability to resist cracking, shrinkage, splitting, and twisting or warping.

Manufactured from thin sheets of cross-laminated veneer and bonded under heat and pressure with strong adhesives, modern plywood has become one of the most widely used building products since it was first introduced in 1850.

APA member mills manufacture plywood in a wide variety of appearance grades from smooth, natural surfaces suitable for finish work and underlayment to more economical grades for wall sheathing and subfloors. Non-overlaid plywood panels come in three levels of surface finish – sanded, touch-sanded and unsanded.

When a plywood panel is performance-rated, it indicates it has been manufactured in line with US official standards. These provide product performance benchmarks, such as load-carrying capacity for designated end uses. US rated panels sold in both the UK and EU countries display an official CE or UKCA mark to show they have also been manufactured to comply with the relevant performance-based standards.

APA’s Global Technical Support Desk

Engineered wood experts are available to answer your questions about the specification and application of APA products. Contact our global help desk or call +1 253 620 7400.

Ply Specification Tool

A trademarked panel carries information on its potential use in one of two ways: 

  • Simply by telling you what it is suitable for eg Rated Sturd-I-Floor, Rated Sheathing or Underlayment, or
  • Showing the veneer grade on the face and back of the panel e.g. A-C or B-B.
  • Check out the Plywood Specification Tool 

Span Rating on a US trademarked panel provides vital information about its structural viability. It denotes the maximum recommended centre to centre spacing of supports over which the panel should be placed, with its strength axis across two or more supports. The further the supports are spaced, the thicker the panel needs to be.

Span ratings and panel thickness need to be considered together when deciding if a specific panel is suitable for your application.  The plywood Specification Tool provides all the info you would need. Use it here.

Different plywood grades have different thickness tolerances. Our plywood Specification Tool  helps you select the right product by giving you the nominal thickness for each grade. 

US  trademarked panels carry the term ‘Performance Category’ as a way of identifying the panel’s nominal thickness. The APA tool provides both the nominal thickness of each panel in mm (converted from US inches) followed by the US Performance Category (given in brackets). The tool gives the midpoint measurement but you can find the complete tolerance ranges in the It takes the info in the APA Construction Guide.

Like all wood products, plywood will expand or shrink slightly with changes in moisture content. If the panels are tightly butted, there is no room for this, and buckling can occur.  APA therefore recommends that you allow a 3mm (1/8-inch) space between panel edge and the end joints.

Sometimes this has already been taken into account by the manufacturer and ‘Sized for Spacing’ is then marked on the panel. This indicates that it has been produced at a length and width slightly less than the usual nominal length and width. 

 

US trademarked plywood panels have a tongue and groove (T&G)  profile on their two long edges. This removes the need for support (blocking) under adjacent panel edges to prevent them from deflecting independently of each other under a load. The material removed to create the tongue normally reduces the width of the panel by 12.5 mm (1/2″),  so that the net width of tongue and groove panels is typically 1206mm (47-½”).

Knowing the species of wood used to manufacture the plywood panel’s face and back are important as they determine its bending strength and stiffness: group 1 is the strongest and group 5 the weakest.  For APA trademarked panels, the main species are:

  • Douglas-fir(Groups 1 & 2) for faces and back; with other species used only for inner plies – Western Larch, Western Hemlock, the true firs (Balsam, California Red, Grand, Noble, Pacific Silver and White), Ponderosa and Lodgeole Pine.
  • Southern Yellow Pine(Loblolly, Longleaf, Shortleaf, Slash) with some minor use of Sweetgum and Yellow Poplar for inner plies.

Plyform is a special type of plywood panel with sufficient strength to support the weight of the wet concrete. It is stronger than conventional plywood, with an exterior bond classification.  Our specification tool will help you select the right grade for your formwork. Check here.

The Washington State History Museum in Tacoma, USA, is a visually powerful example of the bending strength of APA Plyform. Curved forms were integral to achieving the architectural design, and used to form 11 concrete arches, each 55ft. high. The tighter the radius of curvature, the thinner the plywood needs to be to bend to the required shape.

APA offers three Plyform types, all using only the strongest wood species for the face and back veneers. They have an exterior bond classification and come in three strength classes: Structural 1, which is made from the strongest wood (species group 1); Class I or Class II. The lower the grade of veneer, the greater the size and number of defects which reduce strength and stiffness.  

APA’s specific panels for concrete forming:

  • B-B and B-C Plyform: all three strength classes
  • HDO Plyform:  all three classes
  • MDO Plyform: Class I or Class II

An overlay on a panel means the wood veneer is not in direct contact with the concrete. All plyform grades are available with a High Density Overlay (HDO) or a Medium Density Overlay (MDO) on one or both faces. HDO plyform gives a near polished concrete surface and normally produces 20 to 50 pours or more. 

APA’s Concrete Forming Design/Construction Guide is designed for architects, engineers and contractors. Download here.

APA Plywood in Lowest Formaldehyde Release Class

All trees and wood products emit small amounts of formaldehyde. An oak tree, for example, emits 0.009 parts per million (ppm) of formaldehyde.

The adhesive used to manufacture US trademarked plywood is phenol formaldehyde (phenolic)  which does not release significant amounts of formaldehyde. Current scientific data indicates that the maximum formaldehyde level associated with phenolic bonded wood panel products is about the same as background levels present in outdoor air in an urban environment.

For more information, download APA’s Formaldehyde and Engineered Wood Products.

The overall standard in the UK (designated) and EU (harmonised) for wood-based panels, (BS) EN 13986, says that ‘wood-based panels glued with resins emitting either no formaldehyde or negligible amounts of formaldehyde after production should be classified as E1 without needing further testing. E1 is the lowest formaldehyde release class.

Formaldehyde levels in APA panels produced in line with US PS 1-19 and PS 2 -18 standards already meet E1 requirements. Independent tests confirm that formaldehyde emission levels rapidly approach zero as finished US trademarked plywood panels age.

See also: Japanese Agricultural Standard (JAS) for formaldehyde emission and California Air Resource Board (CARB) for measures on release.

Japanese Agricultural Standard (JAS)

Japanese regulation for formaldehyde emissions from wood panels is widely considered the most stringent in the world. The JAS standards for plywood and Structural Panels (OSB) do not directly specify a limit for formaldehyde emissions. But panels meeting the F**** standard for formaldehyde emissions (the most stringent requirement level) are required to have an average emission level below 0.30mg/l as tested to Japanese Standard JIS A 1460.  APA PS 1-19 and PS 2-18 panels easily meet F**** requirements.

California Air Resource Board

APA member products are exempt from the Airborne Toxic Control Measure to Reduce Formaldehyde Emissions from Composite Wood Products.  Section 93210 (c) (8) of the measure specifically states that composite wood products do NOT include:

  • Structural plywood as specified in the Voluntary Product Standard – Structural Plywood (PS 1-19).
  • Structural panels as specified in PS 2-18, sometimes designated on APA panels by our own Performance-Rated Panel Standard – PRP 108.
  • Structural composite lumber (LVL) specified in Standard Specification for Evaluation of Structural Composite Lumber products (ASTM D 5456-06).
  • Oriented strand board.
  • Prefabricated wood I-joists as specified in Standard Specification for Establishing and Monitoring Structural Capacities of prefabricated Wood I-Joists (ASTM D 5055-05)

The CARB regulation was used as the basis for the Federal (US National) Regulation that comes into effect in 2013, therefore, the same exemption exists for the Federal (US National) formaldehyde provisions.

Stability of phenolic polymers

Phenolic glue is manufactured from resins consisting of phenol formaldehyde polymers by reacting together measured amounts of phenol and formaldehyde under carefully controlled conditions. These polymers consist of ‘chains’ of phenol and formaldehyde which are chemically linked together to form polymer molecules. These molecules possess physical and chemical properties which are completely distinct from the properties of either phenol or formaldehyde i.e. these polymers are new chemical entities and not simply mixtures of phenol and formaldehyde.

Once formed, the phenolic polymers are extremely stable and do not break down into their original phenol and formaldehyde molecules. It is this stability which prevents the release of formaldehyde from panels containing this type of water proof adhesive.