How well does untreated timber cope outdoors?

untreated cladding

Timber does not necessarily have to be treated to be used externally. It can provide robust results with an unparalleled aesthetic – free from chemical treatments. In this extract from TDUK’s forthcoming Designing for Natural Durability, we showcase some great examples of buildings whose robust timber is weathering naturally and handsomely.

Exteriors that use untreated timber can stand the test of time whilst developing unique patinas. And using untreated timber increases the opportunities for circular re-use and reprocessing possibilities at the end of the building’s life.

Furthermore, while today’s chemical treatments do sit within the allowable boundaries of nonhazardous waste, these regulatory boundaries may well change over the life of today’s buildings.

Designing to use the natural durability of the timber – where suitable – is positive for the material’s re-use potential and ultimate end-of-life scenario, after all the re-use opportunities have been exhausted.

Best practice when using untreated timber

When designing using timber’s natural durability, it is important to fully understand the unique considerations and potential consequences, both from a maintenance and visual perspective. These must be clearly communicated to the client and end-user.

This is vital to ensure expectations are managed, and these case studies are intended to offer tried and tested examples for this dialogue.

Although each project is different, there are some common general ‘best practice’ considerations:

  • Select a timber species that is durable and appropriate for its application
  • Roof overhangs and canopies will affect weathering, so consider their design carefully. If differential weathering is not desired, consider treating the cladding or using a different material at transitions
  • Provide a well vented cavity behind rainscreen cladding, with heavy duty insect mesh at top and bottom and at window sills and heads. Use open-state horizontal cavity barriers to maintain ventilation
  • Keep the cladding a minimum of 150mm off the ground
  • Be sensible and realistic about the size of cladding boards
  • Allow for movement of timber – due to moisture for example
  • Ensure the cladding boards are fixed the correct side out
  • Use non-ferrous fixings that will not rust, react with timber and cause staining
  • Pay attention to all junctions, ensuring ventilation and water management is well resolved
  • Be aware of potential damage to timber from harmful substances from other building materials, e.g. run-off from certain metal roofs, or claddings located above timber
  • Pay attention to proximity to other buildings, boundaries or notional boundaries as this proximity may result in untreated timber not being accepted due to fire reasons
  • Blend timber shingles from different batches to avoid a patchy appearance (a process similar to blending of bricks)
  • Careful specification of quality selection of the timber cladding (visual grading)

Case study: Collaughtons Ash, Much Wenlock

architype cladding

Original cladding; (all images © Architype)

Callaughtons Ash is a 12-unit social housing Passivhaus development in Much Wenlock, Shropshire. Completed in 2013, the development sits comfortably in its rural surroundings, complemented by a natural palette of UK-sourced materials.

This includes clay roof tiles that have been quarried and fabricated within 25 miles of the site, lime render provided by local company Lime Green and UK-grown thermally modified hardwood cladding – all helping to address the Housing Association’s aims for a cohesive circular economy in Shropshire.

Cladding application

Thermally modified poplar was selected for its inherent durability and potential for even weathlly modified: it has been heated up to 210°C. This means that, no matter what the temperature or level of humidity, the wood demonstrates limited shrinkage or expansion.

Exposure to intense temperatures also means the wood becomes hardier and able to last outside for many decades, without being treated and without risk of rotting.

The cladding was designed in both horizonal and vertical arrangement, using 20mm x 95mm half-lap machined boards. It was fixed with a second fix nail gun rather than first fix gun, using a single 50mm stainless steel brad nail at each fixing point.

Usually this is not recommended but the stability of Brimstone makes this a possibility. This reduced the amount of visual damage at each fixing point.

Collaughtons Ash: findings

architype cladding

Current cladding, 2023

The weathering is very even, largely due to the smooth overall surface of the timber cladding (half-lap joints rather than board on board) and minimal roof overhangs. The original deep chocolate brown colour of modified poplar has been bleached to a pleasing warm grey.

A few localised areas of faint light green algae have appeared. These are mainly above the entrance canopies and along the bottom boards, near the ground level. There are some signs of light, surface insect damage, likely caused by wasps collecting cellulose for nest building.

Collaughtons Ash: specifications

Completion: 2018

Cladding species: UK-grown poplar (Populus nigra)

Specification: Thermally modified poplar (‘Brimstone’), supplied by Vastern Timber

Moisture content at time of fixing: 4–6%

Durability: EN350-2 Very Durable, Class 1

Method of cladding fixing: single stainless steel brad nail through each board to battens at 600mm centres; nails fixed flush with the
external faces of boards

Battens A (for horizontal cladding): 50 x 75mm vertical untreated softwood battens at 600mm centres

Battens B (for vertical cladding): a combination of 38 x 38mm horizontal untreated softwood counterbattens at 600mm centres fixed
to 38 x 38mm untreated softwood vertical battens at 600mm centres.

Note: TDUK recommend that softwood battens are either treated or species are selected based on their natural durability for the intended use-class.

Weathering and discolouration: make sure the client understands

Untreated timber cladding is often desired as an economical and low maintenance solution. However, the designers and clients need to be aware of weathering and discolouration.

All unprotected timber cladding will change its visual appearance over time. The discolouration can be uniform, which can be very attractive in terms of aesthetic qualities. Uneven discolouration can be considered less attractive, and is usually a result of specific design features, e.g. different roof eaves overhangs.

Algae growth can sometimes also develop in parts of the facades (particularly those exposed to higher levels of moisture or those facing away from the sun), but this is something that is hard to predict. Select a timber species that is durable and appropriate for its application.

Weathering and discolouration of untreated timber cladding is totally normal and expected, and should not affect the primary function, which is protection against rain. However, if differential weathering or discolouration is undesirable by the client, the timber cladding can of course be treated with various stains or paints. There are downsides to this: the initial cost of treatment, the ongoing cost of maintenance (re-coating), and potential issues of re-use at the end of life, if harmful chemicals are part of the cladding treatment.

Case study: Herefordshire Archives and Records Centre 

Original cladding

The Herefordshire Archive and Records Centre (HARC) in Rotherwas, Hereford, is a dynamic building with a clear and simple form. The building consists of two simple and distinct masses separated by a ‘buffer zone’ which is articulated into an entrance foyer, reception, information and display area.

The larger of the two masses, an unperforated concrete structural box with vented cedar shingle cladding, holds the main archive and records repository, forming secure accommodation over three storeys and providing thermally isolated and environmentally controlled conditions – tuned and optimised for archival storage.

The other mass is a lightweight 2-storey timberframed system. It houses the offices, public search rooms and education facilities. Specified for their graceful weathering patterns, the material palette is made up of high quality ‘natural’ materials, which sit in visual harmony with the landscape.

Cladding application

shingles architype

Current cladding, 2023

Cedar shingle cladding installed to the larger of the two volumes lends the building a warm appearance, softening the monolithic impact of the three-storey height. The shingles of varying widths have been fixed at 150mm gauge, providing a good variance in appearance. There are no roof overhangs.

Hereford Archives: findings

Weathering of the cladding has been very even to all sides. There are slight colour differences between facades facing different orientations. This is a result of different exposures to UV light and rain. A few localised areas of faint light green algae have appeared along the bottom three rows of cladding to the north façade, likely a result of higher moisture levels.

Herefordshire Archives and Record Centre: specifications

Completion: 2015

Cladding species: Western red cedar (Thuja plicata)

Specification: Grade No.1 Blue Label Certigrade XXXXX shingles

Method of cladding fixing:

  • Setting out: Lay to a lap bond with an overall even appearance.
  • Side lap to course below (minimum): 40mm
  • Gap between adjacent shingle/shakes: 6-9mm for shingles
  • Ends of course: Use wide shingles to maintain bond and ensure that cut shingles are as large as possible.
  • Top course: Head nail short course to maintain gauge.
  • Fixing: Nail each shingle 19-25mm from each edge. Heads of nails to be flush with face of shingles/shakes.
  • Nails: Zinc coated steel, stainless steel or silicon bronze.
  • Nail dimensions: Determine in accordance with BS 5534 to suit site exposure, withdrawal resistance and shingle/ shake supplier’s recommendations.

Battens: 32 x 50mm horizontal untreated softwood battens at 150mm centres.

Counterbattens: 50 x 100mm untreated softwood counterbattens at 450mm centres.

Note: TDUK recommend that softwood battens are either treated or species are selected based on their natural durability for the intended use-class.

Treatment (shingles): none