Weatherboards

Timber weatherboards

Timber weatherboards are a very traditional cladding system and are available in a range of profiles and timber species. Various profiles can be installed horizontally, vertically and, in some cases, on the diagonal. Generally, they are not part of a proprietary system although some total systems are available.

Main timber species are radiata pine, macrocarpa and western red cedar. Macrocarpa and cedar weatherboards are durable when left natural (although they will discolour), while pine boards should be coated with a paint or stain. Dark colours should be avoided, as high UV exposure will cause distortion. 

All timber weatherboards have a degree of absorbency (dependent upon species and surface finish) and therefore can be installed over an absorbent or non-absorbent wall underlay. Some species of timber used for weatherboards need to be treated for durability. Timber treatment requirements are covered by NZS 3602:2003 Timber and wood-based products for use in building.

All timber weatherboards are vulnerable to:

  • thermal movement
  • moisture absorption
  • face splitting.

Bevel-back horizontal timber weatherboards

Bevel-back weatherboards are known as a reasonably air leaky cladding system. Even when the boards are direct-fixed, air can penetrate the assembly at the laps and circulate within the voids created by the lap at the back of the boards. Water that leaks into the assembly can drain down the back of the boards and, in some cases, out through the laps or be dried by the circulating air.

Because they allow air to enter and water to drain or dry, bevel-back weatherboards are considered a robust cladding system with good weathertight performance. 

When using E2/AS1 as a means of compliance for buildings with a risk level up to and including 12, bevel-back weatherboards can be fixed directly. Above this level of risk, they must be installed over a nominal 20 mm cavity. 

Rusticated horizontal timber weatherboards

Rusticated board cladding systems are very air leaky. The thin section of board that overlaps the rustication tends to move and distort, and this allows air to enter at the lap. There is also the potential for rainwater to be driven in at the lap, but this distortion lets water drain out as well.

While rusticated boards have good potential for air entry, they do not have the void of bevel-back boards, as they fit hard to the wall frame (with a high contact area with the wall underlay) and do not have as good a drainage and drying capacity.

Because of this, rusticated timber weatherboards have a higher risk of weathertightness failure than bevel-back boards and can only be direct-fixed on buildings with a risk score of 6 and below when using E2/AS1 as a means of compliance. Above this, they must be installed over a nominal 20 mm cavity.

Shiplap vertical timber weatherboards

Shiplap board cladding systems are very air leaky. The thin section of board that overlaps the adjoining board tends to move and distort, and this allows air to enter at the lap. There is also the potential for rainwater to be driven in at the lap, but this distortion lets water drain out as well.

There is a vertical gap between the boards, and while this is quite small, it does allow some air circulation and vertical drainage to occur. 

While shiplap boards have good potential for air entry, they do not have the void of bevel-back boards, as they fit hard to the wall frame (with a high contact area with the wall underlay) and consequently do not have as good a drainage and drying capacity.

Because of this, vertical shiplap timber weatherboards have a higher risk of weathertightness failure than bevel-back boards and can only be direct-fixed on buildings with a risk score of 6 and below when using E2/AS1 as a means of compliance. As it is difficult to install these boards over a cavity, they should not be used on buildings with higher risk scores than this.

Board and batten vertical timber weatherboards

These systems incorporate flat vertical boards with cover battens over the board joints. The edge of each board must incorporate a weathergroove that aligns with a similar groove on the cover battens – this provides a good capillary break and restricts water entry.

The boards must also be installed with a 5–6 mm gap between them. This gap allows for good vertical drainage for water that has leaked through the system and for air circulation, and it also provides a good capillary break.

Vertical board and batten systems are quite robust and weathertight with good internal drainage potential. Consequently, they can be direct-fixed on buildings with a risk score up to and including 12 when using E2/AS1 as a means of compliance. As it is difficult to install these boards over a cavity, they should not be used on buildings with higher risk scores than this.

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Fibre-cement weatherboards

These are available in a range of compositions, thicknesses and profiles. They are also usually a proprietary system that incorporates a range of compatible components and must always be installed in accordance with the manufacturer’s instructions.

Fibre-cement is a very absorbent material, and the boards must be painted to be made weathertight and durable. Dark colours should be avoided to prevent distortion (although there are some compositions that are more stable).

Fibre-cement boards tend to be more tightly fixed and more stable than timber boards, so the installed system is not as air leaky. Thinner boards offer minimal voids at the laps although thicker bevel-back style boards have similar sized voids to those of bevel-back timber boards. In general, fibre-cement weatherboards have less drainage and drying capacity than timber weatherboards.

Check the manufacturer’s specification regarding installation requirements for weathertightness risk. In general, for buildings with a risk level of 6 or below, fibre-cement weatherboards or planks can be fixed directly. Bevel-back fibre-cement weatherboards are not covered by E2/AS1. However, the requirement for a cavity follows the E2/AS1 recommendations for timber bevel-back weatherboards.

All fibre-cement weatherboards are vulnerable to:

  • thermal movement
  • moisture absorption
  • moisture penetration at board ends if not properly sealed.

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UPVC weatherboards

These are available in a range of profiles and material compositions, are prefinished and come in various integral colours. They are usually a proprietary clip-together system that incorporates a range of compatible components and must always be installed in accordance with the manufacturer’s instructions. 

Because they are affected by temperature, thermal movement and fading through UV can be a problem. To counter this, they are only available in light colours.

The system incorporates voids behind the boards. These voids, combined with the jointing system, make it a very air leaky system – while water may get in, the system provides significant drying capacity. The boards are also non-absorbent, which makes for quite a robust cladding system. However, they do need to be installed over an absorbent wall underlay when they are direct-fixed. 

Check the manufacturer’s specification regarding installation requirements for weathertightness risk. While not covered by E2/AS1, in general, for buildings with a risk level of 6 or below, UPVC weatherboards can be fixed directly. Above this, they are installed over a nominal 20 mm cavity.

All UPVC weatherboards are vulnerable to:

  • thermal movement
  • brittleness and fading (as a result of UV exposure).

Updated: 9 September 2014