Materials have different coefficients of thermal expansion and contraction. Differential movement between components and thermally induced stresses within materials, if not allowed for in the design and construction of the roof, can cause tearing, bending, loss of securement and ultimate failure of the roof. Air Movement Interruption of the laminar flow of wind as it blows over and around a building results in turbulence and areas of positive and negative pressure. This is sufficient force to scour gravel ballast, flip concrete pavers and peel off membranes that are inadequately secured. Keeping a roof on a building is no small task.
Air flow through a building envelope can carry with it significant amounts of heat and moisture.
Air escaping from a building through a 20 by 20 mm hole can, over a heating season, transfer 30 L of water into the building envelope. Depending on the materials in the envelope, this moisture accumulation, as mentioned above, can lead to corrosion, rot, expansion, loss of strength and freeze-thaw deterioration. The component or components in a roof intended to resist air flow must be capable of withstanding the differential pressures that can develop across the roof. Just as variations in temperature will cause expansion and contraction, so also can changes in moisture content. Moisture sensitive materials must be protected from excessive moisture accumulation and movements must be allowed for.