Flat roofs are constructed basically with joists spanning the gap between two walls, covered by sheet timber or other material. In the case of timber a further, waterproof cover is required. The weight of these last two coverings, together with the span, form the basis of the calculation for the size of joists required. A further part of this equation is discovered in Approved document A of the Building Regulations (Structure) where the possible snow loadings, depending on area of the country and height above the ordinance datum, are taken into consideration.