Paxton set the dimensions of this prism by using the length of single pane of glass () as the hypotenuse of a right-angled triangle, thereby creating a triangle with a length-to-height ratio of 2.5:1, whose base (adjacent side) was long. By mirroring this triangle he obtained the gables that formed the vertical faces at either end of the prism, each of which was long. With this arrangement, Paxton could glaze the entire roof surface with identical panes that did not need to be trimmed. Paxton placed three of these by roof units side-by-side, horizontally supported by a grid of cast iron beams, which was held up on slim cast iron pillars. The resulting cube, with a floor area of by , formed the basic structural module of the building.