working details; Manser Associates’ dainty glass barrel-vaulted porch is totally transparent, yet is robust enough to keep out the draughts
The Gatehouse - a two-storey Victorian red-brick building in the grounds of Chiswick House, London - is the headquarters of a retail promotion company. The original main entrance, set in a banded brick arch, used to lead directly into the reception space, causing draughts. Kim Quazi, of Manser Associates Special Projects, has designed a delicate glass porch with glass doors to act as a draught lobby, a solution so transparent that it was acceptable to both the planning authority and to English Heritage. The porch was designed to be as light and delicate as possible - it is made entirely of glass with no visible frame or fixings, the only visible joints being the glass-to-glass silicone joints.
The shape of the porch was determined by the original arch opening. The roof is a glass barrel-vault which follows the profile of the arch and extends beyond the double glass doors as a ‘pen-nib’ shaped canopy. The wall panels which support the canopy are also extended and canted; in the architect’s words, ‘they relieve the ‘deadness’ of a flat arch’ and the extended canopy prevents the porch from looking box-like.
The floor of the porch is laid with limestone paving, and a trench heating system around the perimeter of the glass walls prevents condensation. The original solid entrance door has been removed and replaced. The top and bottom edges of the doors are fitted with Dorma covers for lockplates and pivot restraints.
The barrel-vaulted glass roof rests on two walls of 18mm laminated glass, each braced by a 34mm laminated glass fin, the edges jointed with black structural silicone.
The 10mm toughened glass doors are set in what appears to be a single panel of glass, but is actually a fanlight and two side walls, laminated together with mortice-and-tenon joints. The technique, first developed by Tim Macfarlane to join glass columns and beams (aj 22.7.92), gives the panel the strength to resist the forces of the door opening and closing. The edge of the inner 10mm glass layer of the fanlight panel is cut back to let the outer layers form the mortice, and the outer 10mm glass layers of the side walls are cut back to let the inner layer form the tenon.
The glass has no visible frames: they are concealed inside the inner rebate of the brick arch and in the floor slab. Their design had to allow for the imprecise curve of the arch, and the very precise and inflexible nature of glass. The frame is fabricated from a curved stainless-steel angle screwed to a curved plate; together they form a slot to house the edge of the glass, which is set in Arbokol, a two-part polysulphide. On the arch the curved plate is screwed to the brickwork, stiffened with an mdf packer and covered with a stainless-steel trim. The frame in the floor is almost identical - two angles are screwed together to hold the edge of the glass, and the leg of one is bolted to the floor slab.
The glass for the porch was produced by Firman, which was also responsible for the Hampstead conservatory. The barrel-vaulted roof posed particular difficulties; although it is divided into two panels they were, at 3650 x 1716mm, at the limits of the size possible to bend and toughen at the required thickness (19mm). The glass was laid on a curved mould and heated till it slumped to the shape of the mould, then toughened by stressing the surfaces with cold air from pressure nozzles.