Refurbishment:
Timber and tile or slate cladding

Ideally, insulation should be fixed back to the existing wall using a complete coat of adhesive so as to avoid the possibility of air gaps occurring. Some designers express a preference, when using a frame system, for using insulation batts that are more flexible than rigid – so as to form a tight fit around the frame.
Alternatively, by adding a vapour-permeable sheathing board between the frame and the breather membrane, a kind of container is formed into which loose fill insulation can be poured (see image top right).

Typical U-values*

Cladding materials

Tiles and slates

• Tile hanging using concrete or clay tiles, natural or fibre cement slates provides for a durable and low-maintenance cladding method.
• Tiles or slates are fixed to horizontal timber battens (min 38mm) which are themselves attached to the supporting framework.

Timber

• Timber is the most sustainable cladding material: renewable, recyclable and with low embodied energy.
• Timber cladding can offer a very wide variety of textures, colours and patterns.

Designing with timber cladding

• All timber cladding should be designed along with the presumption that water will penetrate the cladding. Hence consideration should be paid to detailing using a drained cavity backed up by a breather membrane along with carefully designed flashing around openings.
• In most circumstances, cladding is fixed to either horizontal or vertical timber battens (min 38mm). Horizontal boarding has a built-in cavity, but vertical and diagonal boards require the addition of further vertical battens (min 19mm) attached to the supporting framework.
• Water absorption and movement should be allowed for in selecting the cladding type. Systems dependent on interlocking elements (eg t&g), should be used with care to avoid distortion.
• At ground level, ensure that there is an adequate distance between the ground and the bottom of the cladding to avoid the ‘splash line’. The timber should terminate at least 150mm above where risk is minimal, but up to 250mm provides better protection.
• The staining and coating of timber can be avoided by using durable timber (see below)
• All untreated timbers go grey over time – ensure the client understands this.
• Generous eaves: where possible, provide generous eaves – 600mm is the optimum depth.
• Allow for drainage and ventilation. Water penetrating the cavity should be given ample opportunity to drain from the foot of the cavity or evaporate through provision of ventilation.
• Timber boards should be at least 150mm wide
• Battens should be fabricated from durable timber to avoid use of timber treatment.

Types of timber cladding

• Horizontal boards: Square Edge, Feather Edge, Rebated Feather Edge, Shiplap and Horizontal T&G profiles
• Vertical boards: Overlap or T&G profiles are most suitable
• Diagonal boards: Shiplap profile
• Shingles: Cedar or Oak

Species

A species natural durability is rated by BS EN 350. Naturally durable species suitable for untreated cladding are:

• Western red cedar is a ‘durable’ timber from North America. It may be coated with oil to maintain its appearance, with the associated regular costs of re coating.
• European Larch is ‘moderately durable’. UK-grown larch, larch from Siberia or regions where trees are slow-growing and older than 60 years have a similar expected service life with a similar age and growth pattern
• European oak is a ‘durable’ hardwood. Oak is more expensive and needs to be specified carefully to minimise its tendency to warp