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Timber Cladding in Scotland

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Timber Cladding in Scotland

BEST PRACTICE IN SPECIFICATION AND DETAILING
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Keith Hunter

SPECIFYING TIMBER SPECIES AND QUALITY

The National Building Specification for timber weatherboarding (1) cites BS 1186-3:1990 as the appropriate standard for specifying timber cladding and its fixing. This standard does however have several limitations:

  • The timber quality grades in the standard are quite complex and are intended for timber sections to be moulded to fine tolerances. The grading requirements of cladding boards are not usually so demanding and in many cases even ordinary sarking timber is suitable for use;
  • The timber cladding sections recommended in the standard are inconsistent with modern best practice requirements, e.g. the tongues are sometimes too small to be suitable and so some sections are more appropriate for internal wall linings;
  • Some of the timber durability information given is inaccurate (e.g. the durability of home-grown Douglas fir), and the durability and wood preservation advice is not fully consistent with the European standards.

The following notes are intended to supplement the guidance in BS 1186-3:1990, providing points of clarification on timber species and quality grading.

SPECIES

Timber species from a wide spectrum of durability classes can be used quite satisfactorily for external cladding, and specifiers are sometimes unclear as to why such a wide durability range can be employed.

Housing at Graham Square Glasgow by McKeown Alexander Architects makes use of western red cedar cladding in an urban environment.

Three broad approaches exist to deciding the level of natural durability or preservative treatment required. These are implied in the European Standard BS EN 460:1994, although there are considerable difficulties in interpretation. The approaches are:

1 USING NATURAL DURABILITY

Providing the sapwood is removed, timbers classed as moderately-durable or above generally have sufficient natural durability to be used in Hazard Class 3 conditions without preservative treatment.

The service life will vary as a result of both design details and the natural durability of the timber. Poorly drained and ventilated designs will reduce the service life while more-durable timbers will tend to last longer. The draft European Standard DD 239:1998 quotes a minimum 60 year service life for durable or very-durable timbers in Hazard Class 3 conditions and good detailing could be expected to extend this. Moderately-durable timbers might be expected to achieve a similar service life when carefully detailed for durability (2). Where detailing is poor, however, DD 239:1998 cautions that the minimum service life is only 30 years, and it would be prudent to use a durable or very-durable timber for exterior structural uses or where a component would be very difficult to replace or might constitute a health and safety risk if it failed. Alternatively, preservative-treated timber could be used.

From a durability viewpoint, surface coatings are not necessary with these timbers. If the timber is to be left uncoated, however, building owners must be prepared for the inevitable - and sometimes unpredictable - surface changes that will occur as the timber weathers. With correct practice, the use of naturally-durable timbers can give reliable results but this approach is vulnerable to poor timber grading (e.g. failure to remove the sapwood) and to inappropriate species selection.

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2 USING TIMBER PRESERVATION

It is accepted practice in the UK that where timbers used in Hazard Class 3 are designated as less than moderately-durable, or where they include sapwood, the timber should receive preservative treatment by impregnation to the specifications in the European Standards. Where best practice is followed, the Draft European Standard DD 239: 1998 states that preservative treatment can provide a reliable service life of 60 years.

Where preservation treatment or site practice do not conform to European Standards, however, adequate durability cannot be guaranteed from the use of preservatives alone. Stringent quality control is therefore essential at both the preservative treatment and construction stage and, if in doubt, it should be ensured that compliance with the requirements of the European Standards can be demonstrated.

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Approach 1 - using natural durability. Oak shingles on the Koldinghus Museum in Denmark.
Approach 2 - using timber preservation. Preservative treatment has been used on roof and wall cladding at Stavanger, western Norway as the roof is exposed to a considerable decay hazard.
Approach 3 - using careful detailing. Redwood cladding in Stavanger, western Norway. In common with most cladding in the area, the boards are well-detailed for drainage and ventilation and have been given a water-repellent but moisture vapour permeable coating.