Timber Cladding: Species, Treatment & Cost

Timber cladding brings natural variation, texture and warmth to elevations that brick or render cannot match. It is also among the most labour-responsive materials: a well-chosen, well-detailed species can last for decades with little maintenance; a poorly treated softwood in a damp exposure zone can fail in a decade. Species, profile, fixing method and initial treatment together determine how the cladding performs over its life.

Species Overview

Treated Softwood (Scots pine, Spruce, Redwood)

European softwood, typically Scots pine (Pinus sylvestris) or Spruce, is the most cost-effective option. Untreated, its heartwood is rated only slightly durable (Class 4) under BS EN 350, and its sapwood is usually regarded as not durable (Class 5), so it deteriorates rapidly in exposed conditions.¹² To achieve serviceable life as external cladding, softwood must be pressure-treated with a suitable preservative to achieve Use Class 3.2 (above ground, repeatedly wetted) per BS 8417:2024 (preservation of wood, code of practice).³

Tanalised or Vacsol-treated softwood in UC3.2 is widely available and competitively priced. It performs well when details are correct (ventilated cavity, back-primed before fixing, all end-grain cuts re-treated on site), but it must be decorated, typically painted or opaque-stained, and redecoration is needed every 3–5 years in moderate UK exposure.

Cost: roughly £15–£35/m² supply; £50–£95/m² installed (including battens, breather membrane, fixings).

Western Red Cedar (Thuja plicata)

Imported western red cedar is rated durable (Class 2 under BS EN 350), with natural extractives that resist decay without preservative treatment.⁴ Home-grown British western red cedar is rated only moderately durable.⁵ It is dimensionally stable and lightweight, approximately 390 kg/m³, which makes it easier to handle and fix than hardwood alternatives.⁴⁵

Cedar weathers to an even silver-grey within 1–3 years if left untreated; this silvering is a surface oxidation process that does not compromise the wood’s structural integrity. Many architects specify untreated cedar precisely for this weathered appearance, which eliminates ongoing decoration costs. If the natural colour is preferred, a UV-stable oil or clear finish requires reapplication every 4–6 years.

Cedar is well suited to the UK climate, performs in moderate-to-severe exposure zones, and, unlike softwood, does not need pressure treatment. Its main disadvantages are cost and supply chain: most UK cedar is imported from Canada and the USA.

Cost: roughly £50–£90/m² supply; £85–£150/m² installed (estimates).

Siberian Larch (Larix sibirica)

Siberian larch has become the mainstream ‘quality’ cladding timber in UK new-build and self-build, largely replacing western red cedar on cost grounds. Its heartwood is rated moderately durable (Class 3), with a tight grain and resin content that gives better weathering resistance than European spruce.⁶ Slow-grown Siberian larch weighs about 680–700 kg/m³ when dried, and plantation-grown stock is lighter, so it is noticeably heavier than cedar.⁶ It tends to split when nailed, so pre-drilling is recommended, particularly at board ends.⁶

Larch weathers similarly to cedar, progressing from honey-gold through orange-brown to silver-grey over 2–4 years. It can be left untreated or finished with an oil for colour retention. Movement on drying is moderate; well-dried stock minimises gapping and cupping.

Siberian larch from FSC-certified sources is widely available in the UK; European larch (Larix decidua) is a domestic alternative with similar properties and marginally lower cost.

Cost: roughly £35–£65/m² supply; £70–£125/m² installed.

Thermally Modified Timber

Thermal modification (trade names include ThermoWood and Platowood) heats timber to around 190–230°C in the presence of steam, breaking down the hemicellulose sugars that decay fungi feed on and reducing equilibrium moisture content. The process transforms naturally non-durable species (ash, pine, spruce) to durability Class 2 (durable) or better without chemical preservatives.⁷ Lunawood’s durability-class product is modified at 212°C and carries a BRE endorsement for an expected service life of 30 years as cladding.⁷ Thermo-ash and thermo-pine are the most common UK-market products.

Thermally modified timber is darker in colour (mid-brown to chocolate, depending on temperature), more dimensionally stable than untreated timber, and has slightly lower density. It is particularly valued in Passivhaus and low-VOC projects where chemical treatments are undesirable.

The main risk is brittleness: thermal modification reduces bending strength compared with unmodified timber. ThermoWood is not sold as strength-graded timber and must not be used for load-bearing structures.⁸ Fixings are usually pre-drilled to avoid splitting.

Cost: roughly £45–£90/m² supply; £80–£150/m² installed (estimates).

Profiles

Cladding profile affects drainage behaviour, shadow lines, and the total quantity of timber required per m² of elevation.

Featheredge and shiplap are the traditional UK choices for horizontal cladding; open-joint vertical boards are common in contemporary architecture and allow faster drying of any penetrating moisture.

Weathering and Silvering

All timber cladding weathers if left untreated, and the rate of silvering depends on UV exposure, rain washing, and species. On south-facing elevations, silvering can occur within the first year; on north-facing walls, the same process may take several years. Uneven silvering on mixed-orientation elevations is a common complaint.

To accelerate uniform silvering at installation, a dilute iron-tannate solution (‘accelerated weathering’ wash) can be applied. Alternatively, an iron-oxide pigmented wood oil in a silver-grey tone gives an immediately consistent appearance.

If maintaining natural colour, penetrating oil-based finishes (Osmo, Sikkens Cetol, Rubio Monocoat etc.) are preferred over film-forming paints. Oils breathe with the wood; paint traps moisture beneath a film and eventually peels.

Fixings

Western red cedar is an acidic timber that can corrode ordinary steel fixings in damp conditions and cause black iron staining, and tannin-rich timbers such as larch and oak stain in the same way.⁵ Use stainless steel fixings throughout: A2 (304) stainless suits general external conditions, upgraded to A4 (316, marine grade) in coastal, highly exposed or polluted locations.⁹ Coated or plated fixings corrode, staining the boards and failing early.⁹

Hidden fixing systems (clips that engage the board profile from the rear) give a clean face without exposed fastener heads, but must allow for thermal and moisture movement: boards should not be pinned rigidly at both ends.

Back-priming all boards before installation seals the rear face and reduces moisture cycling, which is the primary driver of cupping and splitting.

Ventilated Cavity

All timber cladding requires a ventilated cavity between the cladding and the breather membrane or substrate. The battens create the cavity, and cladding suppliers recommend primary battens at least twice the thickness of the cladding board, so 20 mm boards need battens of at least 40 mm, with continuous airflow at the top and bottom of the cavity.¹⁰ The cavity allows moisture that penetrates gaps or joints to drain and evaporate; without it, cladding remains wet for extended periods and decay is accelerated even in naturally durable species.

Cost Comparison Summary

Supply prices are 2026 UK merchant prices per m² excluding VAT (softwood and larch widely available; cedar and thermowood from specialist merchants), and the cedar and thermally modified figures are estimates. Specialist suppliers quote cladding boards at roughly £25 to £90 per m² plus VAT, depending on species and profile.¹¹ Installed costs add roughly £10–£15/m² for counter battens, breather membrane and stainless fixings and £25–£45/m² for labour, but not scaffold.

Relevant UK Standards and Guidance

• BS EN 350:2016, durability of wood and wood-based products: testing and classification of natural durability (it replaced BS EN 350-2)²
• BS 8417:2024, preservation of wood: code of practice, use class specification (the 2011 edition was withdrawn in August 2024)
• BS EN 335, durability of wood: use classes (UC 1–5)
• NHBC Standards Chapter 6.9, curtain walling and cladding
• BS 1202, nails for use with wood (material specifications)
• Building Regulations Approved Document B, fire spread: on a house up to 11 m high, external wall surfaces less than 1 m from a relevant boundary need Class B-s3, d2 or better, which untreated timber cladding typically does not reach, and taller residential buildings need Class A2-s1, d0¹²

For most UK self-builds and renovation projects in moderate exposure zones, Siberian larch represents the best balance of cost, durability and low maintenance. Cedar remains the premium natural choice where budget allows; thermally modified timber suits contemporary projects where chemical preservatives are undesirable and weathering to a dark tone is acceptable.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.