The right roof for your house will be one that meets your design brief, performs well, and is appropriate to local environmental conditions. A roof that looks great but is wrong for the local climate may be short-lived, with serious consequences for the structure of your home. So whether it’s a new build or a reroof, choose carefully and seek advice from professionals.
Roofing materials: classification
Rooﬁng products are deﬁned as lightweight or heavyweight depending upon their classiﬁcation within the New Zealand Standard (NZS) 3604. A lightweight material is one that weighs under 20 kg per square metre when ﬁxed and ﬁnished on the roof. There are two widely used products within the light rooﬁng classiﬁcation; both use steel as the core – longrun steel and metal tiles. Other products that can fall within the lightweight code include bituminous shingles, ﬁbre cement tiles, wooden shingles and some slate products. Heavyweight materials range between 20 and 60 kg per square metre. They include concrete tiles and most slate products. Concrete tiles generally weigh around 45 kg per square metre.
For all types of rooﬁng NZS3604 ensures there are standards deﬁned for structural performance, ﬁxing standards, etc., thus all of the various mainstream rooﬁng systems can be considered safe. However, achieving these standards may only be accomplished through additional expense in timber and framing – hence it is essential to talk with your designer, builder or architect about the design and structural requirements for the various rooﬁng options available; it may be that they need to reinforce trusses, frames, etc. to achieve sufficient strength. They can show you the speciﬁc cost savings or penalties involved for each rooﬁng product.
Choosing roofing materials
The chief materials used on roofs include concrete and clay (terracotta) tiles; long-run steel (or other metal, such as copper); metal tiles; wooden shingles; and membranes (for ﬂat roofs).
Concrete and clay tiles require less maintenance than most other rooﬁng materials. They also perform well in windy zones, reduce airborne sound, and help provide effective thermal insulation, while allowing moisture vapour to escape. Water runoff from these tiles does not contain pollutants such as zinc or aluminium ions.
Long-run steel is made from coils of steel, usually but not always painted, that are formed into a variety of long-run proﬁles. They extend as one sheet with laps along edges and ridges. There are many more options for proﬁles than just standard corrugated iron. Select the right grade of metal for the location of your house – for instance, if you’re at a coastal location. Note that non-standard colours on colour-coated metal rooﬁng may have longer delivery times and cost more.
Metal tiles are individually pressed from blanks of primed or painted steel. They are available in a range of proﬁles designed to achieve different appearances on the roof, and there are various protective finishes. When coatings are applied after pressing, protection is high, as there is a reduced tendency for microcracking (where the paint cracks microscopically at bent edges and exposes the base metal to direct contact with the elements).
- Make sure the material will perform at the pitch of your roof; some tiles won’t perform at less than 12 degrees to the horizontal, for instance.
- If you’re building a new home, keep the roof as simple as possible; the more flashings and joins that are required, the greater the risk of leaks.
- Similarly, keep holes in the roof to a minimum! Flues, roof windows, chimneys, etc., will all need professional weather-proofing.