The Geffrye Museum has long been a relatively unambitious display of the way people have lived over the centuries. Now it is the client for a major extension from the drawing boards of Branson Coates. Despite a rocky design history it has worked out and behind the modest almshouse facades a few streets north of fashionable Hoxton a spiralling wall of brick uncoils from a hidden garden.
It's round the back of the former almshouses which make up the Geffrye Museum in London's Hackney, an elongated twostorey brick horseshoe with a low pitched roof. It provides the museum with much needed additional exhibition space - and a modestly sensational pavilion in its rear garden.
Sunk into the garden and separated from it by a continuous sunken area, the glass and metal cladding of the lower level is protected by a sloping skirt of sunscreens attached to the base of a stepped concrete edge beam curving around the perimeter of the first level slab. On the north side this skirt merges into the roof of a sloping ramp which rises to ground level and a glazed cafe area which also serves as the link with the Georgian main building. Inside, the first floor is a curving exhibition space with a bookshop at one end and a kitchen plus cafe at the other linking to the old building. At ground/basement level are education rooms, a workshop and a design centre accessed by a dramatic curving staircase at the core of the building's curve.
There's something odd about the curving exterior - and then you get it.
The Flemish bond brickwork, of firstquality multi stocks made from a Sussex clay, is laid in spiralling courses. There is a bit of optical confusion because the wall is a big one but the defining clue is given by the stepped supporting edge beam.
In fact the slope is one in 12 and derives from the slope of the ramp.
Thick thermal flywheel
You can tell from the occasional opening that this external wall is a very thick one. In fact there is a 215mm inner leaf, a 250mm cavity with 180mm of insulation and then a 215mm outer leaf. The wall is a thick one because it was decided to use the structure as a thermal sink in order to iron out temperature differences. The inner leaf, which is mostly plastered, has reinforced-brick piers at six to eight metre centres. They are a brick and a half on side with a steel rod down the middle tied back to the supporting slab and filled with concrete. The function of these piers is to support roof trusses and provide overall restraint for the upper parts of the structure - and to restrain the inner and outer brickwork of the wall. Although the outer wall is actually a continuous 80m long piece of brickwork, it was treated by the engineers as a series of panels spanning between the positions of the internal piers. It is at these points that the outer wall is tied back to the piers by fishtail ties slotted into vertical steel channels. These are supported by substantial stainless steel flats acting as both struts and ties protruding through the insulation from their fixing points in the mortar beds of each pier. The inner leaf of the walls was built conventionally off the concrete slab - the sawtooth lintels actually extend under the cavity and the external leaf and have vertical drain holes.
As Alan Baxter engineer Nick Ling explains: 'It's not really a cavity wall, more accurately it's an internal skin of structural brickwork with piers and a brick cladding which gains some of its stability from the ties at the pier positions. We adopted the fishtail-and-channel approach because with standard horizontal mortar beds on the internal wall and sloping courses on the outer you could never be sure you could line up the ties.
We considered a number of possibilities including standard ties but this was the simplest and most buildable.'
Eliminating expansion joints
The stepped edge beam supporting the outer leaf is actually a connected set of precast sawtooth lintels hanging off the edge of the first floor slab. With 900mm long steps it was possible to set out the brickwork above very accurately. The designers were very anxious that there should not be any expansion joints so the mortar is a 1:2:9 (cement: lime: sharp sand) mix laid as standard 10mm thick joints.
Alan Baxter has some experience with structural brickwork and lime mortars and, as Ling explains 'There were little special features such as not having damp proof membranes - except at the position of the access ramp. This means the wall will not slide laterally and will encourage a build-up of little cracks, which will be distributed across the whole wall. Because the mortar is weak, small cracks will be repaired by the leaching of the lime.'
Doug Branson of design architect Branson Coates says, 'We always thought to do the building in brick. And we resisted the temptation to do it completely differently. The question we asked was, how do you take a traditional material and give it a twist? And that is literally what we did.
'The spiralling courses actually give a sense of movement as the block goes round the corner, ' says Branson. People have said to us that we were being naughty. But you find this kind of brickwork in places like, say, the Barbican's ramps. And if you look a the Victorian buildings opposite you see lots of examples of brickwork following the line of the structure - and no-one says that this is particularly wilful.'
There was a point during the examination of cost implications when it looked as though the sloping brickwork might have to go but the cost analysis showed that it was very little more expensive than standard brickwork. As it was, the brickies were apparently sorry to finish the project - which, says Branson, 'was beautifully built by main contractor Kier's people'.
Holes in the wall
The sloping courses and the unusual 680mm thickness of the wall produce some interesting details at openings. The logic of the sloping beds calls for sloping window and door heads following the line of the brick courses. And that is exactly what the architects designed.
With reveals a brick and a half deep and traditional flat arches with special rubbers, the openings are unusually satisfying - although it turned out that in the cost review it became necessary to do openings with concealed concrete lintels and brick slips. This was also the case with the big arched openings in the two gable end walls. Branson is philosophic. It would have been better to have used traditional construction, but this ends up looking just about right.
There's an oddity, you might think, in the fact that although it costs less to do a replica version, a replica takes a lot more effort and cutting and work and detailing than it does to deploy the time-honoured method.