Timber Cladding in Scotland
CLADDING DESIGN IN MARITIME CLIMATES
It is sometimes asserted that external timber cladding is unsuited to the wet maritime climate of Scotland. The inaccuracy of this statement can be illustrated quite simply by pointing to timber-clad buildings that have performed quite satisfactorily for many years in some of the wettest places in Scotland.
Unlike Norway and Canada, where its use is commonplace, timber cladding in Scotland has a poor reputation for durability in some people's minds. This can be ascribed to a combination of factors:
- Limited examples of good practice -
Well-designed timber clad-structures are comparatively rare in modern Scotland. Timber cladding is most frequently seen on garden sheds, barns and temporary low-cost housing where little attempt has been made to design, construct or maintain for long term durability. As a result their performance has sometimes been poor.
- Lack of skills -
Because there is little recent history of timber cladding in Scotland, design and construction skills are often poorly developed. Even in countries like Norway where timber cladding is commonplace, designers and builders regularly make mistakes. Poor design, detailing or site practice are more common problems than any with the material itself.
- Maintenance -
The UK tends to have a culture of poor building maintenance. Timber is intolerant of poor maintenance and where failures occur, the material may be blamed when once again poor practice is the culprit. Timber external joinery has consequently lost out to uPVC which, because it requires little maintenance in the short term, gives the illusion of being maintenance-free. Properly-maintained timber external joinery will easily outlast uPVC products, but since the plastics industry has been more effective at marketing its products, timber has acquired a poor reputation as an external cladding material.
A late 19th century deer larder at Loch Hourn, one of the wettest places in Scotland. Well-designed and well-maintained timber cladding is suitable for our most maritime climates.
Contrary to popular perception, the main rainfall variation in the UK is not north-south but east-west. The drier eastern half of Scotland has an average rainfall of 250mm over the four summer months, a figure closely matching the drier eastern parts of England and Wales. In contrast, the west of Scotland has much higher rainfall, exceeding 1600mm per annum in many areas and paralleled by the wetter westerly regions of England and Wales. In Scotland the number of days per annum with heavy rainfall (over 10mm in 24 hours) range from 20 on the east coast to over 80 in most of the western Highlands. The highest figure is 120 days of heavy rain per annum around Fort William. Again, these figures are paralleled in the wettest areas further south (1). The west of Scotland thus has a very wet climate though, because of the coastal influence, it is also relatively warm for the northern latitude. Thus Scotland's climate is broadly comparable to other temperate maritime climates such as western Norway and British Columbia in western Canada.
The three areas have very similar climates in the Spring and late Autumn. There are however two significant differences:
- During Winter, Bergen's maximum temperature is below 5C, the minimum threshold for fungal activity. In contrast, the maximum temperature in Onich and Vancouver may at times rise above this threshold. These differences may slightly affect how early fungal activity commences in the Spring.
- Bergen and Onich have broadly similar rainfall patterns throughout the year, whereas Vancouver is significantly drier in the Summer and early Autumn.
As a result of these differences it could be argued that Onich is at a slightly greater decay risk than Bergen or Vancouver. Temperature and rainfall are not, however, the only factors - wind exposure is also very important. Being close to, or on the path of, the Atlantic weather fronts, the frequency of strong winds is greater in Scotland than in less exposed climates such as England and Wales (1). Broadly similar weather patterns are found in the narrow coastal zones of the Pacific north-west of America, southern Chile and Argentina, southern New Zealand, Japan and parts of the extreme western seaboard of Europe. In these areas wind speeds are strongest near coasts and over high ground and so there is a strong gradient from coast to inland exacerbated by surface roughness and mountains. Wind speeds in coastal British Columbia are, for example, much higher than those further inland. Similarly, winds are more extreme in northern Britain than in most of Scandinavia except for the coastal zone centred on Bergen in western Norway (2). It is this combination of a large number of days of heavy rain combined with the strong winds that is characteristic of the weather over much of Scotland and the western coastal zones of Norway and British Columbia. The frequency and intensity of wind-driven rain tends to reduce the importance of the slight temperature and Summer rainfall differences. The three areas are thus broadly comparable in climate, with western Scotland and western Norway being the most similar.
A recent study commissioned by the Scottish Executive (3) predicts that by the end of this century there will be the following changes to the Scottish climate:
- An average temperature increase of 2 - 3C will occur and precipitation will increase by around 20%;
- The coolest individual years in the period 2080 - 2100 are likely to be comparable with the hottest of recent years, e.g. 1990;
- Wind speeds are likely to increase throughout Scotland during all seasons, with corresponding increases in precipitation intensities.
Consequently it is anticipated that more of Scotland will be regularly exposed to periods of severe wind driven rain and the higher temperatures will result in conditions where fungal growth and insect activity could be sustained for a longer period of the year.
Fungal decay and insect attack on timber cladding in Western Norway. This area has very similar decay and insect attack risks to Scotland.
SPECIFICATION AND DETAILING COMPARISONS
The design of external timber cladding varies considerably between different countries. The most important technical difference is whether the cladding is designed according to rainscreen principles with a drained and ventilated cavity behind the exterior boards, or if the boards are simply fixed directly onto the underlying structure without any intervening cavity. In maritime climates, the control of wind-driven rain penetration into the wall is the most important technical consideration in cladding design. Rainscreen cladding achieves this through a two stage approach:
1 An outer rainscreen layer deflects most of the driven rain;
2 Behind this is a ventilated and drained cavity at the back of which is an air seal (the breather membrane).
Rainscreen claddings are used in areas exposed to heavy driving rain, while non-rainscreen cladding is generally restricted to use in drier climates. The rainscreen approach is not new and has long been used intuitively in many countries (including the UK). In the early 1960's, Norwegian and Canadian researchers made serious investigations of wall leakage and proposed solutions that are still the basis of current rainscreen cladding design (4, 5).
THE ACTION OF RAINWATER ON TIMBER CLADDING
Timber is a relatively porous material, and when rainwater continually strikes timber cladding it will tend to be absorbed until finally, when fully saturated, a film of water is formed on the surface. When the rain stops, this surface film dissipates and the absorbed water dries out through evaporation. Where rainfall continues, the surface film of water will - under the combined effect of gravity and wind action - tend to migrate down the boards and also laterally until the flow is concentrated at places where irregularities (such as joints) occur. Consequently the flow of water at vertical joints is much greater than the average flow down the wall. Additionally, near the top of windward-facing elevations, the wind may force rainwater upwards and into horizontal joints. At lower levels on the wall, heavy flows of rainwater can be sufficient to fill horizontal joints. The end result of all these movements is to concentrate the rainwater flowing down a wall at the joints, the most vulnerable point in the external cladding assembly (6)
In the foreground, an open-jointed rainscreen cladding is used to protect the trusses of a timber bridge in Trondheim, Norway while, in the background tight-jointed rainscreens are used to protect the buildings, many of which are of log construction.