Timber cladding secrets explained

Timber cladding has boomed in recent years. However, as early installations now mature it’s time to reflect on what has and hasn’t worked. Tom Barnes of Vastern Timber reflects on the ways to ensure cladding installed today looks good and performs for years to come.

There are many reasons why timber cladding has enjoyed a renaissance in recent decades, not least that it is one of the easiest ways to give a design that ‘something special’. Although an ancient technique, timber cladding declined in the 20th Century as brick and clay dominated house building. However, in recent years, and due in part to a growing appreciation of the importance of sustainability, timber cladding has become commonplace.

Thanks to the number of available tree species and grades of timber used, the range of textures and finishes is literally infinite. On top of this, natural and unique weathering effects will occur dependent on exposure and location. This is all part of the natural attraction of timber cladding.

To ensure cladding delivers on its potential to create an attractive finish with unique character, specifiers need to understand the specific influencers – both natural and human – that will dictate how the cladding will behave over time. Many examples of cladding still look beautiful 5 – 10 years after installation, but in other cases it has not fared as well because the designers and installers did not fully understand or appreciate the influencing factors such as how particular species will react in the modern environment; and the effect that elevation and aspect has on the speed and consistency of weathering over time.

Timber cladding is often used in a natural and unprotected state. Common cladding species typically used are naturally durable and do not need protection if they are not in ground contact. This is an important benefit as the application of surface treatments can prove costly and time consuming. However, when left in its natural state timber will weather more quickly than when protected, and will also be more prone to environmental influencers such as ultra-violet light (sunlight), moisture and even pollution.

Common unexpected changes in the appearance of timber cladding can include extractive staining in woods like oak, sweet chestnut and western red cedar, where soluble organic compounds in the wood migrate in the presence of rain to leave staining. Also, black and green mould is caused by a lack of sunlight and perpetually damp conditions. Particulates in pollution can settle on the cladding surface and react with the natural extractives in the wood to create an unsightly blackening effect. Finally, natural surface greying (bleaching), which can be a highly attractive feature, will be undesirable if it is uneven due to different levels of exposure to rain and sunlight.

Oak and sweet chestnut have become very popular woods for cladding projects. These species are heavy in tannic acid, which is very reactive and brown in colour. In some cases these species have been used to clad the top storey of buildings while the lower storey has featured bright white render. In such cases rainwater can cause the tannins to leach out from the wood, often resulting in staining on the render. Also, ferrous particulates common in urban road pollution will react with these extracts to cause an unattractive blackening of the surface.

Canadian cedar is the most popular species for cladding. In unpolluted rural areas, and on designs that have considered weathering factors, uncoated Canadian cedar will develop a beautiful silver/grey colour. The species itself is not problematic, however, its inappropriate use in the wrong environment and a failure to consider specific weathering factors can result in an unattractive patchy appearance.

Timber cladding is often used on large buildings with numerous elevations and overhangs. With such projects, each panel will invariably face a different set of weathering factors, such as exposure to rain, snow, ice, and differing amounts of UV light and wind. As a result there will be a huge amount of differential weathering to consider, and these weathering effects can be highly noticeable if not enough consideration is given to aspect and elevation, or where detailing serves to compound the issue. It is also important to remember that the inconsistent weathering effects caused by detailing, such as overhangs, recesses and even cills, will be accentuated by the lack of sunlight affecting a northerly elevation.

People often ask, how can they preserve the initial colour of timber cladding? The simple answer is that in the long term, they can’t. A clear treatment, reapplied at the correct intervals, will significantly slow the weathering process, but it is a battle that will eventually be lost. A pigment treatment will protect the wood for longer, but obviously you are no longer looking at the natural colour of wood. Also, care should be taken when selecting pigmented treatments as some are not all that attractive.

There is an alternative timber cladding solution that negates many of the concerns about unpredictable natural weathering and which does not need treatment. Furthermore, it provides stability and more consistent weathering effects with no extractives to bleed or react with pollution, and it maintains clean straight lines, with less surface break down, and less cupping.

Thermally Modified Timber is a relatively new innovation where the timber is super-heated to temperatures between 160° and 210°. The combination of high heat and steam reconfigures the wood to deliver a material of exceptional stability, durability and aesthetic uniformity that is highly resistant to decay. The dark brown colouring of thermally modified products and their performance in exterior applications make them ideal replacements for tropical timber products.

The thermal modification process involves no chemicals and does not rely on impregnating the timber. The most impressive outcome of the thermal modification process is that products are significantly more stable than non-modified timbers resulting in significantly less expansion and contraction when faced with changes in ambient temperature and humidity, making them a superior solution for a range of exterior uses, including cladding. Additionally, the process reconfigures wood that would normally rot when used outside, into products that will last 30 years and more without treatment.

Ultimately, it is important to understand that, as a natural material, wood will change colour and appearance over time. Consideration given to shading due to elevation and detailing, as well as appropriate selection of timber species, will result in a cost effective and long lasting finish that will look beautiful for many decades.

As seen in HouseBuilder and Developer