Going to the wall

Ric Oram reviews a new guide to choosing exterior wall claddings.

Developers are aware of the short-term and long-term economies of the building materials they can choose - and it is important that home-buyers and property investors are, too.

Cheaper materials might allow affordable entry into home ownership or investment, but their lifetime costs or demand for maintenance are usually higher.

Conversely, an owner-occupier might pay more upfront for maintenance-free materials for a leisurely domestic lifestyle thereafter.

The building code requires exterior claddings, for example, to last the lesser of either the specified intended life of the building or 50 years for a structural or bracing element, otherwise 15 years.

Most will exceed 15 years' life with good installation and maintenance and many, such as brick and stone, will match the life of the building.

While the initial and maintenance costs influence choice, a cladding's appearance, durability, strength and weight, fire and seismic resistance, acoustic qualities, a material's availability and the necessary installation skills needed are also important factors.

These are discussed in detail in the Building Research Association's recently published Selecting Wall Claddings, which follows a similar publication last year on roof claddings.

The book generally ignores the relative costs of claddings because that varies on the project's size and design complexity.

Brick: Low maintenance, an almost indefinite life, fire and impact-resistant and offering insulation from noise. It can be reinforced for wind or seismic resistance, but can be damaged by freeze-thaw cycles in cold areas.

Timber weatherboards: Durability varies according to the timber species. Weatherboards cannot be guaranteed to be weather-tight and require a back-up wind barrier. Light-coloured paint will reduce heat build-up.

Reconstituted boards made of wood fibre: Untreated shingles have a life of at least 10 years, but ready-primed weatherboards more than 25 years. They should be avoided for heavy impact areas.

Pl plywood: Treated will last 15 years, but more than 50 if it is treated to H3 and painted. It is a lower-cost material, light, quick to apply and has good impact resistance. Dark paint should be avoided.

Timber shingles: Usually applied to plywood at least 12mm thick and will last more than 50 years in dry conditions (but as little as 10 in the damp), and are not recommended for ground level as they can splinter on impact.

Fibre-cement sheets: These must be coated to keep dry and, if they are not used in high impact areas, will last for more than 50 years. They are light and quick to install, but not recommended in snow.

Glass fibre-reinforced cement: Will not rot and is durable. Suits large projects with a lot of panel repetition.

Concrete: Heavy, durable, can have a structural use, is resistant to noise, fire and impact. Shuttering makes pouring on site time-consuming and weather conditions can affect quality and delay progress.
Pre-cast overcomes such problems, with tilt-up panel construction cost-effective.

Concrete blocks: Well-built masonry has an almost indefinite life. However, single-skin blocks are not weather-tight and need a coating and, in cold areas, need thermal insulation.

Stucco and rendering: Solid plaster applied to a backing will not rot or burn and can last more than 50 years. Time is required for curing between coats and on completion, and three coats of acrylic latex paint are required to seal stucco's inevitable fine cracks. Not recommended where uneven foundation settlement is likely.

Expanded/extruded polystyrene or polymer-modified cement plaster: Can last 30 years and is good heat insulation.

Metal sheets: Usually fitted over plywood in flat, weatherboard, corrugated, trapezoid or trough profiles. They offer little thermal insulation, are dented by hard impacts, need regular cleaning and some need coatings to protect against corrosion.

Zinc/aluminium alloy-coated steel: This comes with warranties of 15 to 30 years, usually factory-painted which requires low maintenance. It should not be in contact with the soil.

Galvanised steel: Less durable than alloy-coated steel.

Stainless steel: Does not need painting (although glare might be a problem) because it does not rust and its lifetime is therefore almost indefinite.

Aluminium: Will last much longer than 15 years. It is more expensive, but lighter and more corrosion-resistant than steel. However, glare from unpainted sheeting might be unacceptable.

Copper: Corrosion-resistant, with an almost indefinite lifetime, and can be formed into complex shapes.

Zinc: Almost indefinite lifetime, durable, can be left uncoated, but needs good ventilation to prevent condensation on its underside.

Glass: No longer confined simply to windows, it comes tinted, reflective, toughened, heat-strengthened, laminated, thermal-insulated, sound-control, opaque, curved, figured, wired or in blocks. Blocks cannot be load-bearing, and consideration must be given to access for cleaning and maintenance.

Ceramic tiles: Depending on the temperature they are fired at, they can be either impervious to water or vitreous. Good quality will last more than 50 years.

Stone: One of the most expensive claddings, it is often now available as a veneer to be fixed to a structural wall. Durability varies according to type. It is not recommended where foundation movement can be expected.

The relative merits and uses of polycarbonate, acrylic, polyvinyl chloride and glass-reinforced plastic sheeting are also discussed.

* Selecting Wall Claddings - a guide to choosing wall claddings materials, $29.95, Branz Publications, 0800 80 80 85, Private Bag 50908, Porirua City.