Home > Applications
> External Cladding
Cladding Board Selection
timber cladding boards are available in either seasoned or
unseasoned materials. Seasoned boards are characterised by being
dressed, having a machined moulded face, and interlocking tongue and
groove (or rebated) joint. Unseasoned boards are more likely to be
rough sawn, have an unmachined face, and the absence of any tongue
and groove joint. Cypress can often be supplied unseasoned, with a
machined face and a tongue and groove joint designed to accommodate
the expected shrinkage. This allows for shrinkage movement in-situ.
The most common options are shown in Figure 6.
for cladding include: Blackbutt, Cypress, Tallowwood, Spotted Gum,
and a mixture of species generally termed mixed hardwoods. Each can
be purchased in specific grades defining natural features of the
cladding. (Refer to Table 1). For example, 'Select' grade contains
few knots and gum veins, while 'Medium' and 'High Feature' grades
contain higher proportions of these natural features. Ranking for
Cypress follows a similar theme. Specific details for each can be
found in AS2796.2 (see note1) and AS1810 (see note 2)
|Figure 6: Typical solid timber claldding
Moisture content in
cladding boards influences overlap requirements. Seasoned boards
should have a moisture content between 10-18% for hardwood and
10-15% for Cypress which ensures relatively little movement once the
boards are fitted –thus allowing a small rebate or
In contrast, unseasoned boards have in excess of 18%
content and are prone to considerable shrinkage movement – thus
creating the need for larger rebates or overlaps.
the following applies (see note 3):
- For seasoned boards less than 150mm in width, a 13mm (min.)
rebate or overlap is required. For boards greater than 150mm, a
20mm (min.) rebate or overlap is required.
- For unseasoned boards a 30mm (min.) overlap is
Nailing requirements also vary
according to seasoned or unseasoned states.
unseasoned boards must be able to shrink independently of each
other. Therefore nailing through overlaps must be avoided but must
still provide restraint to the inner board – as shown in Figure 7.
Seasoned boards also should be nailed well away from edges
to avoid splitting of thinner sections, or where tongue and groove
or rebated overlaps occur. In addition, all board ends should
pre-drilled to prevent splitting and flat head nails should be used
to prevent pull-out.
Nails also need to be chosen to suit
durability requirements and hot-dip galvanised nails are often
adequate. Table 2 provides further details on specific nail sizes
for different board thicknesses.
|Figure 7: Nailing distances|
practices must be undertaken with weather resistance in mind.
- Boards with grooves must be fixed with the groove facing
- Sealants such as mastics should be avoided.
- Butt joints between boards should be minimised to limit
moisture ingress e.g. single or long lengths should be used on
walls exposed to prevailing weather conditions, short lengths
could be used between windows or on sheltered parts of the wall,
such as under eaves and verandas.
- Butt joints should be achieved by slightly over cutting board
lengths then snapping the bowed board into position. For
durability, boards should be end sealed prior to installation
(e.g. with paint or oil based stain).
1. AS2796.2 - timber hardwood - sawn
and milled products, Standards Australia.
2. AS1810 - timber -
seasoned Cypress - milled products, Standards Australia.
Details are taken from AS2796.1 which applies to hardwoods. No
overlap or rebate requirements are given for