GB/T 328.12-2007 "Test Methods for Building Waterproof Sheets - Part 12: Bitumen Waterproof Sheets - Dimensional Stability" specifies the method for determining the dimensional stability of bitumen waterproof sheets. It defines the test apparatus, procedures, and requirements for result calculation. Key apparatus includes an electrically heated air-circulating oven with a temperature control accuracy of ±2°C, a measuring tool with a division value of 0.5 mm, and a device for suspending or laying flat the test specimens. After heating the specimens at the specified temperature, the dimensional changes in both the longitudinal and transverse directions are measured. The dimensional change rate is then calculated to evaluate the dimensional stability of the sheets under heat exposure, providing a quality basis for engineering applications.