Abstract: A demountable building structure comprises a plurality of arches pivoted at their lower ends for vertical swinging movement between raised and lowered positions, and a plurality of flexible transparent plastic sheets extending between and supported by the arches and rendering the space beneath the arches weathertight. The sheets have tension cables along at least one edge thereof, the cables being secured at their opposite ends adjacent the lower ends of the arches. Various embodiments of securement of the sheets to the arches are disclosed.