The most common defect in flat roofs is leaks. Flat roofs tend to have less slope than pitched roofs, which makes them more vulnerable to water pooling and infiltration through seams, joints, or cracks in the roofing material. Other common defects in flat roofs include membrane punctures or tears, blistering, cracking, and inadequate drainage. Regular inspections and maintenance can help identify and address these issues before they cause significant damage to the building.