In a construction context, “prefabrication” usually refers to work undertaken in a factory-type environment to fabricate either complete buildings or structures, or, more commonly, elements of those buildings or structures. By relocating some site-based activities to a factory, and by eliminating or reducing other site risks such as working at height, prefabrication offers the prospect of safer construction.
The word ‘prefabrication’ does have some negative connotations, largely due to historic use. For example, in Europe, post-World War II reconstruction involved erection of many prefabricated temporary buildings, many of which remained in use well beyond their intended service lives. And as these ‘prefabs’ deteriorated, they coloured many people’s views of this type of construction.
But, as construction has sought to reconcile various pressures – low industry productivity, skills shortages, time and cost overruns, the high cost of errors, the human costs of health and safety failures, low levels of standardisation, and under-utilisation of digital approaches – prefabrication is becoming an increasingly common part of the project team response.
For example, entire schools have been constructed offsite as a series of self-contained modules – hence the term sometimes used: modular construction – each complete with flooring, glazing, wall finishes and electrical, heating, and plumbing systems. The modules are then transported to site, craned into position on previously prepared foundations, and connected to water, electricity and other services. Offsite fabrication of reinforced concrete components or steel structures for use in transportation and utilities projects can also shorten time-consuming on-site processes, lessening the disruption to nearby residents or end-users.
Even traditional building finishes such as brickwork can be accommodated. Using ‘brick slips’ (thin cuts of real brick, or purpose-made brick tiles), the appearance of conventional brickwork can be replicated. With skilled bricklayers in short supply and commanding higher wages, this eliminates the expense of onsite bricklayers and the associated delivery, storage and movement of bricks, mortar and equipment to support their work.
Planning and procurement
Prefabrication, sometimes also called pre-manufacture or offsite construction, might be dictated by the project’s location or by constraints relating to the occupants or uses of nearby buildings. Traditional construction techniques generating vehicle movements, noise, dust and vibration might be undesirable where, say, a new building is to be constructed within the campus of an existing operational hospital or university. Some sites, particularly in busy city centres, may also have little or no space for onsite storage of materials. Permission to develop may also be subject to conditions about limiting disturbance to nearby residents or businesses and may be subject to strict conditions about hours of working and environmental impacts.
Prefabrication may also be attractive from a project scheduling perspective. It can dramatically cut the time involved in onsite construction (site preparation and prefabrication can be concurrent activities), while also being less prone to weather interruptions. Constructing a new building within a school site, for example, might be possible within the limited time window of a school vacation, usually when adverse weather issues will also be less likely.
Incorporating the approach, though, is something that needs to be considered early in planning and design processes as it will also dictate how the project might be procured. Increasingly, it is not just about early involvement of the contractor, but also early involvement of the manufacturer; firms specialising in offsite working may have limited production capacity so their lead times will be a factor in when and how they can respond. Moreover, a manufacturer will likely only invest in prefabrication facilities if there is likely to be a predictable demand for products.
Better, safer construction
Prefabrication mitigates many of the quality and safety risks often associated with traditional labour-intensive onsite working.
Factory-based working tends to be safer than traditional construction environments. Skilled work is now delivered in a well-serviced factory environment, and working at height, one of construction’s major risks (for example, around half of the UK’s annual construction fatalities are due to falls from height), is reduced. Workers have ready access to raw materials, tools, equipment, and design information, can deliver a more consistent, higher quality end-product, and are not prone to weather interruptions.
Design for manufacture and assembly (DfMA) must consider the logistics of transport, lifting and installation – cladding components, for example, may be delivered with lifting points and with fixing systems pre-installed ready to attach to the structural frame, while glazing may also have been factory-fitted, eliminating another labour-intensive onsite installation process.
Transportation and installation processes can also reduce onsite activity levels, particularly if a ‘just in time’ approach is deployed. On tightly-congested city centre projects, or where hours of working may be tightly restricted, deliveries to site can be scheduled so that items can then be craned in sequence straight into their finished position and fixed in place (no offloading, storage and relifting). DfMA strategies can also eliminate the need for scaffolding and other temporary works (reducing onsite working at height risks), while installation can often be accomplished by smaller teams, reducing labour interface issues and transport and accommodation requirements.
Prefabrication often results in earlier completion of a building’s external envelope, enabling an earlier start on any building services and internal fit-out tasks, also reducing labour interface issues. Factory-based quality assurance and testing also reduces onsite commissioning and defect rectification, dramatically cutting reworking and snagging (and so reducing site manpower requirements), while also reducing waste.
So, while prefabrication may not be suitable for every job (refurbishments of existing buildings, for example), the approach is being applied increasingly widely, often enabled by digital technologies that connect designers, manufacturers and constructors and ensure a coordinated approach to project delivery. Site, budget or programme constraints may dictate a DfMA strategy, but reduced health and safety risks are also a major benefit.