Are you ready for prefab?

Preece is development manager for the Peabody Trust. His confidence in the scheme's success stems from knowledge Peabody has gained since its first foray into prefabrication with the acclaimed 1999 Murray Grove scheme in Hackney, east London. "Murray Grove was a prototype; it was an attempt to see if we could build a housing scheme using modules. For this project we wanted to take that success to the next level," Preece explains.

The September announcement of the government's £200m Challenge Fund for new housing in southern England has got a lot more social housing providers interested in prefabrication techniques. The fund will be used to deliver 4000 homes in the South-east, of which a quarter will be prefab, and last week it emerged that the response from the sector had been so positive that the fund would be given more money and extended into a second year. But registered social landlords need to give prefab careful thought before taking the plunge. "To get what you want, you have to put in a huge amount of management time – or it will cost a huge amount of money," warns Preece.

"The first decision any developer must make is: what is the most appropriate form of prefabrication for a particular project?" he continues. There's no point in looking to use factory-produced modules if you are only building three houses – a timber-frame solution would be much more appropriate. Darren Richards, operations director of prefabrication consultant Mtech Services, says: "The first thing we do is a feasibility investigation to decide on the most appropriate construction technology."

There are a huge variety of different off-site manufacturing techniques available to developers looking to use prefabrication. These include timber-frame construction – currently being exploited by the Amphion Consortium of housing associations working with timber-frame manufacturer Torwood – and volumetric modular, favoured by burger giant McDonald's. Volumetric construction, the choice for the Raines Dairy project, uses apartment-sized modules, manufactured in a factory and delivered to site complete with the furniture and finishes in place (see "Off-site Options", page 29). However, there are no hard and fast rules as to what is the most appropriate technology.

"We're in the process of drawing up some kind of formal way of deciding which system to use," explains Preece. Factors to consider at the outset include ensuring that the product lives up to expectations. "Many modular products were originally designed for a 20-year life span, but many buildings are expected to last up to 60 years," says Andy Hill, managing director of Hill Partnerships, a contractor specialising in the construction of modular schemes. "Also, ensure the product complies with the performance specification because everybody will be quoting using different products – and check the warranty," he adds.

The choice of system is fundamental to the success of any scheme because, as Preece warns: "The main thing for any prefabrication scheme is to ensure it has third-party approval so that insurers like National House Builders Council or Zurich are happy with the scheme." His advice is to think in advance and use a system that's already approved, otherwise it will be necessary to spend time testing. "This is a horrible process and can take anything up to a year," he says. "It's one reason we used Yorkon's modules for Raines Dairy – the system already has approval."

It is also important at this stage to consider how well the technology will fit with other criteria driven by the Housing Corporation such as sustainability.

Having selected a technology, the next move is to decide who will be the supplier.

"You have to assess the supply chain's capability to ensure they have the factory capacity to do the work," explains Mtech's Richards. As part of this process it is also essential to assess the technical and business risks of using a particular technology or firm. "If the supply chain is very inexperienced, there will be a high risk of business and technical failure," Richards warns.

Whatever the technology selected, the supplier must integrate into the project team at the outset so that a developer can get the best out of the technology. "It is essential to get the supply chain involved at the beginning – not after a scheme has been granted planning permission – if you want to tap into the economics of production," explains Keith Blanshard, managing director of module manufacturer Yorkon. "Think production, not construction," is Blanshard's mantra.

For Peabody, working with Yorkon from the outset on the Raines Dairy scheme has ensured the development exploits modular construction to the full. The modules for Peabody's first modular scheme, Murray Grove, were based on an 8 m long, 3.2 m wide design. For the trust's second foray into volumetric construction, larger modules were used. The increased size of the modules means the apartments' larger balconies can be incorporated into the units in the factory rather than being attached later on site.

However, the most important advantage of increasing the module's size is that it has simplified site work, since each apartment can be assembled from just two modules, rather than the three needed at Murray Grove. Fewer parts mean faster construction and fewer joints to waterproof.

As well as influencing the construction process, the type of prefabrication technology will also have an impact on the most appropriate type of contract for a scheme and may also have an influence on who should run the project.

The Peabody Trust has tried a variety of ways of managing prefab projects. For Murray Grove the trust used a Joint Contracts Tribunal form of contract, which, according to Preece, "put a main contractor between us and the module manufacturer", inhibiting the flow of information. For Raines Dairy a partnering contract is being used which Preece hopes will help Peabody learn more about the process of volumetric construction.

One of the fundamental differences in process between traditional construction and prefabrication is the stage at which the design has to be finalised. "The success of off-site construction is all in the planning," says Mtech's Richards.

His solution is to set an end date for the project and work back. Quicker construction times mean prefabricated projects spend less time on site and clients should use the time saved in the design phase to "plan and plan and plan".

Planning is a theme Preece is keen to emphasise: "The big difference between prefabrication and traditional construction is that with traditional construction you can make changes as you go along, whereas with prefabrication such changes are not feasible. You really have to look at the drawings in detail – not just look at a flat's layout but to try to understand how the flat will work."

One way to ensure the design works before full-scale production commences is to build a prototype. According to Preece, construction of a prototype can cost upwards of £50,000 but, provided there are no major design changes, the unit can be reused in the actual building. Once the prototype has been signed off, the production lines can start to roll.

However, the challenge of using prefab does not end when factory production starts. The next challenge is to integrate a factory-produced unit with work carried out on a construction site. "Prefabrication is not as easy as some people think – at some point you will have to get something that has been factory-assembled to fit onto something produced on site," explains Preece.

At Raines Dairy, the prefabricated modules are supported on a reinforced concrete base frame, which stands 1200 mm clear of the uneven ground to create a level surface on which to mount the modules. Main contractor Wates built the platform to a tolerance of just 5 mm to ensure the modules would be correctly aligned – a tighter tolerance than on a traditional building.

At Raines Dairy each module was delivered and bolted into position in just 30 minutes.

For the contractor, prefabrication has helped compress the construction programme. After a normal lead time for the project, the scheme is expected to take 40% less time to complete. It has also allowed construction to take place close to a railway cutting. "We wouldn't have been able to build this development traditionally because we'd have needed a series of track closures," explains Peabody's Preece.

Preece says there is no cost benefit at the moment in using any form of prefabrication, but, he adds, "there's no cost penalty, either". Andy Hill says it is important to compare like with like when considering costs and says that when his company is working on a modular project, items like his overheads are reduced because he has to deal with fewer suppliers: "Things like the mechanical and electrical services installation are handled by the modular manufacturer," he explains.

But for those determined to pursue the prefabrication route Mtech's Richards offers a final word of warning: "Remember, it's not always practical to use an off-site construction solution," he says. "Well, not yet anyway."