CSLB C-39 Roofing ContractorLow-Slope RoofingHard
A design engineer specifies an inverted roof membrane assembly (IRMA) for a low-slope building, in which rigid extruded polystyrene insulation is placed above the waterproofing membrane rather than below it. What additional installation requirement is necessary because of this insulation placement?
- AA ballast layer, such as gravel or pavers, must be placed over the insulation to resist wind and buoyant uplift
- BThe membrane must be left exposed with no insulation covering to allow inspection
- CA vapor retarder must be installed directly above the membrane before placing insulation
- DThe insulation must be mechanically fastened through the membrane into the deck
Show answer & explanationAnswer & explanation
Correct answer: A. A ballast layer, such as gravel or pavers, must be placed over the insulation to resist wind and buoyant uplift
In an IRMA (protected membrane roof) assembly, the insulation sits above the membrane and is exposed to weather and buoyant forces from water; because it cannot be adhered to the membrane below it, it must be held down with ballast (gravel, pavers, or similar) to resist wind uplift and floating.
Why the other options are wrong
- B. Leaving insulation off defeats the purpose of the inverted assembly.
- C. A vapor retarder is unrelated to holding insulation in place above the membrane.
- D. Mechanically fastening through the membrane would puncture and compromise the waterproofing.
Inverted Roof Membrane Assembly (IRMA)
A roof system where waterproof membrane is installed first, followed by rigid, water-resistant insulation (typically XPS) placed above it, requiring ballast to hold the insulation down.
- Protects membrane from UV, thermal cycling, and mechanical damage
- Insulation must be moisture-resistant (extruded polystyrene)
- Ballast (gravel/pavers) needed to resist uplift since insulation isn't adhered
Memory trick: Flip it — insulation floats above, so weigh it down.