 On the lower level, these larger spaces were covered by the floor above, and on the upper level by longer spans of roofing, but the dimensions of these larger spaces were always multiples of the basic by grid unit.The modules were also strong enough to be stacked vertically, enabling Paxton to add an upper floor that nearly doubled the amount of available exhibition space.Paxton also used longer trellis girders to create a clear span for the roof of the immense central gallery, which was wide and long.