Timber Cladding: Selecting a Species

      
Timber cladding has enjoyed a rapid elevation to the heights of fashion - yet the knowledge to design with this new-old material successfully hasn't evolved at the same rate.

One reason that timber cladding is popular is that it can usefully contribute to a building's sustainable footprint. It's also a powerful visual signifier of the client/architect's commitment to the Green agenda.

The downside of being visual is that selection of the timber, its detailing and specification has to be correct if sometimes-painful public displays of failure are to be avoided.
 

Sustainability issues

  • Imported timber racks-up the embodied energy, so going local can reduce or eliminate that
  • Mechanically dried timber involves heat and bumps up the energy bill
  • Treating timber so that it can be used externally involves chemical treatment and reduces its post-demolition opportunities – altogether increasing environmental impact
  • Treated and coated timber dramatically increases the need for frequent maintenance with its attendant need for access, labour and materials


Untreated timber

Listed below are timber species that because of their durability rating, don't need to be treated to be used for external cladding. The main differences between species are their:

  • Difference in service lives (though the years listed are commonly exceeded by multiples)
  • Durable imported v less durable native versions of the same species
  • Vulnerability to damage
  • Appearance- commonly assessed by number and size of knots
  • Workability
  • Movement and stability- after installation, how likely is the timber to distort
  • Price
  • Availability


What all these timbers have in common is their weathering. Irrespective of a species initial colouration, all species will have weathered to a near-matching silver-grey appearance after around 5 years. Weathering can become a serious issue between the architect and the client. Uninformed building owners having enjoyed the rich colours of freshly installed untreated timber can be rudely shocked by the transformation- particularly since it is often uneven.

 


Western Red Cedar

  • Softwood
  • Currently the most popular of species chosen for external cladding
  • BS EN 350-2 classed as ‘durable’ and does not require treatment for external use, providing only heartwood is used
  • Small movement
  • Available with FSC certification
  • Appearance grade recommended for cladding from BS 1186-3 is Class 1
  • Good for durability, stability and volume to weight ratio
  • Usually imported from North America, but British western cedar is also becoming available, but is rated as ‘moderately durable’ by BS EN 350-2
  • British western cedar matches the durability of the North American cedar but with the added advantage of reduction in cost and of embodied energy
  • Left untreated, like other species, western red cedar fades to a silver grey colour. If specifying untreated wood, ensure that the client is aware of weathering and fading
  • Natural oils in the timber have a corrosive effect on ferrous metals, so galvanized or stainless-steel fixings should be used
  • It can be nailed and screwed easily without splitting
  • The wood is soft and brittle so when designing for use, care should be taken in locating it away from areas where it might be subject to damage
  • It can become stained in polluted areas
  • Ensure that timber delivered to site meets with the specification. Pieces damaged or visually unacceptable should be rejected
  • Check for evidence of sapwood and exclude it
  • Western red cedar is relatively expensive in the UK. Other cheaper species might be considered for doing the same job, particularly if unfinished and left to weather
  • Expected service life of 40 - 60 years
     

western red cedar cladding
House extension using open-jointed western red cedar boards by CAST Architecture, Dublin

 

European Oak

  • Hardwood
  • BS EN 350-2 classed as ‘durable’ and does not require treatment for external use, providing only heartwood is used.
  • Medium movement
  • Available with FSC certification
  • Appearance grade recommended for cladding from BS 1186-3 is Class 1 though Class 2 tends to be more common
  • Green oak is most suited to sawn sections such as feather edge and waney edge which create a natural and quite rustic appearance. Green oak is a relatively cost effective choice when compared to dry oak.
  • When specifying Green oak, detail design should take into account possible shrinkage by as much as 7%. Use in short lengths and fix as soon as possible after delivery.
  • Dry oak is normally used for profiled cladding sections and is either naturally dried or kiln dried to a moisture level of 15 – 25%.
  • Oak is traditionally used untreated. If specifying untreated wood, ensure that the client is aware of weathering and fading to silver/grey
  • Hard-wearing and resistant to impacts
  • Home-grown in the UK or imported from Europe
  • The timber is subject to water staining and considerable tannin leakage (particularly for green oak) during early weathering. Stainless steel screws are recommended for hardwoods. Further, where the oak is being used ‘green’, washers should be considered for fixing security. Possible tanin run-off onto other materials should be assessed and designed-out where appropriate
  • It’s density of 670-760 kg/m3 make it one of the heavier options for cladding
  • Ensure that timber delivered to site meets with the specification. Pieces damaged or visually unacceptable should be rejected. Check for evidence of sapwood and exclude it.
  • Relatively expensive
  • Expected service life of 40 - 60 years
  • Green oak is most suited to sawn sections such as feather edge and waney edge which create a natural and quite rustic appearance. Green oak is a relatively cost effective choice when compared to dry oak.
  • When specifying Green oak, detail design should take into account possible shrinkage by as much as 7%. Use in short lengths and fix as soon as possible after delivery.



Weathered oak in feathered-edge horizonal format

 

Sweet Chestnut

  • Hardwood
  • BS EN 350-2 classed as durable and does not require treatment for external use, providing only heartwood is used.
  • Small movement
  • Available with FSC certification
  • Home-grown in the UK
  • Generally available only in small quantities.
  • Only appearance grade available for cladding from BS 1186-3 is Class 3
  • Used for its hard wearing and stable characteristics
  • Left untreated, like other species, sweet chestnut fades to a silver grey colour. If specifying untreated wood, ensure that the client is aware of weathering and fading.
  • Relatively rapid growth cycle ensuring quicker replenishment. 20-25 years for sweet chestnut : 50 – 100 years for oak and larch
  • Sweet chestnut is similar in its staining risks to European oak. Stainless steel fixings should be specified accordingly, and tannin run-off during the initial weathering period should be anticipated.
  • Ensure that timber delivered to site meets with the specification. Pieces damaged or visually unacceptable should be rejected. Check for evidence of sapwood and exclude it.

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Sweet chestnut cladding on the 'Lighthouse' building at BRE by Sheppard Robson Architects

 

European Larch

  • Softwood
  • BS EN 350-2 classed as moderately durable and does not require treatment for external use, providing only heartwood is used. That said, untreated larch cladding in Europe is very commonly seen with sapwood
  • Small movement
  • Available with FSC certification
  • Used for its robustness
  • Home-grown in the UK and imported from Europe where it is a mainstay cladding species.
  • Home-grown larch, because of its relatively rapid growth, is ideal for traditional cladding profiles where it is used green (wet). Machine profiles tend to loosen and dislodge the knots in dried boards.
  • Generally, UK larch is inferior to larch grown at higher altitudes or in longer winter conditions.
  • Left untreated, like other species, larch fades to a silver grey colour. If specifying untreated wood, ensure that the client is aware of weathering and fading.
  • Stainless steel fixings are recommended.
  • Care should be taken using European larch in exposed coastal areas.
  • Supply of good quality material is sometimes problematic.
  • Variable quality requires careful grading.
  • Expected service life of 30 - 40 years



larch board-on-board vertical cladding

Siberian Larch

  • As for European larch, but of higher quality.
  • Siberian Larch is imported from Russian forests where it enjoys relatively slower growth through short summers and long winters.
  • It is of much better quality than its European cousin. Because of its slow growth, knots are minimised and heartwood optimised.
  • Available from FSC sources
  • Ideal for use where impact damage may occur
  • Appreciably more expensive

 

Douglas Fir

  • Sometimes know as Oregon pine, British Columbian pine or Columbian pine.
  • Softwood
  • Only imported Douglas fir can be classed by BS EN 350-2 as ‘moderately durable’ and used untreated providing sapwood is excluded.
  • Small movement
  • Available with FSC certification
  • Used where impact damage may occur
  • Left untreated, like other species, Douglas fir fades to a silver grey colour.
  • Douglas fir trees grow long and straight – hence greater lengths (up to 5m) are available.
  • Expected service life of 25 - 35 years
     


Douglas fir cladding from Whitney Sawmills

 

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