structure supporting the timber framework of the conical superstructure above, which weighs 80—100 tons.

To change from a square walled room to a circular roof Nash placed diagonal beams of 12 x 12 in. Douglas fir reinforced by a V-shaped cast iron fitch plate; on these he placed a series of 12 x 12 in. secondary diagonals to form an octagon. To reduce the base to a circle, small blocking pieces were bolted in the corners. From this base the conical roof springs with the outward thrust restrained by a cast iron ring beam let into the top of the timber structure. The conical roof is formed by laminated timber ribs sitting in cast iron shoes on the ring beam. Timber boarding is then laid over the ribs. Nash originally covered this structure with a patent Dihl mastic, a form of stucco.

This was watertight when new, and had the advantage of looking like Bath stone. This was due to the linseed oil in its mixture which tended to dry out leaving the material permeable. By 1827 the finish had failed, and water had caused considerable damage. The mastic was removed and replaced with steel plates treated on their outer face with mastic and covered with copper. Poor maintenance subsequently resulted in the laminated ribs suffering, especially at the feet, where 17 of the 20 ribs of the music room needed repair or strengthening, including replacement of timber in 10 cases. The structural engineers also designed steel plates to clasp the ribs and transfer the loads back to the shoes.

However, 80 per cent of the timber structure supporting the ring beam also needed replacement. This was a major undertaking since the ceiling below could not be disturbed. Scaffolding had therefore to be erected, resting on the outer walls of the room to support a structural bridge, from which hangers were dropped to grip the ends of the three laminated ribs, which each rest on a principal diagonal beam. By raising the ribs just 2 mm the main timbers could be removed and replaced one at a time. The base of the

conical structures has tried to spread, causing the bolts holding the segments of the cast iron ring beam together to shear — these have now been replaced. The whole operation is both large and delicate to avoid disturbance of the ceiling which is described below. When the 12 x 12 in. pine beams were replaced it was the first time that anyone had tried to sterilise timbers of this scantling and 40 ft length. It was impossible to predict whether the beams would warp or split. Careful, monitoring, however, indicates that the process was successful.

Dry rot

It was Nash’s use of composite materials which contributed to the problems of dry rot in the building. Apart from water penetration, moisture given off by visitors (especially on damp days) condenses on the cast iron which then feeds rot in

adjoining timbers. Treatment of the dry rot has therefore been of two kinds. In some cases orthodox methods of preventing external damp penetration, drying out timber and masonry, and cutting out rotted wood, and using chemical fungicides were used. In other cases the interior decoration has been too valuable to remove to get at affected areas. A telemetric system of monitoring the damp content in many areas of the Pavilion has therefore been devised by Brian Ridout of Hutton & Rostron. If damp is eliminated and the fabric is kept dry the rot goes into remission. The telemetry alerts staff to high damp levels and the visitor flow can be controlled or localised area of dampness investigated. The boreholes used to investigate the extent of rot in voids have been left plugged rather than filled so that visual inspections with borescopes are still possible. The monitoring apparatus and

Above: Nashs view of the East Front of the Pavilion, 1826. Below: Holland’s designs: the ‘Marine Pavilion”, 1787.

It will unfortunately not be possible for many delegates to the ACO Confer ence to take part in the conservation workshop tours but the skills on display there are evident for all to see in existing work at the Pavilion Subject to natural disasters delegates will see the building for the first time in a decade unobstructed by scaffolding which at one time incorporated the largest self-supporting span erected in this country Don t miss the oppor tunity to see the Pavilion while it looks so good.

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CONTEXT 29