The values of breaking load, elongation and thermal expansion of nine bituminous built-up membranes prepared by roofing contractors under field conditions and by technicians in the laboratory were measured at sub-freezing temperatures.The data indicated that the field-prepared specimens agreed favorably with laboratory-prepared specimens, although the samples from the field frequently reflected higher values for thermal-shock resistance than did their laboratory-prepared counterparts.The higher values were attributed to the smaller amounts of bitumen used between the plies of the field specimens.The results appeared to support the validity of the application of the 'strength-thickness rule' to bituminous built-up membranes.(Author).