Timber Cladding in Scotland
THE DURABILITY OF TIMBER CLADDING
With adequate design, construction and maintenance, timber clad buildings can perform perfectly satisfactorily in Scotland. Virtually all the performance problems of timber products in buildings occur because either the timber was installed at the wrong moisture content for its intended use or because the timber subsequently became wet due to poor maintenance or detailing. In such circumstances, timber may be subject to movement and can also be at risk from fungal decay and insect attack.
THE EFFECT OF MOISTURE ON TIMBER CLADDING
MOISTURE IN TIMBER
Timber is hygroscopic, i.e. it absorbs or loses moisture until it is in equilibrium with the amount of water vapour in its surrounding environment. For any given combination of air temperature and relative humidity, wood has a specific moisture content known as the 'equilibrium moisture content'. Properly maintained and well-detailed exterior cladding has a typical equilibrium moisture content of about 16% (1). It will however fluctuate from around 12% on a hot summer's day to 20% or above during periods of driving rain. Accordingly, BS 1186-3: 1997 recommends that exterior cladding timber is dried to a moisture content of 13-19% before it is installed. The only exception to this is where unseasoned timber such as 'green oak' is deliberately used for cladding.
MOVEMENT IN TIMBER
The cells of wood fibres shrink or expand when they lose or gain moisture. These movements occur mainly across the width of the cell and are usually minimal along the length. Consequently shrinkage or swelling movements along the grain are negligible but can be significant across it. This shrinkage or expansion only occurs when moisture levels change below a critical value known as the 'fibre saturation point'. For most species this value is around 25-30% moisture content. The normal seasonal range of moisture contents experienced by cladding are less than this value, and movement will therefore occur throughout its service life. Different timber species have different movement characteristics and it is important to consider these when selecting a suitable species for cladding: normally described in relative terms, different species are classed as small, medium or large movement timbers. The latter require particular care and attention to detail if used for cladding. In a recent publication, TRADA expressed these movement classes as follows:
As the moisture content of the cladding has increased, the lack of expansion gaps in this western red cedar cladding has resulted in the boards bulging off the wall.
For most purposes, this TRADA classification provides all the guidance required to design for movement in cladding boards. There are two exceptions to this:
- Some types of cladding design - such as close-boarding - are more demanding because there is less capacity to absorb large changes in board width. In these instances, a more careful assessment of movement is required, possibly requiring specialist advice.
- Unseasoned oak with a high moisture content is sometimes used for cladding. This can save around 25-30% on the timber costs but the detailing must allow for shrinkage.
ALLOWING FOR MOISTURE MOVEMENT
In addition to drying cladding boards to a moisture content of 13-19% before installation, movement can be further reduced by limiting the width of the boards. If possible, the heartwood side of the board should be installed to face outwards on the building, to ensure that - even if the boards undergo further in-situ drying - the cladding joints remain tight. This arrangement is particularly important on the outer board of board-on-board assemblies (2). The fixings should leave the board as free as possible to move across its width. Where boards overlap, it is particularly important that nails never go through both boards. Where unseasoned green oak is used, special details such as oversize screw holes should be drilled to accommodate shrinkage.
Timber is at risk of attack from a range of organisms if it remains damp or wet for extended periods - various species of fungi and insects are the principal risks in buildings in the UK. Fortunately these risks are manageable and, with proper specification, detailing and maintenance, should be preventable. There is little risk of fungal decay if timber is dried to, and then maintained at, a moisture content below around 20%. The risk of attack in the UK by wood boring insects is also relatively minor at low moisture contents.
A converted warehouse in Trondheim, Norway is supported on wooden piles which are exposed to attack from marine borers and specialised fungi (Hazard Class 5). The timber cladding, although close, is at much less risk of biological degradation (Hazard Class 3).
Fungi require a suitable combination of moisture, air and temperature to grow. Different species of fungi have slightly different moisture requirements but, in general, wood decaying fungi require a wood moisture content of at least 20% for an attack to start, and most decay fungi will become inactive and eventually die if the wood moisture content drops below this for extended periods. Scotland has low winter temperatures and fungal activity in external joinery generally ceases when temperatures fall below about 5C. That said, it is possible the fungal activity period may in future be extended due to the higher winter temperatures brought on by climate change.
In Scotland, the common furniture beetle or woodworm (Anobium punctatum) is the only insect to regularly attack softwood timber at moisture contents below the fibre saturation point. Damage is usually restricted to the sapwood of certain susceptible timber species. The two other beetles that pose a significant hazard in the UK are the death watch beetle (Xestobium rusovillosum) and the house longhorn beetle (Hytotrupes bajulus). The former species rarely occurs in Scotland while in the UK, the latter is restricted to the south east of England (3). Powder post beetles (Lyctus sp.) can be problem in oak sapwood but as this is normally removed from oak cladding Lyctus attack is not a significant threat.
The optimum moisture content for woodworm is between 18-30%, but it may colonise any susceptible sapwood contained in external timber cladding at moisture contents as low as 12%. The rate of colonisation tends to be slow at this level and, if the moisture content does not increase over time, the infestation will tend to die out. Temperature is also an important factor and rates of growth are 50-75% less at 10C compared to 24C (3). As a consequence, woodworm is generally only a minor problem in external timber cladding in Scotland.
CLASSIFYING BIOLOGICAL DECAY HAZARDS
European Standard BS EN 335-1:1992 groups the end-uses of timber according to the biological decay hazard they are exposed to, thus providing a useful framework within which to summarise the decay risks described above. Like all exterior joinery, timber cladding is normally in Hazard Class 3 and as such, likely to be exposed to some risk of fungal decay and possibly insect attack. However, providing cladding is well designed, regularly maintained and made from suitable timber, the occurrence of rot or insect attack is often preventable. Conversely, very poor detailing or maintenance can significantly enhance the risks and place parts of the cladding in Hazard Class 4.
UNDERSTANDING NATURAL DURABILITY
Different tree species have evolved different degrees of resistance to biological degradation and these are described in the relevant standards. When interpreting natural durability statements, three points should be kept in mind:
- Heartwood and sapwood
There is a difference in natural durability between the inner part of a tree known as the heartwood and the outer sapwood zone. In most species the heartwood is a darker colour than the sapwood due to the presence of various organic chemicals known as extractives. These extractives make the heartwood of some timber species more resistant to fungal or insect attack. In contrast, sapwood does not contain as many extractives and the sapwood of virtually all timber species is classed as being of low natural durability.
- Moisture content
Timber with low moisture content is not prone to fungal attack and the risk of insect attack in the UK is also relatively low. Providing it is kept dry, even timber classed as 'not durable', can thus last for many centuries in the UK without serious decay problems.
- Relative durability
Classifications for durability are mainly derived from long term trials in which heartwood stakes are buried in the ground. In other words, the classification is a description of relative durability performance under Hazard Class 4 conditions. The durability of a particular timber species in other hazard classes may be different. In the case of cladding, the environmental conditions used in such trials are far more aggressive than would normally be found in external cladding.
European Standard BS EN 350-2:1994 groups timbers into five durability classes based on the natural durability of their heartwood against wood-destroying fungi.
Sapwood is less durable than heartwood. In this sweet chestnut log, although the heartwood is classed as durable, the outer sapwood has started to decay after only a few months in a damp environment.
Although the principal of durability classification is relatively simple, it can in practice be quite confusing when applied to specific situations. The natural durability of solid wood depends to a large extent upon the species and upon the presence of heartwood or sapwood. In some cases natural durability may not be sufficient for the particular situation, and improved durability may need to be conferred on solid wood by the use of chemical preservatives forced into the timber. In some circumstances even superficial coatings that help shed water from the timber surface can be important. Moreover, different timber species have varying degrees of permeability to moisture and this can, in some circumstances, also affect their service life. Scope for confusion is further increased because the relevant British Standards are in the process of being integrated with new European Standards. European Standard BS EN 460:1994 broadly integrates the different timber durability specification practices throughout Europe and provides a general introduction to the determination by Hazard Classes of the need for natural durability or preservative treatment.