Timber Cladding

Timber Cladding – Choosing the timber species

If you’re using softwood, whether spruce (whitewood) or pine (redwood) you need good quality, slow growth timber, typically from Sweden and other Nordic countries.

Its moisture content should be between 16% and 19%.

Both pine and spruce need to be preservative pressure-treated to Use Class 3, even if you are going to paint them. Make sure the surface of the wood is thoroughly dry after treatment, otherwise any decorative coating won’t adhere well.

Other softwoods, such as Siberian larch, Douglas fir and Western red cedar may be used untreated, but you will need to specify heartwood timber (the sapwood excluded).

Many hardwoods, such as sweet chestnut, red louro and yellow balau can also be used untreated, as can modified timbers like Accoya™. In the latest version of EN350, European oak has been re-classified as Durability Class 2-4; to ensure oak is suitable for cladding, verify it meets Durability Class 2-3.

Some modified timbers and high tannin species, such as oak require special stainless steel fixings.

Moisture

Wood’s moisture content will change relative to its surroundings. Different species have different degrees of movement and this must be accounted for in timber cladding design.

Timber cladding should have a moisture content of 16% ± 3% at the time of installation. Wood expands across the grain, rarely along it. Oak, pine and spruce will change 1% in dimension for every 4% change in moisture.

Good design and installation practice will help minimize the effects of moisture:

• Use species that are dimensionally stable or with small to medium movement
• Use wood specifically processed for timber cladding
• Use eaves and overhangs to deflect rain – or flashing to protect the board tops
• Finish cladding at least 200mm from the ground or a horizontal surface. Where possible use a surface that diffuses rain, such as gravel
• Board widths should generally be 4 to 6 times board thickness (typically less than 150 mm)
• Design detailing must include measures that minimise water penetration
• T&G boards should have a tongue long enough to stay engaged if it shrinks
• If finishing with a pigmented coating, apply the basecoat and at least one top coat before installing

Styles can vary from one manufacturer to another, so always obtain samples and agree quality parameters before ordering.

If you need a bespoke profile or specific species, contact a member of the TDCA.

Only specify cladding manufactured under a recognised quality scheme such as the TDCA CladMark/ISO09001.

Thickness is largely determined by the profile required. Guidance is available in British Standard BS8605: External Timber Cladding Part 1: Method of specifying.

Direction of boarding

Timber Cladding can be applied in any one of three directions, which will have an impact on the methods of fixing as well as on the overall look of the façade.

Horizontal boarding

Available with overlapped or profiled sections.

Shiplap or feather edge boards should have a minimum 15mm overlap, and 2mm gaps between the up-stands.

Tongue and groove boards should have a maximum face width of 125mm, with a 2mm clearance above the tongue for expansion. Install with the tongue upwards.

Open joint boards should have an 8-15mm gap at the ‘water face’. Chamfered edges allow the boards to overlap slightly, reducing any exposure of the cavity.

Vertical boarding

Board-on-board, shiplap and tongue and groove profiles are used. Tongues need to be long enough to allow for slight movement of the timber so that open gaps do not develop with atural movement. The face width of tongue and groove vertical boards should not exceed 125mm.

The most versatile fixing method is board-on-board. Any overlap should be a minimum of 20mm.

Diagonal boarding

Overlapping boards are not suitable. Shiplap boards are advisable and should be fixed to battens. Use counter-battens to provide ventilation.