Raw logs are graded and debarked in large rollers.
Have questions on Plywood grading codes? What type of plywood should you use and where should you use it. We answer those questions below.
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
Glue Line
Timber Species
Structure (lay ups)
Finish (sanding)
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.
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.
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.
Structural Plywood Grades
High quality face – suitable for clear finish
High quality – surface suitable for painting
Non appearance grade with a solid face – not meant to look attractive
Low appearance grade with permitted defects
Non structural – for packaging use – unsanded rough appearance
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:
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
H3 treated radiata construction plywood. However, beware, as there are 2 different types of H3 treatment each with distinctive characteristics:
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.
Structural plywood remains the only wood based panel that possesses properties that transform into engineering calculations for structural components including:
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.
See how plywood goes from raw logs to final sheets of ply
Raw logs are graded and debarked in large rollers.
The processed logs are then cut to size
The logs are sliced into long veneers
The veneers are cut to size, glued and then stacked.
After glueing and stacking the veneers get placed in a press.
The pressed sheets are then trimmed , any knots or defects are filled, ready to be transported to suppliers
