information/FAQs

Fig 1 Sarking

The use of sarking with external timber cladding

Sarking is a liquid proof but vapour permeable flexible sheet material that is fixed directly behind timber cladding or under roofing material. Its primary function in walls is to prevent any water that may have penetrated the cladding from lodging within the framing. It also provides a draught proof barrier to keep wind driven rain or dust out of the wall cavity.

Sarking must be impermeable to liquid moisture yet still allow the free flow of water vapour from the inner surface of the cladding. If sarking restricts the flow of vapour, timber cladding that has taken up moisture during periods of wet weather may tend to cup as the outside face dries. Only vapour permeable building paper should be used. Polythene film, foil or other non-permeable material should never be used as sarking immediately behind timber cladding. Building papers with fire retardant properties are also available. Note that the manufacturers of perforated foil insulation recommend that it not be used for sarking immediately behind timber cladding.

Where board (as distinct from sheet) cladding is used, it is good practice to use a vapour permeable sarking on the outside of studs, directly under the timber cladding. Sarking is considered essential in walls subject to high winds and wind updraughts and also for those where boarding is fixed diagonally or vertically.

The use of wall sarking is not a substitute for well chosen and properly installed cladding and should never be regarded as the principal means of weatherproofing.

Vapour permeable sarking should always be used and installed according to the manufacturer’s recommendations.

Reference: Timber Datafile FP1 – Timber External Cladding of the NAFI Timber Manual