Construction systems overview
Which construction system is best for your design, location and climate?
The term 'construction system' refers to the way a home is built - that is, the materials and methods used to construct its foundations, floor, walls, and roof.
Timber-framed construction systems are still the most common throughout New Zealand, but there are other options available that have been tested and proven suitable. By understanding these options, you can choose one that provides you with a comfortable, durable, energy-efficient, cost-effective and weathertight home.
The system you choose will depend on many factors including your home's design and location, your finances, the topography, climate and microclimate of your site, and the level of earthquake risk and other hazards affecting the site. You can also mix and match, bringing together different systems for different parts of your home to get the best results.
The Building Code and building consents
All building work must comply with the Building Code, which sets out objectives and performance standards that the building has to meet. For example, one of the code's objectives is to safeguard people from illness or injury that could result from external moisture entering the building. Therefore, you need to ensure that buildings are weathertight. Your designer will determine exactly how to comply with the code.
Most home construction needs a building consent from a building consent authority (usually your local council). The building consent confirms that your building will comply with the Building Code if it is constructed in accordance with the plans the council approves.
Basic building work, such as laying a patio or installing kitchen cupboards, does not require a building consent. But more complicated home construction projects do. If you are considering building or plumbing work, you should talk to your council to be sure. You can also find out what work requires a building consent on the Ministry of Business, Innovation and Employment, Building and Housing website.
Licensing of designers, builders and other tradespeople
From 1 March 2012, some critical design and building work will have to be supervised or carried out by a licensed professional. This includes work relating to the structure (load-bearing walls, foundations, etc) or moisture penetration (roofs, cladding, etc) of homes, and is known as Restricted Building Work.
Foundations, floors, walls and roofs
The main structural elements of your home are the foundations, floor, walls, and roof. Taken together, these are sometimes described as 'the building envelope'.
Foundations or footings provide a firm base to stop the building settling and to keep it level. If foundations fail, the building will slump and tilt, and cracks will appear. For many house designs these problems are costly to repair. Foundations need to be durable.
Solid ground makes it easier to construct good foundations. It is much harder to build on sloping sites, sites on fill, and former swamps or landfills.
Floors need to be flat, smooth, level, dry, and strong enough to support the expected loads. The ground floor must be insulated to prevent heat loss.
Walls support the weight of the roof and any upper storeys. They have to be able to withstand wind and driving rain. Walls can be weakened by spaces for doors and windows. Exterior walls require insulation and weathertight cladding.
Roofs catch the rain and drain it away from the house. They have to be able to withstand wind, driving rain, snow and people sometimes walking on them.
The forces buildings have to withstand
Buildings experience compression, tension and lateral forces. A building must be designed and built to withstand the strongest forces that are likely to act on it over the course of its life.
'Compression' is pushing or squeezing. In buildings, it usually refers to the weight of the roof and/or upper storeys pushing down on the walls. The walls must be strong enough to withstand this force without deforming.
'Tension' is when something is stretched. For example, when someone walks on a suspended floor, the floor stretches slightly. All parts of a building should be able to withstand tension forces with minimum stretching.
Lateral forces push one side of a building element one way while the other side stays put. A strong wind or earthquake exerts lateral force on a building.
To withstand lateral forces, buildings need bracing in walls, floors, ceilings and roofs. Bracing can be:
- steel straps in the roof
- steel or timber fixed diagonally on the wall with sheet material
- sheet (or diaphragm) bracing such as plasterboard plywood and particleboard
- a reinforced concrete wall or floor
- a concrete masonry wall
Weathertightness refers to your home's ability to withstand weather. Few homes are 100% waterproof, and a small amount of moisture in your home's structure isn't necessarily an issue, so long as your home has been designed and constructed to cope with it. However, problems will arise if too much moisture gets in, and/or if the moisture is trapped inside the structure where it can cause problems such as fungal growth and rot.
The Building Code requires that all buildings are constructed to ‘provide adequate resistance to penetration by, and accumulation of, moisture from the outside'. Exactly how this is achieved will have to be determined by a designer.
In broad terms, however, keeping a home weathertight involves:
- good design that takes account of the climate and microclimate affecting your site
- appropriate choice and correct installation of cladding materials.
With many of the leaky homes built in New Zealand in the 1990s, poor design and poor installation of cladding were issues.
While weathertightness is an issue for all construction systems, the consequences of leaks can be greater for some systems than others. Straw bales or untreated timber, for example, will rot if they get wet. When you're choosing a construction system, it's important to take into account climate issues affecting your site.
It's also important to consider construction in conjunction with design. Some design features such as large overhanging eaves can help to reduce the risk of leaks. Other design features such as complex roof design can increase the risk of leaks. A home you see in a magazine may be perfectly designed for its location, but badly suited to yours.
Choosing a construction system
Construction systems for passive heating and cooling
Whatever construction system you choose, it will need to moderate temperature extremes in order to keep your home comfortable.
Some construction systems are, in themselves, good insulators. Others will need insulation added. Under the Building Code, new buildings and extensions must meet minimum insulation requirements. If you exceed these requirements, you'll get a warmer, drier, more comfortable home.
Some construction systems are high in thermal mass - which means they're good at absorbing and storing heat during the day and radiating it into your home when the temperature drops.
All construction systems have different effects on the environment. For example, some use significant amounts of energy in their production or transport (which produces greenhouse gas emissions). Some can be re-used or recycled. Some produce more waste than others, either in manufacture or on-site. Some materials produce airborne pollutants which can be harmful to human health. For more information, see Materials.
Suitability for your site and location
Some construction systems are best suited to level sites. Excavating a site will have an impact on its flora and fauna, and on stormwater runoff. Excavation can also increase the likelihood of erosion and soil instability.
When selecting a construction system, think of the maintenance it will need over its whole life. Systems that have to be painted or plastered to keep them weathertight will need regular maintenance.
No cladding will be maintenance-free.
Other factors to consider include cost, durability, ease of construction, and resistance to earthquakes, fires and pests.
Combining construction systems
You can combine construction systems where appropriate to get the benefits of each. You can have:
- lightweight walls with a heavyweight groundfloor
- heavyweight walls with a lightweight floor
- light and heavy walls on different sides of the house
- any other combination that works.
However, you will need to get good design advice and an experienced builder to ensure successful combining of different construction systems. Combining materials can increase the risk of weathertightness problems if the home is not properly designed and constructed.
From Smarter Homes
- Light steel frame construction
- Concrete construction
- AAC construction
- Straw bale construction
- Earth construction
- Insulated concrete formwork
- Construction site practice
- Exterior design
- Passive heating
- Passive cooling
- using thermal mass for heating and cooling
- The Building Act
- Leaky buildings; this includes information about building for weathertightness
From other sites
The Ministry of Business, Innovation and Employment, Building and Housing's website has information about building law and compliance, including the Building Code, and on weathertightness. You can also download guides to the Building Act and its processes, for example.
- Guide to applying for a building consent (residential buildings)
- Building work that does not require a building consent
- A beginnerís guide to resource and building consent processes
Most home construction needs a building consent from a building consent authority (usually your local council). The Local Government website has links to local council websites.