The original Hyde Park building was essentially a vast, flat-roofed rectangular hall. A huge open gallery ran along the main axis, with wings extending down either side. The main exhibition space was two stories high, with the upper floor stepped in from the boundary. Most of the building had a flat-profile roof, except for the central transept, which was covered by a barrel-vaulted roof that stood high at the top of the arch. Both the flat-profile sections and the arched transept roof were constructed using the key element of Paxton's design: his patented ridge-and-furrow roofing system, which had first seen use at Chatsworth. The basic roofing unit, in essence, took the form of a long triangular prism, which made it both extremely light and very strong, and meant it could be built with the minimum amount of materials.