Complete buildings

By - , Build 139

Recent issues of Build have looked at component, panel, module and hybrid prefabrication. We finish the series by considering the complete prefab building.

Keith Hay Homes complete building.
Keith Hay Homes complete building.
Manoeuvring the building on to its site.

COMMONLY CALLED transportable, portable or relocatable buildings, complete prefabricated buildings are a type of volumetric prefab constructed entirely off-site in a yard or factory and delivered to site.

There are subtle differences in the types of complete building prefabs, with portable referring to small, temporary buildings such as toilets or site offices while transportable refers to larger buildings.

Use has broadened

New Zealand has a long history of complete prefabricated buildings dating back to the 1950s when factory homes were constructed for government hydro scheme housing.

In the 1970s, a number of businesses were established, and this increased the appeal of the complete building as a seemingly instant response for homebuyers. Although initially limited to residential construction, transportable buildings are now used for single and multi-level medium-density developments, corrections facilities, hospitals, education centres and aged-care complexes.

Back to top

Grown in sophistication

As demands for greater individuality and an appreciation of design have grown in New Zealand, so has the variety of options for the transportable building. Gone are the days of the 1970 prefab temporary classroom. Modern prefab buildings are constructed to meet the Building Code or better and are built using technologies that deliver significant benefits in the design and construction supply chain as well as reduced operating costs over the building’s life.

Back to top

Many advantages over on-site builds

Apart from minimising on-site noise and debris, transportable buildings are built on a level and solid working platform that facilitates greater precision during construction. The resulting closer tolerances may produce a tighter building envelope that is more weathertight and makes better use of energy throughout the building’s life cycle.

Complete buildings built under cover also have less disruptions due to weather and provide greater protection for workers. The outcome is more effective construction scheduling, making the overall speed of the build predictable and providing certainty in delivery and budget. This is why many complete buildings achieve a fixed price.

Time is a critical component in any building project, and any savings made during construction mean savings to the project costs. While the construction process for dwellings varies depending on the manufacturer, in many instances, a 120–190 m2 building can be completed and ready for delivery anywhere between 6–10 weeks, depending on the materials used. This compares to the average 14 weeks for on-site builds.

Back to top

Report found savings

BRANZ’s recent Study Report 279 evaluated the impact of prefabrication on the New Zealand construction industry and measured the potential savings for small to medium-sized builders of $1,000–$1,600 per week, equating to savings of $4,000 per building or more.

Although the report indicates that the cost of prefabricated construction is similar to on-site construction, the gross profit margin for transportable buildings is still slightly higher than traditional on-site builds. This is primarily due to the predictability of budgets and the security of the timelines for delivery.

Savings can also be made where greater efficiencies and use of technology are integrated into the design and construction. For example, using other prefab systems such as compressed timber panels or components in the design can deliver a faster build time as the components arrive at the factory ready to be installed.

Back to top

Factors to success

The need for collaboration across the supply chain is another factor in the success of prefabricated projects, but this is more evident in transportable buildings. A number of transportable building companies have established strong relationships with local building consent authorities to assist with more streamlined and cost-effective consenting.

When considering a move to more off-site complete build solutions, there are a number of factors to weigh up. The capital investment involved in setting up a factory or yard and its location is critical. Also, economies of scale are needed for prefab to deliver its well documented greater levels of efficiencies and cost savings.

Back to top

Flexibility and certainty

This does not mean that all buildings are constructed the same. A considered range of designs, materials and finishes provide flexibility for the client and certainty of cost. According to BRANZ, 64% of builders already have a range of plans to show clients. The flexibility is in areas such as the roof (pitch –low versus steep, gable versus hipped),kitchen and bathroom design and the inclusion of fittings.

Before design and construction starts, the designer or architect must consider the modular dimensions required both for construction and transportation purposes.

Manoeuvring the building on to its site.

Back to top

Taking haulage into account

Because finished buildings are designed to be portable, they are engineered to withstand the stresses and strains of movement and are robust. Although this may mean additional costs for initial research and development in the design and for bracing materials, it is countered by the efficiencies gained through the build.

The Heavy Haulage Association of New Zealand has information on its website for engineers and designers regarding height, width and weight specifications, as well as what to consider when moving buildings.

Determining the final design and features such as the external cladding and interior finish of the building also impacts on the cost, time on site and transportation of the final building. For example, buildings designed with weatherboard or lightweight cladding and roofing materials are more likely to be delivered on site with these already installed due to their flexibility and durability for transporting, whereas a tile roof is likely to be installed on site post-delivery of the building.

Back to top

The future

The future increase in the uptake of prefabrication technology in the New Zealand construction sector is a work in progress, but what is clear is that there are considerable economic and social benefits to developing this technology and generating innovative ways to improve the economies of scale.

BENEFITS OF COMPLETE BUILDINGS

  • Greater quality control.
  • Minimal disruption on-site and off-site.
  • Quicker build time.
  • Increased worker safety.
  • Stronger buildings.
  • Reduced greenhouse gas emissions.

Back to top

For more

Visit www.prefabnz.com.

Download the PDF

More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Keith Hay Homes complete building.
Keith Hay Homes complete building.
Manoeuvring the building on to its site.

Advertisement

Advertisement