Durability is the key performance factor involved in the design and specification of timber cladding. The main contributor to durability is the quality of design followed by the ability of a species to resist decay either naturally or through preservatives.
BS EN 350 classes species of timber by their natural durability. Class is based on the ability of the heartwood of that species to resist fungal decay. Sapwood in timber used for untreated cladding is not considered durable and should be excluded. There are five classifications of natural durability:
• Class 1 Very durable (includes tropical Opepe and Iroko)
• Class 2 Durable (Includes Sweet chestnut, European oak and North American Western red cedar)
• Class 3 Moderately durable (Includes European larch, home grown Western red cedar and North American Douglas fir)
• Class 4 Slightly durable (Includes Scots pine/Redwood, home grown Douglas fir, Norway spruce, European Elm Japanese larch)
• Class 5 Not durable (Includes Sitka spruce)
• To specify untreated timber, select species in Classes 2 and 3. Tropical hardwood in Class 1 is an option, but we try and avoid it because of cost and transport.
• Timbers in classes 4 and 5 are not naturally durable and will require treatment with preservatives.
For more information about durability see BS EN 350-2:1994 ‘Durability of wood and wood-based products. Natural durability of solid wood.’
Timber quality and appearance
• Identifying the quality and appearance of the proposed timber cladding is an early decision.
• The desired effect of the appearance of timber can be calculated to vary across a spectrum from ‘rough and rustic’ to ‘smooth and sophisticated’.
• Species and their quality can be matched to the appearance required.
• The appearance of timber varies considerably both within and across species.
• Sawn timber tends to be used where a rough, more textural appearance is sought – usually using boards in an overlapping format.
• Planed timber is better suited to where more control is exacted over the finish. The surface is smooth and the machined joints between boards can be used to produce an overall flattened elevation.
• BS 1186-3 is the specification tool used to set the quality of the timber to be used. In practice, timber is selected by the supplier according to the quality class – and then again by the contractor on delivery to site. In the case of big contracts or where there is doubt about the quality, it’s often wise to have delivered a small sample at the outset so that the specification can be fine-tuned and agreed.
• It’s possible to encounter classification systems used within the industry apart from BS 1186-3. The BS, somewhat dismissively, refers to them as ‘Commercial grading’. Another standard used is EN 975-1 which some argue provides a greater range of finishing features, particularly for hardwood. NBS, however, prefers you to use BS 1186-3.
Quality grades according to BS 1186-3
• There are 3 grades applicable to external timber cladding, mostly concerned about the size and frequency of knots:
• Class 1: Is suitable for ‘high status’ buildings. Using cladding boards of 100 – 150mm width, sound knots are limited to 22.5mm. Most hardwoods are available to this quality, but in softwoods it is limited to imported douglas fir and western red cedar. • Class 2: Is the most common classification for unfinished timber cladding. Sound knots are limited to 35mm. • Class 3: Is generally the traditional class for painted cladding. Knots are restricted to 50mm or no more than the 35% of the board width.
There is also available a Class CSH, though this is more relevant to small profiled trims since it effectively prohibits knots.
BS 1186-3 also sets out limits to a number of other concerns that steer specification quality, namely:
- Resin pockets
- Sapwood (for treated timber only)
- Rate of growth
- Slope of grain
- Exposed pith
- Decay and insect attack
- Laminating and end jointing
- Finger jointing
Temperature and humidity conditions in the UK cause a high degree of moisture content variation in timber cladding. As the cell cavities of the wood fibres lose or gain moisture the wood shrinks or expands causing degrees of movement in the timber.
The point that moisture in timber matches that in its surrounding environment is called the Equilibrium Moisture Content. For properly installed cladding in the UK this point is around 16% moisture content. The figure fluctuates up and down, so on hot days it can be down at 12% and on a very wet day the figure can climb around 20%.
BS 1186-3 recommends that exterior cladding timber dries to a moisture content of between 13% and 19% before installation.
The degree of movement differs according to species. To help describe and specify movement, TRADA devised a simple movement classification:
Across the grain dimension change within a moisture content range of 5 – 30%
1% for every 3% change in moisture content
1% for every 4% change in moisture content
1% for every 5% change in moisture content
The exception to the above is where ‘green’ or ‘unseasoned’ timber is used. Green timber, typically oak as used traditionally, is appreciably cheaper than seasoned timber. However, movement is likely to be more varied and therefore detailing for that movement needs to carefully devised.
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