The roof is a fully welded stressed skin shell structure, made from 4mm thick grade PB102 bronze plate, fabricated off site and lifted into place in one piece, supported at four points which follow existing load paths. The entrance façade leg structures are also formed from 4mm thick facetted bronze plate, with bi-folding entrance gates fabricated in 3mm plate, hung on 25mm diameter bronze piano hinges.
The finish to the bronze has a deep antique patina gently graduating from dark tones within the niche to lighter tones over the highest part of the dome, with highly burnished and lacquered 25mm solid bronze letters mounted on bronze pins to perforated bronze plates above the entrance. Structural gutters are made from 4mm structural stainless steel and float free of the surrounding masonry. The interior is enclosed by a multi-faceted plaster vault, reflecting the geometry of the bronze shell above.
The free standing illuminated double-sided main entrance sign is also fabricated from bronze, a mix of 6mm rolled bronze plate and 25mm thick plate forming the Imperial War Museum logo.
The use of bronze was key to gaining consents within the Whitehall Conservation Area, a nationally significant and culturally historic environment. The design of the building is a fusion of architecture and sculpture, and has been designed to give an ambiguous reading of old and new ; allusions to Churchill and the military design language connected to the Museum below, and a new building of its time, built with high quality, sustainable and long lasting materials which fit into the local urban context - a strong symbol and a new identity for the Churchill War Rooms.