On the other hand, experiments show that the tensile strength of a 4-ply organic felt and asphalt membrane can be as high as 240 lb per in. This suggests that temperature-induced stresses tend to be smaller than the tensile strength of the material itself, and that membranes should be immune to splitting if other factors are not involved. A built-up membrane, composed as it is of felts and asphalt or tar, exhibits the viscoelastic properties of the bitumen and does not behave elastically except when loaded at high speed. Membrane breaking strain, even at low temperatures, far exceeds the strain a membrane is expected to withstand in service. Stresses that could build up in membrane with time are dissipated through cold flow.
Membranes generally have adequate tensile strength.
Methods of Restraining a Membrane To a large extent the method of supporting and assembling roofing determines its behaviour. A membrane neither attached to its support nor restrained along its edges can expand and contract without developing stresses, which generally appear only when resistance has to be overcome. This might well be the ideal system if the threat presented to flashings by a membrane whose size varies continually in response to climatic conditions could be resolved. In practice it is necessary to restrain the membrane by one of two methods. In the first, a loosely laid membrane is restrained along its perimeter so that this attachment transmits all tensile stresses to the structural deck.