Roofs protect the interiors of buildings against rain, snow, wind, and variations of temperature. The roof may be subjected to adverse temperatures, humid or aggressive conditions or fire hazards. It may be used for parking, storage, as a viewing platform, accessway or roof garden. Access for maintenance is required to all roofs and to any ventilation-type equipment and roof lights. Flat roofs usually have self-draining falls to gutters and discharge systems adequate to remove accumulating water before unacceptable loads or bypassing of the weatherproofing results. Thus precautions to avoid blocking of the drainage system are essential, and the degree of fall must allow for elastic and creep deflections and construction tolerances.