Roof wiki:
[he top covering of a building that prevents the ingress of weather into the building interior. Roof designs come in 'pitched', 'low slope' or 'flat' form; however, roofs should never be truly flat. Flat roofs are commonly found on industrial/commercial type structures while low slope is found on pre-fabricated or steel structures such as arenas. Pitched roofs are the primary design found on residential homes. The most common type of flat roof for industrial/commercial structures are the conventional built-up-roof (BUR) assembly and the protected roof membrane assembly (PMRA)The most popular roof membranes are the 4ply (felt and asphalt), the 2 ply modifued bitumum and the single plies such as the elastomeric (EPDM) and thermal plastic (PVC), drains provide run off for rain and snow.
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