protected membrane roof
an insulated and ballasted roofing assembly in which the insulation and ballast are applied on top of the membrane (sometimes referred to as an "inverted roof assembly").
protected membrane roof: upside-down waterproofing that lasts twice as long
A protected membrane roof (PMR) inverts the standard flat roof layering. Instead of insulation sitting above the waterproof membrane, the membrane goes down first against the deck, and rigid insulation boards are laid directly on top of it. Gravel, pavers, or other ballast weight is then spread across the insulation to hold everything in place and shield the membrane from ultraviolet radiation and weather.
The membrane benefits tremendously from this arrangement. Exposed to sun and temperature swings, an unprotected membrane degrades in 15 to 20 years. Under insulation and ballast, a PMR membrane can last 40 to 50 years because UV and ozone cannot reach it. The insulation also protects the membrane from foot traffic damage and mechanical puncture during maintenance work on the roof surface.
Common insulation materials for PMR are expanded polystyrene (EPS) and extruded polystyrene (XPS), supplied in boards 2 to 4 inches thick. Ballast weight varies by application and climate: gravel covers weigh roughly 100 pounds per 100 square feet, while concrete pavers can weigh 300 pounds per 100 square feet. Some specifications call for a separation fabric between insulation and ballast to prevent particle migration.
Trade-offs and proper detailing
The design trades roof accessibility for durability. Ballast and insulation must be removed to inspect or repair the membrane. Drainage must work properly, since water sitting on insulation can cause rot and moss growth. PMR systems require careful detailing at curbs, drains, and perimeter edges; water cannot be allowed to collect between the membrane and insulation. Proper slope, typically 0.5 to 1 percent minimum, and through-ballast drain holes are essential to function.
PMR is commonly used on large commercial and institutional buildings where the roof sees little foot traffic and long service life justifies the material cost and installation care. It is less suitable for roofs that must support mechanical equipment, frequent maintenance access, or vegetated (green roof) systems, which need different drainage and loading strategies.