Serious problems can result if bituminous roofing membranes are allowed to shrink. Base flashings can be torn, metal counterflashings displaced, improperly secured wood blockings pulled out and masonry work along the roof edge damaged. Membrane shrinkage, which often results in roof leakage, appears to be on the increase despite the technical information available on the subject. It may be that the relevant information has not always reached the design and specifying authorities or roofing contractors. It is the purpose of this Digest, therefore, to outline the factors involved in membrane shrinkage and to suggest methods for controlling them.
All materials react to temperature changes by contracting and expanding.
For solid materials this movement is usually reversible, and expansion joints, flexible connections or compressible filler materials incorporated in a structure or building component can accommodate it. Bituminous membranes, though relatively strong in tension, are weak in compression and this difference in behaviour is further magnified by the system itself, which uses adhesives to bond various materials together. Where adhesion is inadequate, the contractive stresses induced in a membrane by falling temperatures may be sufficiently large to break the bond between certain components, allowing the membrane to contract. Membrane shrinkage can therefore be defined as the reduction in size of a membrane as a result of thermal cycling at cold temperatures. It is manifested by inward displacement at the edges.