Protected Membrane Roof (Inverted Roof)

A protected membrane roof (PMR), also called an inverted roof, upside-down roof or IRMA (insulated roof membrane assembly), reverses the usual order of layers. The waterproofing membrane sits directly on the deck, and moisture-resistant insulation boards, usually extruded polystyrene (XPS), are laid loose on top of it. Gravel, paving slabs or a green-roof build-up holds the insulation down against wind and flotation.

Why invert the roof

• The membrane is shielded from UV light, extreme temperature swings, freeze-thaw cycles and foot traffic.
• The membrane stays near the building's interior temperature, reducing thermal stress.
• The top surface can be used as a terrace, podium, car park or green roof.

In the UK, BS 6229 treats inverted roofs as a subtype of warm roof construction.

Typical build-up

1. Structural deck, usually reinforced concrete
2. Waterproofing: hot-melt rubberised bitumen, liquid-applied membrane, modified bitumen or single-ply
3. Optional protection layer
4. XPS insulation boards
5. Water flow reducing layer (WFRL) or filter fabric
6. Ballast, paving or green-roof layers

Design considerations

Because rainwater runs beneath and between the boards, the insulation is partly wet and cold water flowing under it carries heat away. UK practice therefore adds a correction to insulation thickness: BS 6229:2025 calls for at least 2.5% extra where declared and design values are the same, and greater corrections for blue roofs. A WFRL with laps of about 300 mm (12 in) reduces this cooling. The roof must drain at two levels, over the WFRL and at membrane level, and decks must be built to falls without backfalls or hollows. Ballast must be heavy enough to stop boards floating. Condensation can occasionally form under the boards when the membrane is colder than the dew point.

Repair

Leak location is harder than on an exposed roof, because ballast and insulation must be lifted. Electronic leak detection is often used at handover on concrete decks.