Wall claddings – residential
Like roof claddings, wall claddings can be broadly classified by their weight.
Lightweight wall claddings
Failure in a lightweight monolithic cladding in the Christchurch earthquakes. (BRANZ)
For a residential building constructed according to NZS 3604:2011 Timber-framed buildings, a lightweight wall cladding is generally considered to have a mass no greater than 30 kg/m². This includes a range of wall claddings commonly used in New Zealand, such as:
- weatherboard claddings made of timber, fibre-cement, PVC or aluminium
- metal claddings with various profiles, colours and finishes
- sheet and panel materials made from plywood and fibre-cement, with a variety of treatments and facings
- exterior insulation and finishing systems (EIFS)
- maximum 10 mm thick glass curtain walls.
Like lightweight roofing, because of their lower mass, lightweight wall claddings reduce the forces exerted on the structure and foundations when the building is subjected to lateral seismic loads.
During the Canterbury earthquakes, only minor problems were observed with lightweight weatherboard and modern sheet wall claddings. EIFS, a monolithic cladding that generally consists of polystyrene and a plaster coating, also performed well. Where cracking did occur, it was relatively simple to repair without removing or replacing the entire cladding system.
Sheet and panel claddings
Lightweight sheet claddings on houses tend to rely on the flexibility of the fixings to the frame to accommodate lateral distortions of the structure. Cracking may still occur at sheet junctions when the earthquake actions are strong, and this should be expected. Because they can form part of the structural bracing, when these systems are damaged, they require engineering input to adequately reinstate the original bracing capacity.
Medium and heavy wall claddings
For NZS 3604:2011 construction, generally, a wall cladding is considered medium if it has a mass greater than 30 kg/m² of wall area, up to a maximum allowable weight of 80 kg/m². Houses with wall claddings heavier than this and up to 220 kg/m² are classed as heavy. Claddings greater than 220 kg/m² must be specifically designed.
Medium-weight claddings include 25 mm thick stucco and some thicker autoclaved aerated concrete (AAC) panels.
Heavy wall claddings include:
- precast concrete panels, with or without additional facing materials such as natural stone or tiles
- brick and stone veneers on timber or steel framing.
Brick veneer cladding
Poorly secured brick veneer wall cladding can collapse in a large earthquake. Although this does not compromise the structure, it severely degrades the building’s weathertightness. (Andrew Buchanan and Michael Newcombe, Department of Civil and Natural Resources Engineering, University of Canterbury)
A brick veneer wall cladding consists of a panel of clay or concrete bricks tied to a timber or steel frame. The brick plays no part in the structural strength of the building, and it relies on the frame for lateral support.
Brick veneer cladding provides good weathertightness and needs little maintenance. However, historically it has not performed well in earthquakes, usually due to inadequate, corroded or non-existent fixings to the frame. Some poorly installed fixings can also pull away from the mortar joints with the same result.
Modern brick veneer installations must comply with NZS 4210:2001 Masonry construction: Materials and workmanship, which sets out the requirements for the materials and workmanship of clay, concrete and natural stone masonry. For the construction of masonry veneers, the requirements in this standard are to be used in conjunction with:
- NZS 3604:2011 Timber-framed buildings
- NZS 4229:2013 Concrete masonry buildings not requiring specific engineering design
- NZS 4230:2004 Design of reinforced concrete masonry structures.
Testing brick veneers
BRANZ shake table tests showed that both modern clay and concrete brick constructions, when correctly secured, perform well when subjected to earthquake shaking at a level consistent with a design-level event. Once the shaking stopped, cracks almost closed completely, and the brick veneer returned close to its original position. Any remaining cracks could easily and cheaply be repaired by repointing. The full study report is available on the BRANZ website.
BRANZ tests of 1 and 2-storey brick veneer walls concluded the cladding performed well when correctly secured to the framing. (BRANZ)
Even when exposed to extreme levels of shaking, more than three times the design level, tests showed that clay brick veneer can remain intact. However, at twice the design earthquake, concrete brick veneer began to disintegrate into separate bricks after 10 seconds of shaking followed by partial collapse of some parts of the wall.
In addition, tests showed that a well constructed brick veneer cladding provided a useful bracing resistance that could result in less damage to the interior of the building.
Securing brick veneers
Acceptable Solution E2/AS1 requires all masonry veneer wall claddings to be screwed to the frame with a flat tie that is embedded in the veneer according to NZS 4210:2001. This requires the ties to be contained within the mortar bed over the full contact length with a layer of mortar both above and below the tie.
Correct fixing of brick veneer wall cladding to the wall frame.
A field survey looked at the performance of tied masonry veneer homes following the Canterbury earthquakes. It found that the method used to connect the veneer to the structural timber framing is important for the earthquake performance of brick veneer.
The types of ties investigated included screw-fixed ties, nail-on ties, wire ties and other types such as plastic ties. The surveyors found that screw-fixed veneer ties performed better than other types, with the majority of dwellings with this type of fixing exhibiting either no or only minor levels of damage.