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Timber cladding uncovered

Cladding: a word that often makes the construction industry feel nervous following the Grenfell Tower tragedy, but it’s essential we understand what type of cladding is okay to use and where.

Following last summer’s consultation, guidance was released stating that if a building is taller than 18 metres, timber cladding must not be used. This guidance was challenged following the Barking fire earlier this year which saw flames rapidly spread across the timber-clad balconies of a six-storey block of flats, which are less than 18m tall.

It’s led to the timber industry looking into the robustness of Building Regulations guidance on fire performance of timber cladding.

Fire performance

The Confederation of Timber industries (CTI) collated reviews and feedback from the industry on fire risk assessments. Recommended at the project design stage, these assessments can, through design and component specification, deem flame retardant treatment unnecessary. But following the feedback, the CTI recommends that “all timber-based cladding and balcony components should be treated using a quality assured factory-applied flame retardant to Euroclass B, unless shown not to be necessary by an appropriate risk assessment process.”  We’ll explore treatments further on.

Making the case

The Timber Decking and Cladding Association (TDCA) reported earlier in the year that the market for using timber cladding on high-rise buildings is small, so the ban on timber cladding over 18m has not impacted greatly on the market.. But given current concerns, what does the future look like for timber cladding and why would you still use it?

Wood is a natural and renewable source making timber building products a more environmentally-friendly and healthier choice. In some circumstances, timber cladding can help absorb sound, which is particularly useful in taller structures and public buildings.

The blanket-effect of timber cladding, though not to be misconstrued as external insulation, lends itself to homes in areas exposed to high winds and extreme temperatures. Hence timber cladding is a popular choice in Scotland and Scandinavian countries to help protect the building’s external walls.

Aside from the practical benefits, timber is also aesthetically pleasing, and the wide range of available timber cladding can transform the façade of any building.

Choosing your armour

Cladding is often made from softwood such as douglas fir, larch, pine, spruce and western red cedar. This type of wood is perceived as giving a more contemporary appearance and it is favoured for its natural resistance to decay and moisture. It can easily be painted or stained too. TDCA also reported at the start of the year that western red cedar and larch continue to be the preferred timber for cladding.

When architect practice, Foster + Partners, was tasked with creating a homely space for the new Maggie’s Manchester building, Siberian larch was chosen for its light, homely feel. Factory coated, it was ready to be installed as it arrived on site and the white finish gave Maggie’s the clean and fresh look it desired.

Hardwoods such as oak and sweet chestnut have been used for centuries to protect homes, and tropical hardwoods are favoured for being highly durable. Stanbrook Abbey, winner of the Education and Public Sector Award in the 2016 Wood Awards, is clad in oak. This hardy material not only provides an insulated rainscreen but also fulfilled the brief to use indigenous natural materials.

Newer to the timber cladding family is modified wood, a topic we have explored before. This type of wood is created from softwood and treated with a chemical, biological or physical agent to enhance its performance. The result is a durable, non-toxic product that can be used for external cladding, decking and much more.

Down Under in Torquay, Australia, a couple has recently completed a self-build project using modified timber, inspired by the rise in timber cabins in the country. Wood features throughout the property and due to its seaside location, Accoya was chosen for its resistance to rotting, warping, shrinking and swelling.

Applying the layer

Among the recommendations set out in the Hackitt Review, a high level of installer competency is a must for all elements of construction. Cladding is no exception and its installation is under more scrutiny than ever.

Incorrect fixings, poor fixing methods, lack of provision for moisture movement and durability requirements were all poor practice examples experienced by TDCA in 2018. TDCA’s CladMark quality assurance scheme accredits both installers and products to combat poor practice and increase standards.

Considerations for installing timber cladding include choosing between screws and nails -including what size and suitable metals, how they will be fixed e,g. pneumatic fixing gun or screwdriver, ensuring fixings are flush with the surface, and the positions boards can be fixed into. Before any fixing takes place, further consideration needs to be given to the width between the cavity and the solid structure. These elements should be decided at design stage, including how the cladding fits and whether it is vertical or horizontal. The TDCA offers more detailed advice for each of these processes.

To treat or not to treat?

The allure of timber is that buildings can be created from a natural material. However, stripped down and exposed to the elements, timber may need a helping hand to preserve it.

According to the TDCA: “Most naturally durable timber species, modified wood and softwoods treated with copper/organic biocide preservatives can be left unfinished to weather naturally.” As a result, many architects opt for lighter-coloured woods as they will weather better as the building ages.

BS 8417 – Preservation of Wood – code of practice, sets out when preservative treatment should be used and under what circumstances. It’s set out into ‘Use Classes’ and timber cladding falls under Class 3. This means when purchasing timber cladding it must be specified as treated to Class 3. Timber cladding can either be coated or uncoated and coatings range from clear, water repellents to solid stains and paints. Whether the cladding is coated or not, it must have a copper organic preservative. This is water-based and is the only treatment required for uncoated timber. Coated timber may also need micro-emulsion or boron compound preservatives, although the latter is rarely used in the UK. The Wood Protection Association can offer further advice on this.

A timeless appeal

Timber cladding is often the perfect choice for a wide range of buildings under 18 metres. Both durable and renewable, it offers flexibility in design and a timeless appeal. No wonder it is used up and down the country, inland and on the coast. Find inspiration on cladding options for traditional and contemporary buildings in our case studies.

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