The effects of thermal movement of bituminous built-up roof membranes are related to some common failures of built-up roofing observed in field exposures.A procedure for determining the amount of thermal movement of built-up membranes is described.Data are given for the thermal movement of various bitumens and reinforcing felts used in the con * struction of built-up roofs and for the composite membranes over a temperature range construction of built-up roofs and for the composite membranes over a temperature range of +30 to -30F.The data obtained are related to field experience and suggestions are presented which will prove beneficial in reducing the incidence of built-up failure due to thermal movement.(Author).