Tiles, shingles and slate can provide traditional and thermally efficient covering for external walls. Peter Mayer of BLP Insurance considers the options
Tiles, slates and shingles may be perceived as roofing components, but there is a long tradition of using the same materials as the weathering element for external walls. The 1991 English house condition survey, commissioned by the government, suggests that about one in 150 dwellings in England have vertical slate or tile hangings. With the drive to improve the thermal efficiency of existing homes, externally applied insulation combined with vertical cladding may offer an alternative approach to insulated rendering.
Slates and tiles
Clay plain tiles should be to BS EN 1304 for certainty about strength, geometric consistency and performance issues related to durability and fire resistance. Handmade clay tiles should demonstrate durability by meeting the BS EN 1304 requirements, at least for flexural strength, impermeability and frost resistance. The frost resistance criteria may be moderated for vertical hung tiles especially where the wall is protected by wide eaves overhanging. Concrete tiles for vertical wall hanging should be to BS EN 490.
The standard for natural slates is BS EN 12326. The quality or durability of different types of natural slates are not readily distinguished. Predictors of a trouble-free service life are: flexural strength, characteristic modulus of rupture, water absorption, oxidation potential and carbonate content. The most durable natural slates would have a service life of over 75 years.
Shingles have a smooth finish as they are sawn on both sides. Shakes are hand split with one side sawn afterwards to give one smooth face.
Western red cedar is commonly used for external cladding. It is naturally durable, with expectations of a 60-year service life when heartwood only is used. To ensure a long life, suppliers treat timbers with a preservative to BS 8417.
There are four grades of cedar shingles: the best is No 1 Grade Blue Label. Fixings should be stainless steel or silicon bronze. Shingles typically are rectangular, decorative shingles with shaped tail ends are available at higher costs. Care should be taken that acidic run-off from cedar does not adversely affect components downstream from the cladding. Where the external cladding is close to a boundary, additional fire-retardant treatment may be required.
Vertical cladding using small units hung on battens or sheeting may also be designed with a range of other materials such as: fibre cement slates, galvanised and coated steel, copper, zinc and stainless steel shingles. These systems may have an independent third party technical approval, which should give guidance on expected service lives.
Design and cost issues
The code of practice for slating and tiling, BS 5534, gives guidance on minimum batten sizes, preservative treatment to timber, nailing, minimum head lap for expected exposure, design and detailing issues.
Vertical claddings are often specified above ground floor to avoid risk of impact damage. The framework for vertical cladding generally comprises solid or engineered timber sections or galvanised steel or aluminium profiled sections. Where thick layers of insulation are incorporated, additional strengthening to the framework may be required.
The wall structure, including vapour control layers and underlays, should be checked to confirm that there is no risk of detrimental interstitial condensation (see BS 5250).
Claddings work on the rainscreen principle – the design should anticipate some water will penetrate the primary line of defence against driving rain. The design should allow the water to drain down the rear side of the cladding, typically a cavity of at least 19mm to allow for ventilation and drainage behind the cladding. For taller or complex buildings consideration may have to be given to the incorporation of suitable fire cavity barriers at compartment boundaries or openings.
Well-designed details at junctions, edges, openings and penetrations are critical to the long-term success of vertically clad external walls.
Biological growth such as moss, lichens and algae may appear on north-facing walls in low pollution environments where natural drying is slow.
BLP Insurance provides latent defects insurance for buildings, www.blpinsurance.com
- Other, Size 0 kb