Moisture Barrier: The primary function of a damp roofing membrane is to stop the ingress of water and water vapor. Unlike a breather membrane, which allows vapor to pass through, a damp-proof membrane (DPM) is 100% watertight.
Protection of Insulation and Structure: It is often installed to prevent moisture from reaching and compromising insulation materials like mineral wool or cellulose. By keeping insulation dry, the membrane helps maintain its thermal properties. It also protects the roof structure, such as timber, from rot and decay.