If you are a commercial or high-rise building owner in the Greater Toronto Area, you may face unique challenges when selecting or restoring a flat roofing system. Everything from weather and building design to weight limits and future restoration can play a role in this decision. Many building owners are familiar with built-up roof (BUR) systems, but aren’t as well-versed in inverted roof systems.
With Inverted Roof Membrane Assembly (IRMA), also known as a Protected Membrane Roof Assembly (PMRA), the insulation sits above the waterproofing layer. This protects it from temperature changes, freeze-thaw cycles, and physical damage. With a traditional BUR, the waterproofing layer is placed on top of the insulation, leaving the membrane exposed to the elements.
Built-up roofs are more commonly used, but depending on the needs of your structure, flipping that order around can pose several advantages.
Today we’ll examine the differences between IRMA and BUR and why an “upside-down” roof may be more beneficial to you as a property owner in the Greater Toronto Area.
IRMA or PMRA Roofing Systems: Defined
An inverted roof membrane assembly, often called an “upside-down” roof, reverses the traditional order of roofing components to increase protection of the waterproofing layer. Commonly used on high-rise and commercial buildings, here is a look at the layering system of inverted roofs:
- The waterproofing membrane is installed directly on the substrate to protect the building from moisture, UV exposure, temperature swings, and mechanical damage.
- Insulation is added to regulate temperature and reduce thermal stress.
- A filter fabric layer is added over the insulation to encourage proper drainage and to prevent ballast from getting underneath the insulation.
- A ballast layer of crushed limestone or river stone is installed on top, holding the system in place and guarding against wind uplift.
Known for their durability, long service life, and protection during major restoration work, inverted roofs are especially valuable on mid-rise to high-rise residential and commercial office buildings where extra protection is necessary. Because the membrane is shielded, equipment is not planted directly on it; rather, it rests on the padding and insulation. Also, the concern with traffic over exposed membrane is eliminated.
Built for long-term performance in demanding environments, inverted roofing systems bring many benefits.
Benefits of IRMA and PMRA Roof Systems
From longer life expectancy to cost savings through reuse, there are many advantages associated with IRMA systems.
Longer Membrane Life Expectancy
- Because the waterproofing membrane is installed beneath the insulation, it is protected from weather extremes and direct UV exposure.
- Foot traffic, tools, and rooftop activity don’t pose as much of a risk of punctures and abrasions.
- The membrane doesn’t age as quickly thanks to less direct exposure to the elements.
- You can be assured of a longer lifespan for your roof with IRMA versus BUR and other traditional roof assemblies. [Source: CMHC]
Can Accommodate Rooftop Equipment and Restoration Projects
- Heavy equipment such as scaffolding, outrigger beams, and solar panel systems don’t rest directly on the membrane. Instead, they are supported by insulation and ballast layers.
- This reduces the risk of damage to the membrane during construction or long-term equipment use.
- IRMA roofs can better support the demands of major restoration projects involving window replacement, concrete repairs, or railing upgrades.
- This maintains the integrity of the waterproofing system even when the roof used as a working platform.
Cost Savings Through Reuse
- If the existing insulation is in good shape (dry, structurally sound, etc.), it can be reused.
- Building owners can qualify for credits or reduce material costs during restoration or reroofing projects.
- Labour and disposal expenses associated with removing and replacing large amounts of insulation and ballast can be reduced.
- When an inverted roof system is already in place, it is more cost-effective to maintain the same assembly versus converting to a different roofing system.
Limitations of Inverted Roof Systems
As with any roofing system, there are some drawbacks to using IRMA versus BUR.
Structural Weight Requirements
- Ballast weight can be high – between 10-12 lbs. per square foot.
- Traditional BUR systems, on the other hand, average closer to 7 lbs. per square foot — even with gravel.
- Not all buildings are designed to handle the added load and stress of an inverted roofing system. Supports may need to be added.
Limited Suitability for Roof Conversions
- This can be a labour-intensive and costly process due to stone hoisting and installation.
- If a building already has an inverted roof, it is more cost-efficient to maintain it rather than convert.
- Likewise, converting a traditional roof to IRMA/PMRA is not always practical due to the heavy stone that needs to be brought up.
Other Limitations
- Vegetation growth can occur between the insulation and membrane.
- Poor drainage may then cause the insulation to float.
- Displaced insulation can lift off and become airborne, posing a projectile risk. This is of particular concern with a flat substrate.
Note: these challenges can be prevented with attention to proper design, drainage, and fastening methods.
Built-Up Roofing (BUR): Instances Where it May Be a Better Choice
From easier visual inspection to lower weight load, there are a few reasons why a built-up roof may be a better selection for property owners.
Issues Are Easier to Spot
- Because the membrane rests on top and is fully visible, it’s easier to identify damage such as cracks and blisters.
Lower Weight Load
- BUR roof’s weigh less per sft compared with inverted roofs.
- This is better for buildings with structural limitations.
Materials Commonly Used in IRMA and PMRA Systems
Single-ply membranes are typically not used and are often not warrantied for use with inverted roof systems. Modified bitumen is the most common, preferred, and reliable solution.
Making the Decision: IRMA versus BUR
If you’re unsure whether your building can benefit from IRMA versus BUR, it’s important to start with an inspection by a qualified roofer in the Greater Toronto Area. Elite Roofing can weigh all factors, from building height and structural capacity to future restoration plans and existing roof type to recommend the best solution for you.
We know it can be challenging to decide on an ideal roofing system for your commercial or industrial building. But rest assured, we have been designing roofing systems for more than six decades, with a focus on attention to detail, quality craftsmanship, and long-lasting customer relationships. That’s the Elite Advantage!
Book a Consultation With Elite Roofing Today
IRMA versus BUR: which would be best for your building? We can help you determine the right solution. It all starts with a conversation and an inspection. Contact us today for a personalized consultation and expert recommendation.
