This is the second post in a series on prefabrication and offsite construction in New Zealand. Sticking with the basics, here's a go at demystifying prefab and the myriad of terminology that is used in and around offsite construction. You can call it the prefab dictionary...

What is prefab?

Prefab, short for prefabrication, literally means any building part that is made (fabricated) before (pre) it is used at the final building site. This typically occurs in an enclosed building, a factory, or a controlled construction yard. Because it is something made off site, the term offsite is also used interchangeably with prefab.

Other terms that seem to be used in a similar way include: offsite construction (OSM); modern methods of construction (MMC); non-traditional construction; ready-made and innovative construction. Yet more terms include: pre-built; pre-configured; pre-designed; pre-planned, and; pre-assembly. In fact there are too many ways to describe it, which is why we are advocating a return to our roots, and to the contemporary revival of the simple term, prefab.

The most important thing to remember is that prefab refers to a process, and not a product. Prefabrication is a way, an approach, a system and does not for one instant infer a set outcome. If you get this, then you can be open to the possibilities that the prefab approach enables.

What does prefab look like?

Prefab buildings can be categorised according to the size of its parts. It is widely agreed that there are five main types, or levels of extent of prefabrication. These five typologies of prefabrication are component (stick and sub-assembly), panel (non-volumetric), module (volumetric), hybrid (module-plus-panel) and complete buildings (box-form).

Component

Component-based prefab includes stick and sub-assembly prefabrication. Stick refers to lengths of timber or steel which are pre-cut, pre-sized or pre-shaped puzzle-type pieces brought to site. Sub-assemblies include windows and doors, fixtures and fittings, and structural members such as pre-nailed roof trusses and wall frames.

Panel

Panelised, non-volumetric or two-dimensional prefabrication comprises manufactured panels that are transported as a flat-pack. They can be classified as closed panels, complete with doors, windows, services, cladding or lining. They can also be open panels, made up of framing components. Some architects refer to closed panel systems as cartridges or cassettes.

Module

Modular, sectional, volumetric or three-dimensional prefabrication refers to a three-dimensional structural unit being constructed away from the site and referred to as a volume, module, or section. Cores and pods are non-structural units used inside conventional buildings or modules.

It’s worth noting that the modular home term was brought into common usage in the 1970s by the United States modular housing industry. It is a type of building that meets building codes, is factory assembled in full-dimensional units and then fixed onto a permanent foundation at site. It is a more permanent type of building than the mobile or manufactured homes which also gained popularity during that era.

Hybrid

Hybrid prefabrication is the handy term used for combinations of systems, such as hybrid module-plus-panel or semi-volumetric systems. These systems use a mixture of  volumetric units for the highly serviced areas such as kitchens and bathrooms and construct the remainder of the building using panels or by another means.

Complete Buildings

Box-form or complete buildings are commonly known as portable, relocatable or transportable dwellings in New Zealand. They are a type of volumetric prefabrication where entire buildings are constructed in a factory or yard and then moved by a heavy haulage vehicle to site where they are attached to permanent foundations.

There are subtle differences between the four terms of portable, transportable, mobile and relocated: portable refers to a small temporary building that is light and easily moved repeatedly such as a toilet or site office; transportable means a larger building that is moved once from place of construction to its final site; mobile indicates a tow-able caravan-like structure on a permanent chassis which can potentially be moved repeatedly; and relocated can be applied to any type of building of any age and built using a variety of methods that is subsequently moved either in parts or its entirety to a new location. Any of these can be prefabs, but are not necessarily so.

Suitably confused by now? Hang in there – more info coming soon about the merits and challenges of prefab.