In the case of timber roof a further, waterproof cover is required. The weight of these last two roof coverings, together with the span, form the basis of the calculation for the size of roof joists required. A further part of this equation is discovered in Approved document A of the Building Regulations (Structure) where the possible snow loadings on roof, depending on area of the country and height above the ordinance datum, are taken into consideration. It is therefore important, before building your roofing structure that you have the desired building regulation approval. There are 8 “maximum span” tables in the regulations, which deal with the loading and roof span of the flat roof together with any slight pitch over 10 degrees that it may have to dispose of rainwater.