Exterior timber cladding is becoming increasingly popular on new buildings in both the private and public sector - houses, schools, offices, and developments such as leisure centres.

There is also a movement toward the use of more home-grown woods such as oak, Douglas fir, and European larch for timber cladding, all of which are grown in Scotland and can usually be milled locally.

Oak is classed as a durable hardwood (excluding sapwood) and oak cladding requires no preservative treatment – oak being generally impervious to preservatives.  It has an attractive figure and like all untreated timber, weathers to a silvery grey colour over time, at a rate determined by exposure to wind, rain etc. Medium movement helps make it suitable for exterior cladding, where there is inevitably considerable variation in atmospheric humidity.  Oak has a significantly higher density than the softwoods, with a range between 670 and 760 kilograms per cubic metre (kg/m3), so it is a heavier option on the wall.

European larch (UK grown) is also durable, although care should be taken in exposed coastal areas.  Larch heartwood is orangey red in colour, sometimes paler.  Regular and prominent knots are characteristic of the species, although their frequency is influenced by growing conditions.  It is a resiny timber, and denser than Douglas fir, the average density is 600kg/m3, with a range between 470kg and 650kg/m3.  The movement class of European larch is small. Supply of good quality material is not always guaranteed.

Homegrown Douglas fir is moderately to slightly durable, and will therefore need some preservative treatment.  Douglas fir is a tough timber, with good resistance to impact, and an average density of around 510kg/m3 in a range between 470 and 520 kg/m3.  The movement class of Douglas fir is also small. It can be available in longer lengths.

Attention to detail in the use and location of timber cladding on a building is essential if you wish to maximise the longevity of the timber.  The following points should prove helpful.

  1. Ensure that the timber is not in touch with the ground and damp earth/leaves etc.  By using a small plinth for the base of the wall in concrete, stone or brick, this can easily be avoided.  The detail should be such that the timber will overhang any such plinth and thus keep the stonework clear of damp.
  2. Ensure that there is an adequate overhang at the roofline.  This is so that the top is protected in the same manner as the bottom to prevent ingress of moisture and also to protect the walls from the effects of large downpours. 
  3. Ensure that windows and doors are set well into the walls to preserve then from the rigours of the weather and to give room to adequately detail the cladding where it abuts the windows and doors.
  4. Pay proper attention to joins and ends.  For example, in horizontal cladding, how are the boards going to be butted?  Will you need extra battens?  Do you need a standard length of board?  Remember that the end-grain absorbs water more readily.
  5. Think about the weather.  It’s important to what the prevailing weather will do to the timber, i.e., how weathering will occur and over what time scale.  This should be a major influence on design especially of buildings that are left untreated. 

If you want to know more about design issues, a good place to start  is the downloadable “A guide to Timber Cladding”  by Ivor Davies published in Wood 2004, available on the Napier Centre for Timber Engineering website.

Recently, more research work on timber cladding in Scotland has been carried out or is ongoing, which is expected to yield useful information for all those involved in timber specification as well as the design and construction of timber-clad buildings.  Some of this information is downloadable from the Napier Centre for Timber Engineering website.

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