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External Cladding

Cladding Board Selection
Solid 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.

Species used 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) respectively.


Cladding - table 1


Figure 6: Typical solid timber claldding profiles
Cladding - Figure 6: Typical solid timber cladding profiles


Moisture Content
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 overlap.

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.

Given this, 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 required.

Nailing requirements also vary according to seasoned or unseasoned states.
For instance, 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
Cladding - Figure 7:Nailing distances


Cladding - table 2


Installation Practices
Installation practices must be undertaken with weather resistance in mind.

  • Boards with grooves must be fixed with the groove facing downwards.
  • 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.
3. Details are taken from AS2796.1 which applies to hardwoods. No overlap or rebate requirements are given for Cypress.