When considering new or replacement roofing, there are some critical decisions to be made that include performance and cost.
Geographical location, environmental conditions and good design are key factors when considering roofing options, as are durability, longevity, maintenance and the pitch of the roof.
Getting all the elements right the first time could save thousands down the line.
There are many factors to consider when pricing a roofing project. Each home is different, and variations on costs can occur for many reasons, including site access, elevation, scaffold hireage, and unusual roof shapes.
It should be noted that for all roofing jobs a scaffold or edge protection is mandatory. This can add a significant cost to the project, so make sure this cost is factored in up front.
If you are changing a roofline or an existing roofing product on a house you must apply for a building consent.
Conversely, no building consent is necessary if a “like-for-like” roof cladding is being installed, or for repair work or normal re-roofing where a roof is more than 15 years old.
The traditional galvanised corrugated iron has largely been superceded by zinc/aluminium alloy-coated mild steel. It’s lightweight, easy to install, and comes in a wide range of colours and profiles.
While each design profile will have specified minimum slopes, some metal roofs can be laid on as little as a 3° pitch.
As a simple rule of thumb, if you can see the sea from your house it’s best to use an aluminium substrate. Avoid using copper/brass and stainless fixtures with galvanised steel, zinc/aluminium coated steel or pre-painted steel as water travelling from copper or brass can hasten corrosion.
Metal roofing materials also come as pressed tiles. They’re typically made from G300 grade zinc/aluminium, alloy-coated mild steel, formed into individual shingles or tiles, or into modular panels that mimic a row of shingles or tiles, with a natural stone chip protective coating or a pre-painted finish. They are suitable for roof pitches over 10°.
Butyl rubber membranes are suitable only for flat roofs (between 1° and 10°). Membranes can also be used on curved roofs.
They are lightweight, tensile, easy to repair, and resistant to UV rays, ozone and weathering. Some products are 100% recyclable, thus environmentally-friendly.
Butyl or the wider EPDM rubber is fitted in two sheets over a plywood or concrete substrate. They can be lap joined by rolling, or “welded” using hot air.
Torched-on bitumen products have elastic polymers and a fibreglass component for extra strength, are thicker, and are joined with a naked flame, leaving a seamless finish.
These membrane sheets range from 2.1 to 6 metres in width. Larger sheets reduce the amount of seams and the cost of labour.
Concrete and clay
Concrete and terracotta clay tiles are extremely durable, require less maintenance than most other roofing products, and are conducive to all environments and steep pitched roofs over 10°. A wide range of tile colours and profiles is available.
Concrete and clay tiles do not rust, warp or corrode. They are impervious to frost and ice, and can handle high winds.
Another plus is that they are up to 30 decibels quieter than iron, which is handy in high density housing situations. The downside is that they are 30% heavier than iron/metal, so extra truss costs should be factored in.
Concrete tiles are not suitable for curving roofs, and walking directly on the tile is not recommended.
The thermal mass of concrete roofs will effectively reduce heat loss. The laps in a concrete tile roof allow the roof space to breathe so moisture can escape. They are naturally inert so, therefore, any drinking water collected from them will not contain zinc or aluminium ions.
Slate tiles are hewn from actual stone. They have been used for centuries all over the world and, as such, have stood the test of time in saltwater locations and extreme temperatures.
Slate roofing meets building code requirement in all corrosions zones. It provides good fire protection, is low maintenance, resistant to rot and insects, and can last up to 400 years.
Available in different sizes and colours, slate is very heavy and requires expensive extra support. It’s suitable for roofs with a minimum pitch of 25°.
A lighter weight and cheaper alternative to slate is synthetic slate. It’s made from 80% post-industrial recycled rubber and plastics.
Synthetic slate tiles are at least as strong and possibly more durable than traditional slate tiles.
They meet the Class C fire resistance code, and can sustain winds up to 160 km/h. They require a plywood substrate, and are suitable for steeper pitches.
Asphalt shingles are one of the fastest growing roofing choices on the market. They are made from fibreglass-reinforced asphalt, with non-combustible fibres and a ceramic-coated metal or stone chip surface.
Resistant to corrosion and rot, they can weather harsh conditions and are generally low maintenance. Some heavier weight shingles boast warranties of 25-plus years.
A 15° to 18° pitch is recommended as a minimum, but with special installation they can be laid to pitches as low as 10°.
They are installed over a plywood substrate (with stainless nails and a bitumen-based adhesive/sealant), and can be physically bent into shape to provide workable solutions for complex roof profiles.
During installation, temperatures need to be at least 10°C or they can become brittle and crack.
Timber shakes and shingles are similar products cut from treated or naturally durable timbers.
Shingles are tapered with relatively smooth front and back faces, while shakes have a more textured, rustic surface.
While most timber shakes and shingles are imported western red cedar, locally-produced, first grade ACQ-treated radiata pine products are also available.
Both provide low maintenance durability with low thermal and moisture movement properties.
Timber shakes and shingles may have a limited lifespan of between seven and 10 years in damp conditions, but will last much longer in a drier climate.
They can be laid on 18° or steeper pitches (25° to 30° pitches are best for effective water dispersion).