TIMBER WEATHERBOARDS AND WEATHERTIGHTNESS
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Cladding a home with Timber weatherboards is a very traditional cladding system which is available in a number of species, grades and are offered in a range of profiles. Timber Weatherboards are installed both horizontally, vertically and, on occasion diagonally.
Main timber species are Californian Redwood, treated radiata pine, macrocarpa and western red cedar. Redwood, 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.
When cladding your home with timber weatherboards you need to be aware that all weatherboards have a degree of absorbency and this can vary from species and surface finish. Which is why timber weatherboards can be installed over an absorbent or non-absorbent wall underlay and it is good practise to use a cavity system for good ventilation. Some species of timber used for weatherboards need to be treated for durability namely pine, and some timber weatherboard species will have a high level of natural movement like pine or Red beach.
CLADDING WITH TIMBER WEATHERBOARDS CAN BE VULNERABLE TO:
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thermal movement – Pine and Red beach are well known for this.
moisture absorption – all species but with the right cladding system this is minimal.
face splitting – made worst by some local climates but in all instances Pine and Red beach will split.
Bevel-back horizontal cladding for your homes timber weatherboards
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Bevel-back timber weatherboards considered to have a good level of air flow or an air leaky cladding system. (Air leakiness is a desirable attribute to aid in drying). Bevel Back Timber weather boards have excellent airflow even when the boards are direct-fixed to the subframe, air will penetrate the cladding system at the laps and circulate within the gaps created by the lap at the back of the boards. When water leaks into the cladding system it will drain down the back of the boards and out through the laps or be dried by the circulating air.
Because Bevel Back timber weatherboards allow air to enter and water to drain away and dry, bevelback weatherboards are considered a strong cladding system with excellent weathertight performance.
When using building code E2/AS1 for compliance for buildings with a risk level up to 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 cladding for your homes timber weatherboards
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Rusticated board cladding systems are very air leaky. The thin section of board that overlaps the rustication tends to move and this allows air to enter at the lap. There is also the potential in extreme weather for rainwater to be driven in at the lap, but this distortion lets water drain out as well.
Rusticated weatherboards have good potential for air entry, but do not have the void seen in bevel back weatherboards due to fitting hard to the wall frame creating a high contact area with the wall and do not have as good a drainage and drying capacity.
Because of this rusticated weatherboards have an increased risk of weathertightness failure when compared to bevelback weatherboards. It is best practise to install with Cavity battens which increases air flow and decrease contact of boards to framing. You can only be directly fix 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.
Vertical Shiplap - cladding your home with timber weatherboards
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Shiplap weatherboard cladding systems are very air leaky from the thin lap of that board that overlaps the adjoining board, this tends to move and this allows air to enter at the lap. In extreme weather this profile also have the potential for rainwater to be driven in at the lap, but this distortion lets water drain out and being a vertical system it drains very well.
The vertical gap between the weatherboards also allows air circulation and vertical drainage to occur.
Shiplap weatherboards have good potential for air entry but again do not have the gap bevelback weatherboards have, as they fix flat to the wall frame giving a high contact area with the wall and consequently we recommend using a cavity batten system to improve drainage and drying capacity.
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Board and batten vertical cladding your homes timber weatherboards
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These systems incorporate flat vertical weatherboards with battens that cover over the weatherboard joints. Weatherboards 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.
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Drying rates in walls
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The most striking finding of a large BRANZ experiment into drying rates of walls with different claddings was the large difference between drying rates from deep inside the wall (in the framing) and from the back of the cladding. Water dries 100 times faster from the back of the cladding than from framing. This is due to the lack of air flow and because water diffuses slowly in timber framing. This illustrates the importance of isolating the back of the cladding from the framing
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If the cladding is absorbent, then drained and ventilated and open rainscreen walls recover quicker than those with direct-fixed claddings. On average there is a 3-to-1 advantage here for the ventilated cavity walls. It is worth noting, though, that weatherboard walls are an exception because they are naturally ventilated and have always been the fastest drying of the direct-fixed walls on the BRANZ building.
Another result was that the ventilated cavity walls deal with water trapped on the back of the cladding or on the building wrap more effectively than the non-water managed direct-fixed walls. However, the cavity designs have not helped to dry water from framing in these experiments, because the drying rate is still limited by moisture transport rates in timber. This emphasises the importance of preventing water from reaching the insulated cavity.
