There are a number of questions which help to identify the most suitable group or species for use in specific timber cladding situations:
What is the anticipated maintenance regime?
Regular maintenance allows a wider choice of species and finishes to be used. If maintenance is likely to be poor, it would be prudent to use a relatively durable or preservative-treated timber.
What is the desired service life?
The service life of external cladding is difficult to predict since many factors affect it. However, preservative treatment or a durable or very-durable timber should always be chosen for buildings requiring particularly long service lives.
How exposed is the site?
Sites prone to severe weathering may require a more-durable timber than sheltered sites. Alternatively, preservative treatment by impregnation might be specified to protect the timber.
How easy or acceptable is it to replace boards that have started to decay?
Durability may not be so important if building owners are happy to undertake repairs, should they become necessary.
What type of detailing is used?
Simple traditional details generally cause fewer decay problems. Other types of detailing may require preservative treatment or a more-durable timber. Movement is an issue, as some types of detail can more easily accommodate shrinkage and swelling than others which may require the use of a low movement class species.
Will timber be coated or left to weather naturally?
All uncoated timber eventually weathers to a silver-grey colour, with the speed of weathering varying between species. If cladding is to be left
uncoated, then the heartwood of at least a moderately-durable species should be used. Alternatively, the timber could be impregnated with a preservative that is immune to leaching.
Is preservative treatment acceptable?
Where preservatives are acceptable, only timbers of low natural durability generally need to be used. However where preservatives are not acceptable then
either a more durable species should be used, or the cladding could be de- tailed to allow ready-replacement of boards if there are problems. In such instances,
designers should ensure that building owners are aware of – and accept – the potentially greater risks of timber decay and increased maintenance costs.
Do you wish to use home-grown timber?
The principal moderately-durable or durable timbers which can be sourced are European Larch, European Oak, Sweet Chestnut and Western Red Cedar. A wide range of lower-durability timbers are available and in some circumstances these may be suitable as cladding.