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Plywood Questions

How is plywood graded?

Plywood veneers for general purposes are graded from A grade being the top grade to D grade being the bottom or lowest grade. B grade is slightly lower than A. C grade has any knots, splits etc filled and sanded. D grade can have open knots and splits and can be unsanded. The face grade of plywood is always the first designated:

A – A grade has two A (good) faces

A – D grade has an A face and a D back

B – B grade has two B (good) faces

C – D grade has a C face and a D back

D – D grade has two D (rough) faces



Structural plywood is a series of timber veneers glued and pressed at right angles to each other to form a rigid board.

Radiata pine veneers are normally used in structural plywood

A water proof glue is used in structural plywood. The glue is usually identified by it’s red/brown/black colour. The glue bond is tested by a 72 hour boil test.

The veneers are coated with glue then hot pressed under extreme pressure to set the glue and structural plywood is the result of this process.

Structural plywood is normally pressed in uneven layers of veneers. Different thickness veneers are utilized to achieve varying plywood thicknesses.

As a rule of thumb:

  • 3 ply = 4mm, 7mm, 9mm
  • 5 ply =9mm, 12mm, 15mm
  • 7 ply = 17mm, 19mm, 21mm
  • 9 ply = 25mm
  • 11 ply  = 33mm

Structural plywood will not deteriorate when it gets wet

Structural plywood standard size is 2.400 x 1.200

Some products available in   2.700 x 1.200


What is the difference between Marine and Structural plywood?

Glue Line

  • Marine plywood
    Has a permanent type A
    phenolic bond
  • Structural plywood
    Has a permanent type A phenolic bond

Timber Species

  • Marine plywood
    Can be made from timber selected for its density, bending strength, lightness, surface finishing characteristics & impact resistance
  • Structural plywood
    Is normally made from exotic pinus species plywood which have a stringy tough characteristic. These species are not naturally durable but are provided in treated form for extreme situations

Structure (lay ups)

  • Marine plywood
    Is normally layed up with high quality veneers throughout all the veneer layers.Voids, gaps or knots are not allowed
  • Structural plywood
    Can be layed up with low grade veneers in the middle layers. Can have gaps, knots & voids

Finish (sanding)

  • Marine plywood
    Fine sanded both faces
  • Structural plywood
    Can range from unsanded both sides, sanded one side to sanded both sides

Note: Tanalith type treated plywood has been successfully used in many marine applications. Many people ask for marine plywood when tanalith treated structural plywood would provide a more practical & economical solution. Tanalith plywood reacts adversely with aluminium.


What plywood should I use outdoors?

H3 treated radiata construction plywood. However, beware, as there are 2 different types of H3 treatment each with distinctive characteristics:

  1. Water borne treatment. CCA (Copper Salts, Chrome, Arsenic)
    Trade name Tanalith & Impretec.
    Normally green in colour as some of the ingredients in these are copper salts, chrome & arsenic.
    There are a few DON’TS:
    a . Don’t use near aluminium, particularly in aluminium boats (creates electrolysis)
    b . Don’t use for housing pets that are prone to chewing. Do USE for:
    a . Do USE for: substrates under butyl type membranes, shingles, stucco, fibre cement products etc
    b . all those outdoor projects.Compatible with marine glues and fibreglass resins
  2. White spirit based treatment LOSP (Light Organic Solvent Preservative).
    Trade name Vascol or Protim.
    Contains solvent,oil & tin.
    Has a distinctive spirit smell when fresh. Is normally colourless.Again, a few DON’TS:
    a . Don’t use under butyl membranes (incompatible with solvents)
    b . Don’t use with marine glues and fibreglass resins (incompatible with solvents)DO USE for:
    a. all those outdoor projects.
    b. Both treatments have H3 classification (suitable for exterior use 300mm above ground)


What plywood should I use for decking?

When choosing a material for outside decking you can’t go past plywood. The ultimate would be tongue and grooved plywood. Ensure the plywood you use is H3 treated.

Always fix with stainless steel screws.

CCA water borne treatment (tanalith) must be used under butyl overlays.

You can also cover your finished deck with coatings such as ‘deck tread’, fiberglass and all approved proprietary coatings.


What plywood should I use for flooring?

Because of it’s stability and strength tongue and grooved plywood flooring is the best choice. The tongue and groove may be the traditional profile or the newer double groove with a plastic slip tongue.

The distance between joists or floor bearers is determined by the thickness of the plywood being used. Minimum thickness 12mm. In some situations H3 treated products would be advised.

Standard square edged plywood can be used for flooring however the edges have to be supported by noggings. Plywood flooring with tongue and grooves on the long edges eliminates the need for noggings.

When laying the plywood sheets on the joists always run the length of the sheets across the joists.

Always lay the sheets in brick pattern. Use a full sheet first on the first row then a half sheet first on the second row.

Always use stainless screws for fastenings.

Plywood must be used (as opposed to particle board) if an under carpet heating system is to be used. Using plywood in these circumstances eliminates the problem of formaldehyde emissions.


What plywood should I use for cladding?

  • Virtually any H3 treated structural plywood is suitable for exterior cladding. There are however some speciality cladding products manufactured.
  • 9mm plywood is normally the thinnest recommended for cladding purposes.
  • Lower grades can be used for a board and batten finish resulting in a rustic look.
  • Some specialised claddings have a bandsawn finish which has a practical purpose as well as enhancing the appearance.
  • Some of the specialized claddings have grooves in the face at varying depths and centers.
  • The decision for or against is a personal choice.

In all multi sheet applications such as in cladding situations provision must be made for expansion (ie a minimum of 3mm gap left between sheets). Some of the specialised claddings have a built in provision for this phenomena.

Claddings also provide effective bracing for structures.


What plywood should I use for bracing?

Structural plywood remains the only wood based panel that possesses properties that transform into engineering calculations for structural components including:

  • Stressed skin panels
  • Boxed beams
  • Portal frames
  • Bracing (shear) walls
  • Gussets
  • C and I section beams

Plywood is a practical solution to many basic building bracing problems with many advantages over alternative bracing systems. Can be used in the bracing of walls in timber framed buildings and also in sub floor situations. In roofing applications the plywood substrate provides effective bracing as well as the base for many roofing products.

Plywood bracing can in fact provide the cladding requirements as well as providing the bracing requirement in a building.

D-D grade structural plywood provides a satisfactory performance/cost ratio. Higher grade plywood’s provide more aesthetically pleasing finished surfaces.

Manufacturers and the NZ standards provide detailed information and should be referred to for specific instructions.


Call scooters on 09 438 6565

Our friendly, helpful and knowledgable team are here to help sort your plywood needs.