This means they need to be able to stand up to high winds and heavy snow and rain falls.

Roof structure

A roof has structural components which 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 and the timber trusses transported to the site. This has several advantages:

• trusses are designed by engineers
• trusses are accurately cut and securely assembled
• the roof is completed quickly and builders can work under cover
• waste is minimised.

Roof slope

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.

Eaves

These overhang the walls and provide some weathertightness protection from rainfall.

Gutters

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.

Roof leaks

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

Flashings are strips of metal or other material that cover joints and gaps where water can get in. They help stop water getting in, and help to drain it out when it does.

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.

Roof cladding

Many roofing systems, such as metal and tiles, 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.

If you want to use water from the roof for household use, check the roof system is compatible with water collection.

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
• bitumen membrane
• profiled impregnated wood fibre sheets.

Metal

Metal is very strong and comparatively light. A metal roof can be constructed very quickly.

All metal 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. Unpainted zinc-coated steel may contribute zinc in collected rainwater for drinking.

These days a zinc/aluminium alloy is the most common base material for domestic roofing.  There are compatibility problems when using this on alterations where it can come in contact with galvanized iron. An electrolytic reaction results in premature rusting.

Overlapping pressed metal tiles with stone chips may need to be sprayed 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 rubber 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, UV-resistant and long-lasting.

It can be used on roofs of different shapes, including flat roofs.

It should be applied by a specialist.

Shingles

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 not be too flat.

Membrane roofs

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 metal roofing.

The surface can weather and may need repainting to the manufacturer’s specifications, using a bitumen-compatible water-based paint.

Asbestos

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 is hazardous if it starts to weather. Call in specialists if you want to demolish or remove an asbestos cement roof. Do not waterblast asbestos products.

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.

Wind can also put pressure on the roof structure. Wind traveling up a hill will tend to lift the roof by pressure underneath the eaves. Or wind may push part of a roof down while sucking another part of the roof upwards.

For these reasons, it’s important that roofs are designed and built to withstand strong winds.

Wall cladding

Wall cladding must protect the interior from water penetration under all conditions, including rain driven by high winds. It should also stop draughts.

Cladding types

Some wall cladding options are:

• weatherboards
• fibre cement
• plywood
• masonry veneer
• concrete
• monolithic systems
• metal

Weatherboards

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.

As well as timber, weatherboards can be made from materials such as fibre-cement, metal and vinyl (PVC).

Fibre cement

Fibre cement exterior wall coverings come in the form of panels and weatherboards. They may be used as the exterior wall covering, or as substrate for monolithic claddings.

Plywood

Plywood panels may be used as cladding. Gaps are covered with battens or flashings. You can also get plywood weatherboards.

Masonry veneer

Masonry veneer is a system where a timber-framed home is clad with bricks, stone, or thin concrete blocks. The masonry is connected to the timber framing through flexible wall ties.

Concrete

Concrete blocks or poured concrete may act as both the structure and the cladding.

Monolithic systems

Monolithic cladding systems have a seamless appearance. They have become popular in recent years, but have to be designed and applied properly or they will leak. The ‘leaky home’ problem is largely to do with incorrectly constructed monolithic cladding. Ongoing maintenance is essential.

The traditional monolithic system is stucco. Cement-based plaster is applied over a variety of backings including fibre-cement and plywood sheeting. 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 insulation and reinforced plaster. There are several different proprietary systems available.

Fibre cement sheets can also be plastered to give a monolithic effect.

All monolithic claddings rely on the final coat for waterproofing, and this needs to be well maintained. 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 prevent any moisture which does enter the wall cavity, or has got in behind the cladding, from getting into the framing and interior lining.

Flashings

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.

More information

From Smarter Homes

• Exterior structure
• Passive heating
• Passive cooling
• Insulation

From ConsumerBuild

• The Building Act
• Leaky buildings; this includes information about building for weathertightness
• Construction
• Exterior maintenance: roofs
• Exterior maintenance: exterior walls

From other sites

The Department of Building and Housing’s website has information about building law and compliance, including the Building Code, and on weathertightness.

All home construction needs a building consent from a building consent authority (usually your local council). ConsumerBuild’s Council finder web page has contact details for New Zealand local authorities.

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 bookshop).