Timber Cladding: Technical Design: RIBA Plan of Work Stage 4 - Swedish Wood

Timber cladding should provide adequate weather protection but not all wind driven rain would be deflected.

Free draining cavity

So, a well ventilated free draining cavity should always be included in the detailed design. This should be a minimum of 21mm whether for a solid masonry or timber frame structure.

However, the more open the cladding style the wider the cavity required. On timber frame buildings, the minimum sized batten (21mm) may be used, so long as its position coincides with wall studs.

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Battens

Specifying softwood battens that have been pressure treated for a used class 3 application. Battens should be at least twice the thickness of the boards, battens are normally spaced at 600 millimetre centres. For high moisture content species like green Oak use 400mm centres. Use double battens where two boards are but ensure horizontal battens don't impede ventilation and drainage of the cavity. Use counter battens to improve ventilation and ensure that the top surface is of horizontal battens are tapered to a drainage away from the cladding.

Insect Mesh

All openings into the cavity should be fitted with insect mesh.

Breather Membrane

A breather membrane is not essential for cladding fixed to a masonry building with cavity walls.

Refurbishment - Waterproofing

Where cladding is fitted to an existing building with solid walls, the walls should be given a waterproof coating, membrane or wax treated insulation board.

Timber Frame

For timber frame properties, the inner wall structure should be fitted with a durable and tear resistant breather membrane, in accordance with Type 1 membranes in BS 4016.

Installing Horizontal Cladding - New Build

This drawing is for horizontal cladding fixed to a timber frame structure. For a masonry wall the same batten and cladding configuration is used.

Installing Horizontal Cladding - Existing Buildings

Cladding may be fixed to an existing property in one of three ways.

1. To cladding battens and counter battens over secondary battens or studding attached separately to the wall.
2. Or to cladding battens that are fastened directly to the outer wall through the vapour barrier and non-compressible insulation using special fixings.
3. Or where there is concern about additional loads being attached or the wall is uneven or out of true, the cladding may be fixed to battens attached to a self-supporting treated timber frame.

When not using a proprietary clip fixing system these are the fixing methods for different profiles.

Use the table for design detail recommendations for boards up to 145mm in width. All dimensions are minimum sizes. 

External Clading Application Tongue and Groove  Boards

T and G boards should always be installed with the tongue uppermost. Only specified T and G boards that have been manufactured for external cladding applications, with a minimum tongue size of 10mm or ideally 12mm.

For open jointed cladding, the gap between boards should be no more than 10mm to prevent UV degradation of breather membrane material. Specify a gap of at least 3 millimetres when installing horizontal profiles to allow for movement.  The timber should have a moisture content of from 16% to 19%.

Nail Flush

If you are nailing through the face of the timber, ensure the nail sits flush with the surface. For most softwoods use small headed annular ring shank nails, larger heads are used for low density species such as Western Red Cedar to avoid the nails being pulled through. Higher density softwoods such as Siberian larch and Douglas fir and hardwoods should be drilled and screwed to avoid splitting. For high tanning species such as Oak use stainless steel fixings to avoid staining.

If you are using green timber such as Oak, drill oversize fixing points and use washers to allow for movement. Nails and screws should be a minimum 2.5 times the board’s thickness.

Installing Vertical Cladding - New Build

These drawings of a vertical cladding fixed to a timber frame structure. For masonry walls the same batten and cladding configuration may be used. Counter battens are an essential design detail for vertical cladding to permit unrestricted drainage and air circulation in the cavity. Counter battens must be at least 16mm thick. Cladding support battens should be at least twice the thickness of an individual board.

The top edge should be machined prior to preservative treatment, to an angle of 15 degrees, sufficient to shed water running down the back of the cladding into the ventilation cavity. Battens should be preservative treated to use class 3. A cavity of at least 21mm is required to permit air circulation and unrestricted drainage. All openings into the cavity should be fitted with insect mesh.

Installing Vertical Cladding - Existing Buildings

Cladding may be fixed to an existing property in 1 of 3 ways.

• To cladding battens and counter battens over secondary battens or studding attached separately to the wall.
• Or to cladding battens that are fastened directly to the outer wall through the vapour barrier and non-compressible insulation using special fixings.
• Or where there is concern about additional loads being attached or the wall is uneven or out of true, the cladding may be fixed to battens attached to a self-supporting treated timber frame.

Solid Walls

Solid walls must be protected from water penetration by either a water repellent coating or breather membrane. Fitting a breather membrane between cladding battens attached to a cavity wall structure is not essential where water repellent insulation is used.

Battens

Battens should be at least twice the thickness of the board profile. They must be preservative treated to use class 3. Battens at 400mm centres should be used for diagonal cladding or where boards with high moisture content are to be used, green Oak for example.

External Corners For Horizontal Cladding

Corner cover strips are available in solid section or preformed profiles as shown in figure 4B and 4C. Figure 4C also includes machined grooves on the reverse face to prevent the movement of moisture by capillary action.

Leave an expansion gap between the board ends and the corner finishing detail. The size of this gap may vary depending on the type of wood being used. Traditionally this gap is between 8mm and 10mm but, when a low movement modified wood is used it can be 4mm-6mm.

Leave a 3mm gap between the upstand and/or rebate of horizontal cladding to allow for expansion.

External Corners ForT&G Vertical Cladding

Here is a diagram showing the details for external corners for T and G vertical cladding.