Roofs and cladding
Correctly designed, installed and maintained roof and cladding systems keep your home weathertight.
This means they need to be able to stand up to high winds and heavy snow and rain falls.
Roof structural support the roof covering. For domestic construction, the structure is usually made of timber or steel framing.
Sometimes a roof structure is constructed on-site by the builders but often it is prefabricated with the timber or steel trusses transported to the site. This has several advantages:
- trusses are designed by engineers or by software programmes designed to comply with Building Code requirements.
- trusses are accurately cut and securely assembled
- the roof is completed quickly and builders can work under cover
- waste is minimised.
Roofs should slope to allow water to drain off them.
Gently sloping roofs are safer than steep roofs when people need to walk on them. However, a steeper slope is necessary if heavy snow falls are possible. This can make the roof more expensive to construct because it requires more material and possibly a stronger structure.
These overhang the walls and provide some weathertightness protection from rainfall. Wider eaves also provide shade during hotter summer months.
Gutters are fixed at the bottom of the roof slope to catch water from the roof. They carry the water to downpipes which lead to stormwater drains or rainwater storage tanks.
Roofs should overhang the sides of the building so that any overflow from gutters falls to the ground. Some buildings have gutters in the middle of the roof or hidden in some way at the top of the walls. These internal gutters are frequently associated with leaky homes as they can be a source of leaks if the gutter fails or gets blocked.
A leak in a roof sometimes shows up when water starts dripping into the rooms below or stains appear on the ceiling or wall. The water may enter in one place, move sideways a long distance, and then find a hole to drip through.
Or the water may pool in the ceiling, rotting timber and causing damp, but not dripping through.
Wind-driven rain from certain directions may find gaps that vertical rain doesn’t penetrate. This can be a major issue around poorly flashed skylights and gently sloping roofs.
Finding the source of a leak may be a difficult job and any leak is potentially expensive.
Flashings are strips of metal or other material that cover joints and gaps where water can get in. They are designed to stop water entering the roof and to track water away from the roof penetrations (for example, skylights or vents).
Flashings must be installed correctly or they don’t perform effectively.
Flashings must be installed at the gables, hips and barges of a roof, and around any pipes or chimneys that penetrate the roof.
There are different profiles of flashings to suit the various places they are used.
Allowance must be made for thermal expansion and contraction of flashings, and, where they are joined, the pieces must overlap.
Many roofing systems, for example, tiles roofs rely on overlapping cladding components to keep the water out. If there is too much water, or in very windy conditions, water may enter through the gaps.
If wind-driven rain is likely to be a problem the roof must be specifically designed to prevent this.
The roof covering can be made of:
- profiled metal roofing (e.g. corrugated)
- pressed steel tiles
- clay or concrete tiles
- synthetic butyl rubber sheet
- shingles or slate
- bitumen membrane
- profiled impregnated wood fibre sheets.
If you want to use water from the roof for household use, check the roof system is compatible with water collection.
Roofs are not generally designed for foot traffic, and there is usually at a height of at least 3 metres to the ground, they are best left to trained roofers.
If you need to walk across a roof, take extreme care and walk where there are visible fastenings (as there will be rigid battens / purlins underneath the cladding). Do not walk up valleys as they are easily damaged and leaks could result.
Metal is very strong and comparatively light. A metal roof can be installed very quickly.
Title, shingle or slate roofing systems rely on overlapping components to keep the water out.
Domestic metal roofing has been traditionally coated with zinc (galvanised iron). Plain zinc-coated steel needs to be painted every few years to prevent rust.
These days a zinc/aluminium alloy is the most common base material for domestic roofing.
Early pressed metal tiles with stone chips may need to be re-coated from time to time to prevent lichen becoming established.
Clay or concrete tiles
Clay or concrete tiles are heavy and need additional framing.
Tiles overlap each other to keep the water out.
Tiles may need to be sprayed from time to time to prevent moss and lichen growth.
Synthetic butyl sheet
Synthetic butyl rubber sheet is used for roofing of commercial buildings and sometimes in domestic situations. It is usually laid on a plywood base.
It is flexible and resistant.
It can be used on roofs of different shapes, including flat roofs.
It should be applied by a specialist and must be applied by an accredited installer for warranties to be valid.
Shingles can be made of materials such as asphalt, concrete, clay, metal, butyl rubber or timber.
Shingles overlap to keep the water out, so the roof pitch must be over 15 degrees. It will generally require underlay to comply with manufacturers' specifications.
Sheet membrane roofing is often used on low-slope roofs, including decks or balconies that form the roof of a floor beneath. Maintenance of membrane roofs is often crucial to ensuring your home remains weathertight.
Bitumen-impregnated cellulose fibre
Bitumen-impregnated cellulose fibre is a pre-finished sheet product used in both wall and roof claddings. It generally comes in a corrugated profile and is lapped and fixed in a similar way to traditional tile roofing.
The surface can weather and may need repainting to the manufacturer's specifications, using a bitumen-compatible water-based paint.
Old roofs (before the mid 1980s) were sometimes made from asbestos cement sheets. These look like a thicker version of corrugated steel.
Do not walk on an asbestos cement roof as it is brittle and may crack or fail without warning.
Asbestos cement can be hazardous if it starts to weather. Contact a roofing specialist if you want to demolish or remove an asbestos cement roof as there are occupational health and saftety requirements which must be complied with. Do not waterblast asbestos products.
Asbestos roofing can continue to be durable when well encapsulated in a coating system.
Wind damage to roofs
Roofs are the part of the building most exposed to the wind. In severe storms, roofs may be damaged or lift in the wind.
For these reasons, it's important that roofs are designed and built to withstand strong winds.
The wall cladding is the primary defence against the weather.
Some wall cladding options are:
- fibre cement
- masonry veneer
- monolithic systems
This is the traditional cladding material for New Zealand homes. Weatherboards are usually shaped planks fixed horizontally and lapped over each other. Rainwater drains down the outside and can only get inside if it is forced upwards between the boards. it then runds down the backs of the weatherboards, exiting the wall at the bottom.
As well as timber, weatherboards can be made from materials such as fibre-cement, metal and vinyl (PVC).
Fibre cement exterior wall coverings come in the form of panels and weatherboards. Vertical sheet joints can be made with plastic jointers or covered with vertical timber battens. It is important that manufacturers’ jointing and flashing recommendations are followed.
Where the sheets have flushed joints and a plastered surface, the cladding is known as a monolithic cladding (see Monlithic systems).
Plywood panels may be used as cladding. Gaps are covered with battens or flashings. You can also get plywood weatherboards.
Masonry veneer is a system where a timber or steel-framed home is clad with bricks, stone, or thin concrete blocks. The masonry is connected to the timber framing through flexible wall ties.
Concrete blocks or poured concrete may act as both the structure and the cladding.
Monolithic cladding systems have a seamless appearance. They have become popular in recent years, but have to be designed and applied properly to manufacturers’ specifications or they will leak. The ‘leaky home’ problem is principally associated with monolithic claddings, where claddings were installed without appropriate jointing / flashing details and appropriate coatings. Ongoing maintenance is essential.
The traditional monolithic system is stucco or solid plaster. Cement-based plaster is applied over a variety of backings including fibre-cement and plywood sheetings with mesh. It is then painted. This is the oldest of the three types of monolithic cladding and has been used in New Zealand since the 1920s. Exterior Insulation and Finish Systems (EIFS) are multi-layered systems, using polystyrene boards with a plaster and paint finish. There are several different proprietary systems available. Fibre cement sheet has also been widely used as a monolithic cladding.
All monolithic claddings rely on the final coat for waterproofing, and this needs to be well maintained. Today, these systems are now required to be fixed over a 20 mm drained cavity which makes the system more robust.The ConsumerBuild website has more information about monolithic cladding systems..
Other features of wall construction
Drained and vented wall cavities
Some cladding systems work on the assumption that some water will inevitably penetrate the outer skin of the building. A cavity between the outer wall covering and the interior lining allows water to drain away through drain holes and air to circulate.
With most types of cladding, in all but low-risk situations, a dry cavity is now required under the Building Code.
Wall underlays or building paper
Building paper and synthetic wraps are designed to help limit entry of wind and moisture to the wall or roof cavity, and they provide temporary weather protection during early stages of construction. They also keep any moisture that gets past the cladding from direct contact with framing timber and insulation until it can drain and dry
Flashings are strips of metal or other material that cover joints and gaps where water can get in. They are used around window frames, external doorways, and on top of exposed walls, to help stop water getting in, and help to drain it out.
From Smarter Homes
- The Building Act
- Leaky buildings; this includes information about building for weathertightness and a section on how wall claddings contribute to leaky homes.
- Exterior maintenance: roofs
- Exterior maintenance: exterior walls
From other sites
A building consent from a building consent authority (usually your local council) is needed for all new houses and most alterations and additions. The Local Government website has links to local council websites.
You can buy New Zealand Standards relating to various construction systems from the Standards New Zealand website.
You can buy BRANZ publications about construction systems from the BRANZ website (click on the link to the BRANZ shop).