All residential roof claddings can be divided into two broad groups – lightweight claddings and heavy claddings.
Lightweight roof claddings
Generally, for residential structures built according to NZS 3604:2011 Timber-framed buildings, a lightweight roof cladding is considered to have a mass no greater than 20 kg/m².
A wide variety of lightweight roof claddings that meet this criterion are available in New Zealand including:
- steel, aluminium or copper sheet steel with various profiles and coatings
- metal tiles
- asphalt and fibreglass shingles (usually fixed to a backing sheet)
- sheet membranes on plywood sheet.
A structure with lightweight roof claddings has less earthquake load on it than one with heavy roof claddings. The lower mass generates smaller inertial forces when the structure is exposed to lateral seismic loads. Lightweight roof claddings also require less roof bracing to resist gravity and seismic loads.
During the Canterbury earthquakes, no significant problems were observed with lightweight long-run metal roofing or pressed metal roof tiles – two of the most widely used lightweight roof claddings in the country.
As a lightweight cladding contributes to lower bearing pressure at the site, lightweight cladding materials should be promoted whenever there is the potential risk of liquefaction, as this facilitates repair should such an event occur.
Heavy roof claddings
For NZS 3604:2011 structures, a roof cladding is considered heavy if it has a mass greater than 20 kg/m² but less than 60 kg/m2 of roof area. Heavy roof claddings include:
- concrete and clay tiles
- slate and stone.
While heavy roof claddings do pose additional challenges, they can perform well if correctly designed and detailed during installation.
Tile roof claddings
Heavy concrete and clay tiles did not perform well in the Canterbury earthquakes and were vulnerable to being displaced or dislodged unless they were sufficiently fixed to the frame of the building. The effects ranged from no damage through to almost complete loss of the tiles. Clay roof tiles were nearly always dislodged to varying degrees, with many roofs losing their weathertightness as a result.
The failures were caused by a variety of factors. These included inadequate design where fixings were not specified, inadequate maintenance where fixings had corroded or been removed or inadequate workmanship where fixings were specified but never installed. One aspect of the ground movement during the February 2011 Christchurch earthquake – a vertical acceleration in excess of 1 g in combination with high lateral displacements – caused unsecured tiles to lift off the roof framing and either dislodge or fall from the roof entirely.
Securing tile roof claddings
All tile roof claddings must be adequately fixed to the structure below to prevent them being displaced if the roof framing distorts under lateral loads or if high vertical seismic accelerations occur.
At least every second tile (every tile is even better) should be fastened to the battens using wire or one or more metal clips, nails or screws. Some tiles use proprietary fixing systems, and it is important to use the correct type. To remain effective, fixings must also be maintained and not allowed to become loose, rusted or broken. Damaged fixings should be replaced with the same style of wire, clip, nail or screw.