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Off-site manufacture

Off-site manufacture continues to generate debate in the construction industry and in the property world at large.

One of the principal recommendations of the 1998 Egan report 'Rethinking Construction', what used to be called 'prefabrication' is something of which we are all now supposed to be in favour. So why is it taking so long to catch on? In this Briefing we look at the role of off-site manufacture, its advantages and its drawbacks The problem with prefabrication The Government has been trying to encourage the use of off-site manufacture, especially in social housing projects, and has said that, although it will not provide subsidy, it will insist on greater off-site manufacture quotas in public housing projects of the future. The private sector is expected to follow suit.

Although Government intervention and encouragement is focusing attention on off-site manufacture, a poor understanding of what it entails and what its advantages and disadvantages might be prevents off-site manufacture from being seen as a viable form of construction.

Proponents of off-site manufacture tend to regard the term 'prefabrication' as unhelpful, as it often conjures up negative images such as the collapse of Ronan Point, the system-built tower block in east London, or of the chalet-type 'temporary' bungalows erected on bombsites in post-WWII London.

The general view continues to be that off-site manufacture is costly and, because of its poor image, unattractive to tenants and purchasers.

What is off-site manufacture?

The term off-site manufacture refers to any new development where a component part or an element of a building is constructed in a factory and then transported to the development site. If the size of the project and conditions dictate, construction or fabrication of the component or building element can even take place on-site.

'Off-site manufacture' therefore has a broad meaning. It can refer equally to the building structure, the envelope, or to elements within it, such as precast concrete staircases. One area where off-site manufacture has enjoyed a lot of recent success is the manufacture and pre-assembly of mechanical and electrical equipment for delivery and rapid installation on site.

Off-site manufacturing systems There are two main types of off-site manufacturing systems for buildings:

nThe modular/volumetric system Usually part of a pre-finished building, fabricated from lightweight steel and linked together to form individual dwelling units, the latter are then stacked to provide the required accommodation and the modules can then be clad externally in different ways. This system is suitable for small, compact innercity sites and modules can be stacked to provide medium-rise multi-storey developments without a need for a supporting structural frame.

nPanel systems Usually made of lightweight steel, timber or concrete, they do not give the same level of prefabrication as modular construction. Timber and metal frame panellised systems are capable of being used on buildings of up to six storeys, are quick to construct, lightweight and reduce foundation loading and on-site labour requirements. Concrete panel systems can accommodate varying designs and offer excellent sound insulation.

The benefits The advantages of using off-site manufacture, compared with traditional construction methods, include:

nImprovements in quality and reliability due to fewer defects or call-backs nReductions in waste on site of up to 70 per cent nReduced construction time. In the right circumstances, off-site manufacture can be 40-60 per cent quicker than traditional construction nUp to 30 per cent reduction in the weight of structure nLightweight steel modules allow designers to offer innovative solutions to residential development problems such as building over and on top of existing buildings nEconomies of scale for volume production can result in reduced capital costs of approximately 10 per cent nProven to be a safer means of construction, particularly in the case of modular construction nReduced site labour is required and better productivity can be achieved nOffers better predictability of delivery nLess energy is often required to develop the modules or off-site manufactured components nDismantling modules during demolition is easy and the modules can then be recycled nSustainability is increased because sound and thermal insulation is improved nOff-site manufacture meets Egan targets.

The drawbacks It would be wrong not to acknowledge that off-site manufacture has its disadvantages:

nLow volume = high cost. For off-site manufacture to be cost-effective, the size of the project in most cases must exceed 30-35 dwellings. A study by Monk Dunstone Associates of various-sized residential schemes of up to six storeys identified that, to achieve appreciable economies of scale, 40-80 dwellings are required and further savings result from schemes of 120+ dwellings nLack of capacity. This means that costs are only marginally less for off-site manufacturing. In its current state, the modular sector could only cope with approximately four per cent of the new homes needed nGeography. Most production plants are located in the Midlands or further north because demand in the south is not yet sufficient nChoice. No great variety of products available due to low demand nTechnical risk. Systems are still being developed and, with the exception of timber frame, none have had the chance to stand the test of time nImage. The public perception of off-site manufacture (the prefabricated buildings of the 1950s and 1960s) acts as a barrier to its general acceptance as an alternative means of construction nFinancial risk. Financial institutions and mortgage lenders see 'risk' in financing projects constructed using off-site manufacture, albeit there are signs of improvements here nEnvironmental impact. A large number of journeys are necessary in the transportation of the component parts of buildings prefabricated off-site.

Pointers to success A development team's approach is instrumental in the success of off-site manufactured projects.

The following are essential:

nClose involvement of client in assessing business-related benefits nClient and design team must be strongly motivated towards using off-site manufacture at the outset nDesign decisions need to be made early in the process before production commences nDirect involvement with the supplier of the chosen off-site manufacturing system by client and design team is essential nMethod of procurement of the development must take account of the off-site manufacturing process of the chosen system nCareful programming of site delivery, including transportation constraints and site access is necessary to achieve the full benefit of off-site manufacture The future With the exception of timber frame systems, almost all projects to date undertaken using off-site manufacture are in the social housing and, to a lesser degree, in public health sectors.

It is rarely considered for private residential or commercial developments.

It has been predicted that from 2004 a quarter of The Housing Corporation's estimated £1.4 billion investment budget will be directed to off-site manufacture (approximately 5,600 homes).

Only when demand reaches these levels will the component manufacturers be able to increase production, offer a greater number of products and consider locating production plants in the south-east, where demand for social housing is greatest.

These developments will decrease the costs of off-site manufacturing.

To enable expansion of off-site manufacture developers, funds and the construction industry as a whole must be educated about it.

Increased Government intervention will also help.