Off-site construction (OSC), also known as prefabrication, off-site assembly, factory assembly and off-site manufacture, describes a variety of systems where building components, or whole buildings, are manufactured in a factory away from the construction site. OSC is an application of MMC and represents a fusion between building and manufacturing. OSC systems include: sub-assemblies (eg staircases); frames; panels (either open or closed); volumetric or modular systems; and hybrids.

OSC has seen steady growth across the globe and is well established in North America, Europe, Japan, Australia, New Zealand and Malaysia. Important emerging markets are China and India.

New building performance quality thresholds – such as improved thermal and air tightness performance, customer perceptions and various government support initiatives, as well as the interest and success of various high-profile build projects, have all helped to raise awareness of the potential for off-site construction.

Compared to traditional methods of construction, OSC has a number of advantages – these are commonly reported to be: lower environmental impacts; greater resource efficiency; and improved waste management. The move away from sitebased construction methods (which are slow and component-based) to a factory built (systemised, quality-controlled approach) is well suited to timber based solutions. Timber based solutions can offer potentially sustainable, efficient, high-performing, lightweight and structurally sound solutions.

Wood based panels such as cement-bonded particleboard, fibreboard, OSB, particleboard and plywood, as well as solid wood panels such as cross-laminated timber (CLT), can be incorporated as structural components in OSC systems for build projects. Low-density wood fibre boards can also be used to provide some of the insulating elements in roof, wall and floor panels.

So what are house manufacturers looking for from the wood based panel products they use?

This will depend on the position of that panel within the build-up and the function it is performing: Racking strength; how well it takes fixings near to edges without breaking; toughness; and impact resistance, particularly to minimise damage during transportation and construction phases. Moisture and flame retardance are also important – both during the construction phase and also within use. If the board is to be used as a carrier board – either internally or externally – it needs to be compatible with the finish.

Wood panel manufacturers have started to respond to the opportunities in the OSC sector by making innovations in the form of improvements, refinements and new developments of products to improve performance in use as well as increases in efficiency of construction, either off- or on-site.

For wall, floor and roof sheathing applications, panels that provide a combination of strength, moisture-resistance and flame-retardancy are important for applications such as low and medium-rise buildings, hotels and schools.

One way to improve thermal performance is to improve the air tightness of the building. Interlocking panels, and panel surfaces that take sealing tapes, are now available to help reduce air leakages. While this helps to improve air tightness, there can be knock-on implications for indoor air quality – particularly for moisture management and VOCs. Low- or noformaldehyde panels can help address some issues around VOCs, while ‘breathable’ membranes, panels and wall build-ups can help manage indoor moisture.

Levels of sophistication for OSC can vary from very simple, mostly manual, systems through to semi-automated and on to (occasionally) fully-automated systems. Often, OSC companies have developed from construction companies which have no background in manufacturing techniques. Modern wood based panel manufacturing facilities are, by comparison, highly-automated and technologically sophisticated. It will be interesting to see how the sector responds to the emerging opportunities offered by OSC.