Flashings must provide watertight connections between the various roof and wall elements. To do this, they must have flexibility, resistance against deterioration and physical damage, and compatibility with adjoining surfaces. Flashings can be classified into expansion joints, gravel stops and eave flashings, flashings at roof penetrations, connections of horizontal with vertical surfaces, and valleys and gutters. Flashings can be made of metals, built-up ingredients, or elastomeric materials.

Frequent failures of flashings can be caused by omission or improper use of expansion and relief joints; inadequate attention to the mechanical properties of materials involved and their thermal and moisture properties; disregard of design details; use of inadequate quantities of waterproofing materials; inadequate and careless workmanship; and exposure of materials to moisture and other damage before and after installation. Recommendations for proper design and construction of flashings are given.