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By: Ben Powell

September 30th, 2021

Over the past several years, the popularity of the IRMA (Inverted Roof Membrane Assembly) has grown significantly. Also known as a Protected Roof Membrane Assembly (PRMA), the roof design refers to a low slope roofing system where the waterproofing materials are installed below the insulation boards rather than on top. Additionally, flat roofing systems can have gravel, stone or pre-cast pavers set on top of adjustable pavers.

Typical Inverted Roof System Diagram

To understand its origin, the first IRMA system was created by The Dow Chemical Company and was brought to the public in the late 1960s. The design was to install a glaze coat after three layers of organic felt were applied to the roof deck. A flood coat of asphalt was poured and cooled followed by the extruded polystyrene insulation. Finally, crushed stone was set over the insulation boards at a minimum of 10 pounds per square foot. Over the years, the original design developed several modifications as membrane manufacturers eventually created their own systems. Products such as modified bitumen and thermoplastic sheets helped reduce application problems and incorrect installations.

The IRMA system has grown increasingly popular due to its numerous advantages and workarounds for possible limitations. If high winds are an issue, gravel can be replaced with concrete pavers which still protect the membrane from pedestrian foot traffic, ultraviolet rays, and thermal shock. For roof replacements where flashing height restrictions limit the use of standard insulation, the inverted built-up system allows for customizable finishes in products while still protecting the life cycle of the roof membrane below.

Other advantages of the IRMA system include overall longevity in life cycles ranging from 20-40 years. Leaks and other issues are easier to identify which helps reduce the cost of repair projects. There are many options when it comes to aesthetics and choosing between sizes, textures, and color. We have also seen an increase in utilizing the combination of both precast pavers and vegetative roofs systems to enhance the visual designs of certain roofs or terraces. Vegetative roofs provide many more advantages, such as providing a rainwater buffer, reducing ambient noise, and increasing solar panel efficiency.

Roof with IRMA System

While there are multiple benefits for this roofing system, there are a few disadvantages that are important to note. Ponding water can lift the insulation due to its buoyancy if the roof pitch is not installed correctly toward drains, which can damage the waterproofing layer. Due to the heavy protection of the IRMA system, the weight requires approximately 15-20 pounds per square foot which can be a significant increase in dead load.

The possibilities for IRMA roofs are constantly growing, materials are improving, and new products are always being developed. Next time your building is preparing for a roof replacement, you may consider the functionality and fine aesthetics that an IRMA roof system can provide.

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About Sullivan Engineering, A Rimkus Company

Sullivan Engineering provides high-quality building envelope restoration and compliance solutions.

We partner with facilities managers and account executives to provide technical expertise and project management for building envelope restoration, compliance, and maintenance.

Our solutions reduce the overall building life cycle maintenance costs by creating long-lasting, high-quality work for years to come.


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