How to not bow weatherboards

By - , Build 90

In a perfect world, perfectly straight weatherboards would stay that way when fixed in place. In the real world they behave otherwise, unless a few precautions are taken.

The cause of these badly bowed weatherboards can be seen in the lack of adequate expansion gaps at the top of each board.
Badly bowed and cracked timber weatherboards are caused by moisture and lack of expansion gaps.
The cause of these badly bowed weatherboards can be seen in the lack of adequate expansion gaps at the top of each board.

The moisture content of timber is the cause of many faults and failures, particularly with rusticated or rebated weatherboards. Timber always contains some moisture. The actual amount will vary with changes in temperature and humidity, causing the timber to shrink or swell. This movement can cause cracking, distortion, opening of joints and laps, and stress on surface coatings.

On site

Materials should not be delivered to the site until just before they are needed. On arrival, timber cladding must be carefully unloaded and placed on timber dunnage well clear of the ground (see BUILD June/July 2004, pages 42–43 for more about site storage). It is important to keep the material protected from the weather by keeping it well covered. If the timber is unusually dry (less than 12% moisture content) it may need time to come up to site conditions.

Installation

When installing weatherboards the moisture content must be near to the average moisture content expected in service. In most parts of New Zealand the moisture content for timber claddings is about 16%.

If sufficient expansion gaps are not provided and moisture content is too low at the time of installation, bowing can result, especially with rebated profiles, as shown here. Table 1 lists the expected amount of shrinkage or expansion for a typical 200 mm weatherboard for various wood species.

End grain is far more absorbent than face grain and must be well sealed with paint or wax to prevent both rapid moisture uptake and drying out. Critical end-grain locations are corners, mitres, openings and running joints.

Table 1: Typical moisture-change movement in timber weatherboards (assuming a 6% change in moisture in a 200 mm board).
    Shrinkage or expansion (mm)
Species Radial (quarter sawn) Tangential (flat sawn)
Radiata pine 1.4 mm 2.6 mm
Macrocarpa 1.2 mm 2.1 mm
Douglas fir 1.9 mm 3.3 mm
Larch 1.3 mm 3.3 mm
Western red cedar 1.5 mm 2.8 mm

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The cause of these badly bowed weatherboards can be seen in the lack of adequate expansion gaps at the top of each board.

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