Good morning Teabag,
Re your enquiry about lap in roof tiles etc. A flat roof membrane, such as felt, butyl, rubber, pvc etc relies on keeping out water through the fact that it's composed of impermeable material, so like a drum skin it is only effective if it's free from punctures. Being continuous and impermeable it can be employed at low roof pitches 15 deg. or less. Pitched roof coverings perform in a different manner, being composed of many small rectangles/ squares which overlap to form a barrier to rain and snow. The overlap of one tile (slate or shingle) over another is referred to as lap and the amount of lap depends on the pitch of the roof, generally the higher the pitch the less lap required. This stems from the fact that overlaping tiles are subject to capillary attraction, i.e. water trapped between them will tend to migrate up hill. This effect is more pronounced at lower pitches, as gravity has less counter effect. The problems of capillary attraction are worse where standing water is concerned, such as a large thawing snow load on a roof. The lap and pitch of the covering also dictates the roofs ability to resist wind driven rain. so depending on exposure some coverings and roof pitches are more suitable than others. It is for all the reasons outlined above that tiles and slates are unsuitable for use on low pitches, as no guarantee can be given that moisture won't pentrate into the interior of the roof space. Incidentally the felt membrane (sarking) beneath tiles and slates acts as a second line of defence where any moisture does find it's way past the tiles/ slates. Hope that explains it for you.