What is offsite construction?
Building construction around the world, and now in Australia, is steadily moving to greater use of offsite systems – but why, and what does offsite really mean? Kevin Ezard explains.
It seems the answers from contractors and builders as to ‘why’ are basically the same all over the globe, with responses in recent studies confirming similar trends in most countries.
The number one reason given is the lack of skilled workers with many companies struggling to find adequately trained staff. The reasons for this vary, but obviously the lack of interest in manual work by younger generations is a common factor in our changing world, along with inadequate access to training programs.
Another common reason given is the speed of construction using prefabricated and pre-assembled components, which overcomes the skilled labour shortage, and also saves in overall cost with less activities that need to be conducted on the building site.
Studies have also revealed that when the popularity of offsite systems increase it has the effect of improving outcomes from the method becoming better understood, with factors such as maintaining strict construction schedules, reducing waste, less local area disruption, and familiarity with the system all helping to create even greater cost savings.
In Australia we have adopted most of the offsite construction techniques over the past decade, and they can be defined as building systems or assemblies that are manufactured or prefabricated away from the building site prior to installation.
For decades the most common timber frame system in use has been prefabricated truss roofs and wall frames, with prefabricated floor cassettes now entering the market. These are termed ‘open’ frame systems as they are purely structural elements with the windows, cladding, insulation and plasterboard all yet to be fixed manually on site.
The next stage for timber framing is ‘closed’ wall frame systems which are supplied to site complete with windows, cladding, insulation and internal lining, with plumbing and electrical services installed ready for connection.
In addition, floor cassettes are usually part of the system which are delivered as large panels to be craned into position, ready for wall assembly above, which can enable savings of up to 10 weeks in completion of dwellings.
The primary building applications for timber offsite systems in construction are detached dwellings, multi-residential construction, and commercial residential construction which are summarised below.
Defined as low-rise one and two storey houses, predominantly built with prefabricated ‘open’ timber wall frames and truss roofs, now with more offsite ‘closed’ timber framing with cassette floors and wall panels completed and ready for assembly and connections on site.
Defined as mid-rise mainly three to five storeys and predominantly built with concrete, however timber offsite systems are being used the same as in detached dwellings with ‘open’ timber framing, and the new “jumbo ply” offsite panel material Cross Laminated Timber (CLT) now appearing in the market with major advantages in construction time, occupant liveability, and more environmentally sustainable outcomes.
Commercial residential construction
Defined as mid-rise three to eight storeys and high-rise above eight storeys predominantly built with concrete and steel, however recent major property developments in medium and high rise apartment buildings have used offsite CLT construction, and more are currently in progress.
In the decades ahead these systems will change the way we build to take advantage of the many opportunities such as faster construction and lower cost, and we will join the rest of the world in greater utilisation of our renewable timber resource.
And as international architect Michael Green stated:
“Wood is arguably one of the best tools architects and engineers have for reducing greenhouse gas emissions… and introduces a major opportunity for systemic change in the building industry. For the last century there has been no reason to challenge steel and concrete as the essential structural materials of buildings.”