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A NEW CURVED ROOF FOR A HISTORIC TOWER

WORKING DETAILS - MARTELLO TOWER

The tower has been restored and converted, with living and dining areas at roof level opening on to a terrace.

The roof measures 13 x 8m and its geometry is based on a 168mm-diameter CHS, which forms a ring beam at the perimeter, supported by five pairs of canted 50mm-diameter Macalloy bars. Each pair creates a V-shape, which sets up a rigid triangle to provide support and lateral restraint.

The three-dimensional form of the roof is created by curved 178 x 102mm UBs bolted to the ring beam at 1.8m centres. Timber rafters and firrings set between the UBs act as the base for a plywood deck - two skins of 9mm ply laid diagonally.

It is insulated with a double layer of rigid insulation, also laid diagonally and covered with a single-layer membrane. Three circular rooflights are raised above roof level on upstands of bonded plywood rings.

The roof tapers at the perimeter to a fine edge, supported by tapered steel brackets fixed to the ring beam.

The edge overlaps the glazed clerestory of single sheets of frameless curved glass held in place by aluminium channels.

The wall tops are protected with a DPC and a covering of zinc sheet with standing seams; the sheet accommodates both the variations in thickness and the slope of the vast walls.

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