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Heart of glass

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MetalWorks Little gems

BB Partnership's sensitive extension of Highgate Pavilion maintained the integrity of the original Lorenz/Arup design

There is a house in Fitzroy Park in London's Highgate that has an architectural history.

Designed in 1958 by Erhard Lorenz with the help of his friend Ove Arup, it is a brick, concrete and glass pavilion with a shallow butterfly roof. It was reckoned good enough to be featured in both The Architectural Review and Architectural Design.

In 1995, the AR focused on Eva Jiricna's internal remodelling and her lightweight steel and (mostly) glass leisure facilities.

Three years later the owner made an unsuccessful planning application to enlarge the house, then sold it.

Balancing recent traditions

In 1999, the new owner asked BB Partnership to reinvestigate the possibility of extending the house.

In looking through the previous application and adjudication, BB Partnership's Henry Busiakiewicz discovered that the earlier application was for an extension in front of the house and that the inspector had actually reckoned that the number of bedrooms was disproportionately small for the size of the house and its plot. The architect cautiously explored several possibilities with the planners and they finally agreed to the current scheme at the back of the house.

Says Busiakiewicz: 'We were looking to create a balance between the theme of the Jiricna glazing and transparency and the original brick and concrete and glass house - bearing in mind that for guests there was a need for privacy.' Because the new structure is a guest suite, there is an all-glass link back to the main entrance hall. It is all glass because it encloses big windows to the existing study and living room.

Simple arrangement

The pavilion's plan is simple enough: a central bathroom with a kitchen recess, an open bed area at the back and a living area at the front, where two big sliding glass doors lead out to a small terrace covered by a cantilevered extension of the single-ply Sarna roof.

Structurally it is a concrete slab on stub pillars with internal load-bearing bathroom walls and a steel roof structure. Although there are several panels of brickwork around the perimeter, they are just that: panels.

With the exception of one short section at the back, all have clerestory windows above them so that the roof appears to float. Except for the two sliding doors at the front, the glazing under the roof edges is frameless; in fact, they are standard double-glazed units.

Busiakiewicz says: 'Solglas helped with the specification for the glass and our engineer, Alf Campion, did definitive work on structural glass.'

There is no structural support from the external walls - not that this would have been particularly effective because the brickwork is all stack bond, which acts as a series of connected columns rather than spreading the load like conventional brickwork. There are two 114mm diameter steel columns set back inside behind the line of the glazing. They support the front section of the roof. At the back a single column lines up in the corner of the bed space to carry one of the two main longitudinal beams.

The longitudinal beam on the other side is supported at the back by a bracket off the existing house wall. The roof structure is more complicated than that because it also involves diagonal bracing and deploys the loadbearing bathroom walls for some lateral and longitudinal support. The outer 300400mm of roof is supported on welded jack beams. You do not see this because the internal walls and ceiling have a hard plaster surface.

One of the complicated structural issues was the conflict between the need for an adequate steel depth for the root of the cantilever, the need to maintain a 225mm roof-edge depth and the fact that there was a 150mm deep slot in the ceiling where the roller blinds were hidden directly behind the glazing - including the glazing under the cantilever. In addition, there was a smaller slot for the sliding mechanism of the doors at the front, which further weakens the cantilever beam and adds to the load. Solving this problem involved some ingenuity and the use of welded stiffening plates.

The main factor in this problem was that, externally, the architect was anxious to maintain as narrow a roof edge as possible.

They had created a kind of second roof on top, set well back behind the visible edge.

This rather thicker roof build-up accommodates insulation, water pipes, electric conduits, air-conditioning ducts and the like.

So, close-up at least, the edge of the roof appears to be a 225mm by 75mm stainless steel U section on its side. In fact it is decorative. Somewhat in the tradition of the Seagram Building, it is a steel facing to a steel structure - without, here, the need for intervening fire protection. Busiakiewicz says: 'It was a difficult thing for the fabricators to achieve on site. There couldn't be any face fixings or welded attachments on the back.

Eventually, the engineer devised a complex arrangement of fixings and the channel came on site in relatively short long sections.

Each of them had to be aligned vertically and horizontally with very fine joints.'

Campion engineer John Paul says: 'The architect didn't want a painted or anodised finish, so we couldn't weld anything because you can't rely on polishing to remove weld bloom - although, in the end, we did have the corners welded and buffed. The fabricators reckoned we could get away with polishing out the discoloration in this special circumstance. To fix the stainless steel channel, we used a 3M adhesive to fix a smaller channel to its back and bolted that on to the edge of the main steel roof structure.'

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