Stack volumetric boxes and you get unusable space between each module plus a buildup of thickness requiring materials to skin redundant space. If you plan to build 2,000 homes a year, which is the capacity and business plan of Advance Housing, Barratt's joint venture with Terrapin, that's a cost that has to be designed out. Advance's response is Hybred, a mix of cassette wall panels, floors and roofs, with volumetric bathrooms and kitchens.
The structure, framed in extruded lightweight steel, is a true cassette in the sense of being a closed panel. So the soffit of the first-floor cassette is finished with a robust cellulose fibre-reinforced plasterboard, prepainted with two coats of emulsion (complete with ceiling rose and light fitting). The tops of the panel are finished with chipboard, and wiring looms are closed within the void.
Connections between the panels depend on a push-fit electrical system that allows them to be wired together. Porthole-type inspection hatches, typically about 14 in a two-storey three-bed house, allow access through the floor forming the top of the cassette. Wall panels come with radiators already fixed and fenestration pre-installed.
There are pods for factory-finished bathrooms and kitchens pre-fitted with white goods. These fit into cut-outs of the ground-floor and first-floor levels. Tolerances for foundations are about +/- 5 mm and for walls +/- 2 mm, but the finished look suggests absolute perfection because the house designs eliminate visible joins. Each wall is faced with a single sheet of the tough plasterboard and the joints are where the room partitions fall. This means all visible surfaces are smooth and clean - and the plasterboard is also bonded to the steelwork using a proprietary system to eliminate nail popping or screw holes.
Whereas the best-run Barratt sites take 16 weeks to finish a three-bed house, Advance can construct a house in 10-12 weeks. Barratt programmes four weeks from planning permission to completing foundations, the same lead time for Advance to take an order and produce components to ship to site. It then takes less than two days to erect them.
The scaffolding goes up, the suspended ground floor is laid, using a patented system of castellated concrete insulated with expanded polystyrene, and a crane drops in the kitchen and bathroom pods. The panels are then dropped in sequence, with the room in the roof cassettes (complete with batten and cross-batten) lowered on top. Advance schedules six weeks to face the house with bricks, tile the roof, landscape the gardens and give exposed surfaces a finishing coat of paint.
There is additional flexibility to the system in that either the roofers or bricklayers can work first, according to availability.
The Advance factory has a 2,000-homesa-year capacity. The company, which has had house plans approved by the Housing Corporation to streamline grant bids, says architects can change standard plans but should be aware that changing depths of plans or facing materials presents little problem, but widths and heights are set by manufacturing jigs, and alterations will be expensive - unless you are designing for volume.