Inverted Roof Membrane Assembly “IRMA” or Protected Roof Membrane Assembly “PRMA” Explained

Inverted Roof Membrane Assembly "IRMA" or Protected Roof Membrane Assembly "PRMA" Explained

Project pictured: 650 Parliament Street, Toronto.

We know what you’re thinking; IRMA or PRMA Roofing System? Sounds complicated, maybe not for my building. Inverted systems may sound complex, but the name simply refers to a flat roofing system where the waterproofing layer is beneath the thermal insulation. Typically when it comes to flat roofing systems, the thermal insulation is above the waterproofing membrane, but it can sometimes be advantageous to flip that around, depending on the needs of your structure.

IRMA or PRMA Roofing System roofing systems are advantageous because they protect your roof from expansion and contraction caused by weather fluctuations. These kinds of inverted roofing systems are especially valuable in Canada, where the weather fluctuates rapidly and routinely. Inverted roofing systems are also durable, and offer substantial protection for your building. The question is, is this kind of roof right for you?

Click Here for the full specs for this project.

Here are our top considerations when looking at an IRMA or PRMA roofing system.

The Advantages of IRMA or PRMA Roofing Systems:

Thermal Performance:

Inverted roof membranes use insulation, which is resistant to water absorption and can cause condensation damage during freeze and thaw cycles. These membranes are unaffected by weather change issues generally.

High Traffic Durability:

IRMA and PRMA systems are built to withstand surface traffic as well as protect from sun damage and UV rays.


IRMA roofing systems last longer than traditional built-up roofing systems. These systems have a minimum life cycle of 40 years, making them efficient and less likely to need frequent repairs.


One of the great advantages of an inverted roofing system is that leaks and other roofing issues are easily detected, which means repair costs are kept lower. They are also suitable for roof gardens or terraces, which can add value to your structure.

Extra Protection For High-Rise Buildings:

Inverted roofs are worthy of consideration for high-rise projects because they protect the membrane from damage during restoration projects.

Disadvantages of An IRMA or PRMA Roofing System:

Cold Weather Concerns:

Repairs on this kind of roofing system can be challenging during cold weather. When the ballast is frozen, repairs become challenging and complex to execute.


Inverted roofing systems can be prone to leave/dirt build-up, but this kind of issue can be managed with regular maintenance. Usually, algae and vegetation are only problematic with this kind of membrane if there is also ponding water.


The structure of a roof with an inverted membrane must be capable of holding the weight of the roof ballast, which can range from 10-12 pounds a square foot. The installation of this kind of system also requires 15-20 pounds per square foot of load capacity, which means it may not be suitable for all buildings.

Ponding Water:

Ponding water is an issue with any roofing assembly system but can be particularly damaging for an IRMA or PRMA roofing system. Ponding water lifts the insulation due to its buoyancy which in turn displaces ballast and can lead to insulation blow-off risks.

Choosing a new roofing system for your commercial or industrial structure is a big decision, and at Elite Roofing, we’re here to help you make the right one. Our experts have been designing roofing systems for over six decades, and make sure that our clients receive the right kind of roof for their individual needs. If you’re looking for more information on roof membrane types, click here, or contact us here. Find out why our clients have the #eliteadvantage.