Learning about off-site construction

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The New Category of Home project has undertaken two trials, with varying methods of off-site construction and designs, which have provided some key learnings on successful off-site construction.

 

1. Off-site construction needs to be design-led

Both trials used existing house designs which proved counter-productive to the off-site manufacturing process.  Complex architectural ‘features’, such as wing walls, can add considerable time (and cost) both off-site during construction and on-site during installation.  Pre-set material choices, set in the original resource consenting process, were also not well suited to optimising off-site manufacturing.  Vertical cedar cladding, for example, commonly used for aesthetic purposes, required additional timber framing attached to the steel framing for fixing.  This added cost and complexity.  A design-led process would have prioritised a cladding system more suited to efficient off-site manufacturing and the steel structure in this instance.  This is an issue that deserves further exploration given that New Zealand’s planning framework often ties homebuilders and designers into certain design forms even at the earliest planning stages such as overall subdivision or resource consent stages.

 

2. Working off-site brings benefits but needs to be well organised

Leasing and setting up a factory was relatively simple and cost effective, and using a good process engineer helps to establish streamlined and efficient ergonomic construction processes.

Quality and productivity was substantively better in the factory than on-site.  Having the correct materials and tools available in the weather-proof, single level environment of the factory has significant potential to accelerate construction.  Typical on-site construction can suffer from a lack of workforce consistency with building contractors alternating between other jobs.  In comparison, the factory benefited from a dedicated, experienced workforce with set work hours.

Working in a factory has the potential to improve safety and further reduce costs of compliance.  In particular, installation of second storey windows was able to happen at floor level without the encumbrance of scaffold.   Slips, trips and falls can be reduced through the combination of weather-proofing, working at low level, and having equipment set up safely (e.g. bench saws, power leads, drill chargers etc.)

Factory set-up can also reduce site theft through use of a secure environment, and improve goods and services delivery.  A ‘goods inwards’ system was set up to keep track of construction material, enabling greater scrutiny of materials used and more efficient supply and delivery.

 

3. What’s the best scale to deliver off-site construction?

Testing various scales of off-site construction (entire 3D house, 2D walls, 3D modules) enabled some high level conclusions on efficiency of scale.  The 3D components (upper stories) proved to be significantly more complex to construct and transport than the panelised components. They also took up significantly more factory floor space.  However, working in 3D means there is more potential for a larger degree of finish to be completed in the factory before moving to site. The HIVE house, for example, was fully enclosed and lined in the factory, reducing on-site labour time and potentially improving quality.

Transportation of the 3D components, even as modules, was comparatively inefficient given the volume being transported and the need for specialised contractors.  Wall panels, however, could be transported by industry standard hi-ab trucks, a more efficient and cost effective transport option.

 

4. Need to extend from factory to site to change business as usual

A number of challenges arose from delivering an innovation using an already busy work force with business as usual constraints.  The natural tendency to resort to ‘the usual way of doing things’ works against the introduction of innovation and the search for ‘out of the box’ solutions.  The result was that the on-site construction phase was less efficient than expected.  There were good reasons for this, and it is something that could be improved with further delivery to scale; but the conclusion is that off-site construction needs to cover the entire process, from design to on-site fit-out. 

 

5. Dealing with perceptions of ‘prefabrication’

The experience of selling the HIVE house brought home a crucial barrier to off-site construction: the general perception from the market that a prefabricated house should be cheap.  The real estate agents reported that people viewed a prefabricated house as being a step above a portacom, despite being well-appointed. 

This would suggest that while there are significant productivity, efficiency, quality and cost benefits from whole house prefabrication, a partial prefabrication approach (e.g. wall panels) may be a more acceptable option within the market, while still realising some of the benefits. 

It also suggests that a major learning for the industry is that we need to develop more sophisticated language to describe these innovations. Using the term ‘prefabrication’ may be less preferable than talking about off-site construction; and the New Category of Home partners are dropping the term prefabrication from their lexicon.


  • 03-Sep-2014 (Conference paper PRES/46)

    Off-Site Construction Challenges (PDF 6MB)

    Verney Ryan

    Presentation to the Building a Better New Zealand Conference 2014.  Covers learnings about off-site construction during the New Category of Home project.


Working in the factory

Working in the factory


Pod constructed off-site

Upper storey pod constructed off-site