Module behaviour

A precise six-storey stack of steel-clad boxes gleams in the sunlight among the post-war industrial premises, decrepit Victorian bedsit conversions and down-at-heel shops.

The steel wall marks the construction site of the Peabody Trust's Raines Dairy development. This is the housing association's second foray into volumetric construction after its acclaimed Murray Grove scheme in 1999. This development of 30 flats was constructed using factory-built modules, each complete with a fully fitted interior, that were slotted together on site.

Adam Preece, Peabody's development manager, puts the case succinctly. "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."

Work started on site in February and, when the 61 apartments are complete in early 2003, Raines Dairy will be the UK's largest factory-assembled affordable housing project. Architect Allford Hall Monaghan Morris has designed the six-storey scheme in a T shape. At ground level, eight live–work units act as a buffer against the nearby busy road. Stacked above these are five levels of two-bedroom apartments. To the rear, forming the crossbar of the T, is a wing of three-bedroom family accommodation overlooking a landscaped courtyard.

Squeezed onto a cramped triangular site, this second-generation volumetric development is rapidly taking shape, but the site's constraints mean that the apartments are being delivered in three phases. The modules that form the six-storey rear wing have been craned into position, and half the modules that form the live–work units and two-bedroom accommodation are also in place. The next delivery of modules is scheduled for October and the final delivery of six modules, which will form the flats adjacent to the railway line abutting the site's eastern boundary, will be put in place towards the end of the construction programme under a track possession notice.

The modules are supported on a reinforced concrete base frame. Piled foundations carry the frame, which stands 1200 mm clear of the uneven ground, creating a level surface on which to stack the modules. Main contractor Wates built the platform to a tolerance of 10 mm to ensure the room modules were correctly aligned. "It's a tighter tolerance than for a traditional build," says John Walsh, Wates' project manager.

Because the modules arrive fully fitted out, one of the most disconcerting things about the site is its lack of construction personnel. Aside from the building's framework base, the only large areas of work up to now have been some preparatory groundwork, underground drainage and the construction of the building's steel-framed access core, which houses a precast concrete staircase and also contains the scheme's two lifts.

The steel-framed accommodation modules have been manufactured by Yorkon, the firm that Peabody worked with on Murray Grove. Flat panels for walls, ceilings and floors are attached to a welded, rolled hollow-steel frame on the supplier's production line. The modules are then lined internally with a double layer of plasterboard and externally with galvanized steel sheeting. Insulation and cabling is included in the panels. Before they leave the factory, the modules are fitted out so that when they arrive on site they are complete with kitchen and bathroom furniture and all electrics – they even contain an oven.

Installing the modules is a remarkably quick process. One of the problems with Murray Grove was the inaccessibility of some of the bolts to join the modules together. For Raines Dairy, the location of the bolts has been changed. "It only takes 30 minutes to crane each module into position and bolt it into place," says Simon Allford, a partner in architect Allford Hall Monaghan Morris. No additional structure is needed – the module's integral frame is sufficiently rigid to support the six-storey structure.

Modification of the bolt system is one of the less obvious improvements to the system; more apparent is the increased size of the modules themselves. The units for Murray Grove were based on an 8 m long, 3.2 m wide design. For Raines Dairy, the modules are 600 mm wider and up to 3.6 m longer. "We worked together with Yorkon from the start to arrive at a maximum module size," explains Allford. At 3.8 m wide, the new modules are the maximum width that can be transported by road without a police escort.

This was the first modular scheme architect Allford Hall Monaghan Morris had designed.

As well as picking Yorkon's brains, the designers also visited Murray Grove's architect – Cartwright Pickard – before starting on the design. They must have been well advised: the architect's subsequent layout for the apartments cleverly exploits the space within the constraints imposed by the standardised modules to produce an efficient floor plan. A typical two-bed apartment will comprise two units: one module for the living–dining and kitchen areas, and another for the bedrooms and bathroom. The only link between the two is a connecting door.

Increasing the size of the modules has simplified the site work. The two-bed flats at Murray Grove were assembled from three modules. Here, the two-module method increases the amount of wiring that can be done in the factory and cuts down on the number of interfaces between units, which have to be sealed on site. "Larger modules mean fewer joints to weatherproof," says Allford.

The increased size of the modules means that the apartments are larger and allows balconies to be incorporated in the factory rather than having to be attached later on site. "The more things built in the factory, the better," says Peabody's Preece. Other minor changes included more accessible cable and plumbing ducts and the addition of a ceiling void to conceal cables.

However, not all work on the 127 modules has been completed in Yorkon's factory. Surprisingly, the apartments' wooden laminate flooring will be installed on site to limit movement in the floor as it absorbs and releases moisture, which would have otherwise affected the floor warranty. This means that the skirting boards will have to be removed in every flat before the floor can be installed.

The precast concrete external walkways will also be installed once the modules have been assembled. And, just as in Murray Grove, the development's rainscreen cladding will have to be installed on site because the detailing was not finished early enough in the programme. "In an ideal world, the modules would have been pre-clad," says Allford. Instead, the contractor will have the time-consuming task of applying the timber and profiled zinc cladding panels from a cherry-picker.

Perhaps the biggest change of all, however – and the one that will have the biggest impact on the future success of volumetric construction – is that a different form of construction contract has been used for the Raines Dairy scheme. Murray Grove was procured under a JCT form of contract, which, according to Preece, "put a main contractor between Peabody and Yorkon". For this project, Peabody has changed to the partnering contract PPC 2000.

Preece hopes that this will help the trust learn more about the use of volumetric construction. "At the end of Murray Grove, we found that we hadn't learned as much as we could have done, whereas this contract has got us close to everybody," he says.

In addition, the architect is producing a document for Peabody on what it sees as the project's successes.

For the project's main contractor, Wates, however, the decision to use volumetric construction has helped overcome some of the site's little challenges – notably its proximity to a busy railway line and a main road classified as a red route, which means that it has to be kept clear of construction traffic at all times. Using prefabricated modules reduces traffic to and from the site, which in turn minimising congestion and the impact on the site's neighbours.

For Walsh, the use of volumetric construction has helped compress his construction programme to 50 weeks after what he calls "a normal lead time" for the project – that is, about 40% less time than it would have taken if the building had been constructed traditionally. It has also allowed construction to take place close to the 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.

But perhaps the biggest benefit in using modules has been for the scheme's future tenants. As Preece points out: "The end users will be getting a superior product, with minimum defects, for less money. It's a win–win situation."