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Prefabrication grows up

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Peabody Trust's commitment to improving construction efficiency has led to its second major off-site housing development The government is championing modular construction as a key solution to the UK's housing shortage. If that is to be the case, a major market is likely to be in high-density, urban developments where both speed of construction and minimising disruption are deciding factors.

High density brings the repeatability that manufacturers need to optimise the manufacturing process, and also enables designers to take the characteristics of modular construction and develop a strong architecture that celebrates these characteristics.

This approach has been applied successfully to a site in Stoke Newington, north London, where the second of Peabody Trust's modular housing schemes has just been completed.

Raines Court, built on the site of a former dairy, is a large, T-shaped development ranging in height from three to six storeys.

Its dramatic 'chequerboard' appearance was developed by architect Allford Hall Monaghan Morris as a response to the modular construction form chosen by the client.

Peabody commissioned Raines Court after the success of its first modular development, Murray Grove, also in Stoke Newington, which opened in 1999. That development had been designed by Cartwright Pickard, which has since gone on to work with Peabody on other projects.

Keen to widen the pool of designers with knowledge of off-site manufacture, however, Peabody appointed a different architect, AHMM, for Raines Court. The same manufacturer, Yorkon, was retained, though, to ensure that lessons learned from the previous project could be carried forward.

Raines Court consists of 41 two-bed units, 11 three-bed units, one one-bed apartment and eight one-bed live/work units. The scheme was first envisaged as a development made up entirely of two-bed units, maximising the repeatability aspect of the modular work, but other sizes were brought in during the planning process.

The key lesson from Murray Grove was to incorporate more of the building's interior and exterior into the modules, but also to cut down the number of cross-module connections. At Murray Grove, each two-bed unit was made up of three modules, creating 8m x 9m of internal space.

AHMM partner Simon Allford says:

'Modules are inherently efficient when pushed to their outer limits. The manufacturers use the same size beams whatever the size, so there are benefits in taking them to their maximum height and length. The modules have a high initial cost, so the cost per square metre goes down as they get bigger.'

Those cost benefits reduce still further if a single apartment can be made from just two modules instead of three, and there are additional savings in the cost of cross-module connections, transport and cranage of using fewer individual modules.

Having settled on a larger module size of 11.6m x 3.8m - the widest that can easily be transported by road - AHMM then set about developing a standard plan that would create sufficient space for a two-bed unit within two modules without sharing spaces between the modules. 'A module is very robust, ' explains Allford. 'For the modules to work at their best they should be closed and solid.'

Another key aim was to incorporate both a front balcony and an internal courtyard for access within the module. At Murray Grove, balconies were added on site and access was via conventional walkways, which was something AHMM wanted to get away from.

'We want each apartment to have its own entrance space to give some privacy, ' says Allford. 'When there are walkways outside, residents have to create privacy by putting up curtains. Here we've done it through the architecture, not through furnishings.'

A standard two-bed unit, then, consists of two modules side by side, one containing the kitchen and living room, the other two bedrooms and a bathroom. Entry is through the internal courtyard into a very small circulation space that opens out into the kitchen and living room, with the balcony at the far end. There are only two openings between the modules - the front entrance itself and a door leading from the living room into a small lobby that has both bedrooms and the bathroom leading from it.

This plan is very different from the standard 'corridor-style' layouts adopted in traditional apartment design, but Allford says that, having developed it specifically to optimise the modular format, the practice has since taken the same layout into non-modular housing as it is so efficient and creates large open spaces.

Visually, the balconies offer light and shade, contributing to the chequerboard effect that is emphasised by the use of colour. Each flat has its own colour, used on doors and cladding panels, chosen from a palette of blue, green and yellow.

Raines Court has been built using 127 individual modules, all fabricated at Yorkon's factory in York. The basic module is a monocoque structure made from a galvanised steel frame fitted with stud walls, sealed floors and ceilings. Balconies were laid to crossfall and waterproofed with a membrane that continues up the external walls.

Kevin Jones, operations manager at Yorkon, says: 'The main driver for the design of the modules is acoustic performance.'

Internally, the walls are lined with either one or two layers of plasterboard, depending on the acoustic performance required, and tests at Raines Court prove the apartments meet the forthcoming requirements of Part M of the Building Regulations. The construction also contributes towards thermal insulation - but, says Jones, this must be supplemented by an element of insulation within the external cladding system in order to meet the new Part L of the Building Regulations.

The Raines Court modules have shiplapprofiled zinc panels on their front facades, with zinc cover strips to mask jointing.

AHMM was keen to use materials that would be both attractive and cost-effective to build and maintain.

Side elevations are finished in larch vertical timber cladding, a highly sustainable material with very good thermal insulation properties. The balconies and internal courtyards are lined with cladding board with a painted finish.

It was the design team's intention that cladding should be fixed to the modules in the factory so as to cut down the use of scaffolding on the site and maximise the benefits of off-site manufacture. However, agreeing a detail for this element of the work that would enable the cladding to appear continuous across the joints between modules became problematic within the time frame, and most of the cladding was actually fixed on site.

At one end of the development, however, cladding had to be factory-fixed because the block is adjacent to the railway line and there was no space either for scaffolding or powered access. 'That focused everyone, ' says Allford, 'and proved that it could be done.'

However, Jones says: 'At every interface there is going to be something to do with cladding, so it's all about coming up with a detail for that cross-module detail. If you want a continuous band of one type of cladding you have to accept there is going to be a break at the joint.'

He suggests that, as a compromise, it would be easy to supply the modules with the insulation already fitted and just fix the final layer of cladding on site if agreement could not be reached about interrupting cladding at the joints.

In all, approximately 55 per cent of the project was constructed off-site - a figure that Allford would like to see rise to 85 per cent or 90 per cent, with just the substructure and service connections done in situ.

However, this is still an improvement on Murray Grove, and Yorkon is determined to increase the percentage. 'Without doubt we wanted to do the maximum, ' says Jones. 'The whole point of this is to maximise factory work.'

As it was, all the internal partitions, doors, windows, patio doors, tiling, plumbing, electrics, kitchens and bathrooms were fitted in the factory, and the inclusion of the balconies and courtyards is a major step forward.

Yorkon built a full-size demonstrator unit within the factory that all sides found useful. 'It does help to see a space; we actually changed the size of the windows as a result, ' says Allford. Jones adds: 'The demonstrator is useful for ironing out any little problems or misunderstandings with the drawings, and we get good feedback from the guys who are building them.'

Internal floor finishes - carpets in the bedrooms and wood laminate elsewhere - were done on site. 'Carpet is very quick to fix, so that's fine to do on site, ' says Jones, 'but we would have preferred to fit the laminate in the factory.' Like the cladding, the wood floor hit the obstacle of agreeing a suitable detail to take the material seamlessly across modules - this time internally.

At the outset, AHMM also hoped to build access decks within the modules, but encountered problems of cold bridging and acoustic transmission. Instead it has designed an elegant solution that sees precast concrete units spanning from cleats on the modules to secondary steelwork that supports the stairs. It is still a predominantly off-site solution, but Allford intends to incorporate the decks within the module on his next off-site project.

All in all, though, every party to the project believes the scheme has been a success, and residents certainly agree. The units were all snapped up immediately they came on the market, many of them as shared-ownership properties. 'In the end it's a success for the people who live in them because it works, it looks good and they have generous spaces in a solid building, ' says Allford.

And even with only 55 per cent of the work carried out off site, construction still took far less time than it would have done with traditional methods. The modules were built in the factory at the same time as foundations were going in on site and, when they did arrive, could be erected at a rate of between eight and 10 a day. As a result, the £4.9 million development was built in just 50 weeks, a saving of about 40 per cent on traditional site-based construction.

The success of Raines Court vindicates Peabody Trust's commitment to off-site manufacture and its choice of architect.Dickon Robinson, the trust's director of development and technical services, says: 'I think it's an important project in terms of off-site manufacturing - in particular, volumetric construction - because it demonstrates that highquality architecture can be achieved using that form of construction. I think it's a fine building.Modular construction does impose a certain discipline, but this shows that a capable and confident architect can still, without any great difficulty, design a very successful building.'

Raines Court was a repeat commission for Allford Hall Monaghan Morris, but its first using the modular form.'They had worked for us before and we thought their particular aesthetic language would suit this project, ' says Robinson.'They have done some very interesting things with colour that pick up on the modularity of it.'

But the success of the scheme is not simply in the design.'A crucial thing was to prove that modular construction is popular and that mortgage lenders would be quite happy to lend on them, 'explains Robinson.Raines Court is the first volumetric development to incorporate apartments for sale, and mortgage lenders have been reluctant in the past to recognise non-traditional construction methods.

'Nobody wants to have a form of construction the banks will not lend on, ' says Robinson.No such problems were encountered at Raines Court, setting an important precedent.

The range of different unit types incorporated within the development also demolishes another argument against modular: that it is not flexible.With the one-, two- and threebed apartments and live/work units all made from the same size module, Robinson says Raines Court demonstrates that the form is extremely flexible.

Next on Peabody's agenda is a smaller modular development in King's Cross, currently going out to tender.'Up to now we've worked with Yorkon and we always said we would give them two schemes so they could take the lessons forward, ' says Robinson, 'But now we want to see what's out there in terms of the other suppliers.'

The trust also has a planning application in with the London Borough of Southwark to develop modular 'microflats' for key workers, and is keen to build more homes using lightweight steel cassettes.

'In the past, going back to the '60s and '70s, when people hit snags and problems they were easily discouraged and didn't dig deep, ' says Robinson.'These problems have been challenging for us.They haven't necessarily been easy, but we're going to quietly get on with it and keep working with the manufacturers and suppliers to improve the process.'

CREDITS CLIENT Peabody Trust ARCHITECT Allford Hall Monaghan Morris MAIN CONTRACTOR Wates Construction MODULAR SPECIALIST Yor k o n STRUCTURAL ENGINEER Whitby Bird & Partners

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