A laboratory study was undertaken to determine the load-strain properties and watertight/strain levels of aged polyester reinforced built-up roof membranes. Tests were run at 0°F on field samples of roof membranes ranging in age from three to five years. One sample of a glass and polyester reinforced membrane was also evaluated. Strain energy values were determined for all specimens tested. A three point watertight integrity test was devised, using a prestressing level of first yield, average ultimate strain, and end point strain.