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PLYWOOD
QUESTIONS
How is
Plywood Graded?
What is
structial Plywood?
What
is the difference between MARINE & STRUCTURAL
plywood?
What
plywood should I use OUTDOORS?
What
plywood should I use for DECKING?
What
plywood should I use for FLOORING?
What
plywood should I use for CLADDING?
What
plywood should I use for BRACING?
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
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What
is
STRUCTURAL
PLYWOOD?
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 utilised
to achieve varying plywood thicknesses.
As a rule of
thumb:
-
3 ply = 4mm, 7mm, 9mm
-
5 ply = 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
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What
is the difference between marine and
structial 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.
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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:
-
Water
borne treatment.
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 . substrates under butyl type
membranes, shingles, stucco,
fibre cement products etc
b . all those outdoor projects.
Compatible with marine glues and
fibreglass resins
-
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:
all those outdoor projects.
Both treatments have H3
classification (suitable for
exterior use 300mm above ground)
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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',
fibreglass and all approved proprietary
coatings.
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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.
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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 specialised
claddings have grooves in the
face at varying depths and
centres.
-
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.
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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 plywoods provide more
aesthetically pleasing finished surfaces.
Manufacturers
and the NZ standards provide detailed
information and should be referred to for
specific instructions.
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