PREFABRICATION VERSUS IN-SITU SITE COSTS

Prefabrication got a bad name for itself in the 1960s, when elements such as prefabricated concrete sometimes failed to meet the grade. But the techniques have advanced considerably since then and prefabrication now offers several advantages over in-situ construction under certain circumstances.

A 1993 CIoB study, 'Prefabricated modules in construction', found that prefabrication was advantageous, especially where: 'there are tight construction programmes; there is restricted site space; high quality finishes are specified; and complex construction interfaces exist.' Prefabrication has the added advantage of being constructed in controlled factory conditions, where everything can be tested before reaching site.

Many elements of a building can be prefabricated - the frame, cladding, toilet/bathroom pods, plant rooms and a range of M&E services. Buildings with many repeated features are particularly suited to prefabricated, modular construction such as hotels, student accommodation, prisons and large retail chains. Initial costs may be higher than in-situ as better quality materials are often used, but time on site, labour and wastage are cut considerably.

Fast-food giant, McDonalds, has even gone the whole hog and developed an entirely modular construction system for some of it outlets with partner Britspace Modular Building Systems. Construction time was cut to a mere 15 days from green field site to fully operational restaurant in one instance.

Bob Akhurst of Construction Marketing Services is an outspoken advocate of prefabrication. His company manufactures a range of fully furnished room structures in Accrington and markets prefabricated bathroom/toilet pods made by Danish firm EJ Badekabiner.

'Modular structures finally came of age in the UK about four years ago and are now here to stay. Mistakes were made in the 1960s and we just stopped in the UK. The Europeans looked at it, changed, went on and are now far ahead of us. What are the R&D departments of architects and contractors looking at? Innovation has been driven by the manufacturers needing to sell their products,' said Akhurst.

Akhurst points out that prefabrication can speed up the construction programme, increase quality and cut costs. His view is corroborated by the CIoB study. This found that, while the initial outlay of prefabrication was 10-30% more than in-situ, when a full cost comparison was made the prefabricated option was found to be considerably cheaper.

Peter Adams of Costain Building Products which manufactures prefabricated concrete structures, explained: 'If you compare prefabricated concrete to in-situ concrete it is more expensive. You have to build it and then transport it and pay for high factory overheads. But cost savings are made elsewhere. Prefabrication reduces site requirements - there is less mess, less falsework, formwork and scaffolding. You need fewer people and you spend less time on site because it is so much quicker to install.'

Adams feels that prefabricated concrete is shaking off the bad name it got in the 1960s. Whereas before clients and designers used to look at prefabrication as a secondary alternative, they are now looking at it as the first choice option.

A good current example of a building using prefabrication is Land Securities' Eland House being built by Mowlem for the DoE near London's Victoria Station. Prefabricated elements include reinforced steel pile cages and steel stairs which are dropped into place as each storey goes up and then used for access, cutting out the need for scaffolding.

The toilet pods are also prefabricated and completely tested and de-snagged in the factory in Scotland.

The cladding is prefabricated and comes in 7.5m panels, fully glazed with automatic blinds controlled by a computer linked to solar sensors on the roof. All are pre-tested before being delivered on site, as are the stand-alone plant rooms.

Much of the pipework is also prefabricated and the welds x-rayed in factory controlled conditions instead of on site where access can be difficult for inspectors. The lifts are supplied as a single package that can be assembled easily on site.

Mowlem's project manager for Eland House, David Rogers, enthused about the benefits of widespread prefabrication: 'We can programme to get things at the right time, fully tested and snagged. All we have to do is connect it. We've had 240 workers on site, but if we had done it all in-situ we would have needed 500. The cladding took eight weeks to build and nine weeks to install. If we had built it all on site, it would probably have taken five months.'

Not everyone is entirely convinced yet. Greycoat's development director, Chris Strickland, said: 'We have always been advocates of prefabrication. There is no doubt that the efficiency and quality of factory-made products is better. On the face of it, the more you can make in the factory the better it is.

'That said, if you have a very tall building it is sometimes difficult to get high quality of design - it can get so repetitive. You still have to have a high quality of aesthetic design, but you don't have to sacrifice artistic integrity to achieve a 30% saving in the cost of construction.'

Colin Stoker, a partner at Gardiner & Theobald and director of engineering services added a further caveat: 'There have been considerable successes in prefabrication, but there are potential risks. Workers are used to bricks and mortar, know what the stresses and strains are. Pods flex sometimes and bits fall off.

'With traditional materials people tend to know what materials can carry what weights. You can get caught out sometimes - I know buildings with podded plant rooms that turned out to fail acoustically, they were just too noisy.'