Which is the Correct Roof Assembly Type for My Commercial Building in Toronto?

roof replacement with a 2-ply SBS Modified Bitumen membrane. roof assembly for commercial buildings.

Determining the correct roof assembly type for your commercial building in Toronto is not a cut-and-dried decision. The building owner, architect and roofing company have many factors to consider to ensure the roof will meet building’s structural needs, exposure to wind and sunlight, equipment on the roof, expected roof traffic, reliability, scheduling, and other factors.

Like all projects in the construction trades there is no single answer to what is right. The parties must consider the pros and cons, make trade-offs, and find the right overall balance.

Below are descriptions of the most commonly used roofing materials and installation techniques for commercial roofing in Toronto. These basics will provide an understanding of what to consider before moving forward with your new construction or roof replacement project.


4-5 Ply BUR for Commercial Buildings

Overview: 4-5 ply BUR (Built-Up Roof) provides a puncture-resistant watertight roof. However, there are problems that can occur with odours, and the larger staging areas required at ground level.

The Roof Assembly

4-5 ply roofing systems are reinforced by many layers of roof felts, each embedded in a full mopping of asphalt bitumen for adhesion and waterproofing. Stone ballast protects against Toronto’s stormy weather and the sun’s UV (ultraviolet) rays.  It is also a strong weathering surface.

4-5 Ply BUR Roofing Systems


The roof assembly’s durable gravel surface protects against everyday wear and tear, reducing maintenance requirements. Multi-ply redundancy also offers the advantage that water or objects must penetrate through multiple layers, protecting against leaks and damage. These membranes are highly wind-resistant and have a Class A fire rating.


The bitumen used in 4-5 ply BUR assemblies can create an odour during installation only, even if this is mitigated using low-odour asphalt. Tight temperature controls and flavour additives can also help combat odours. However, while workarounds are available, building owners should understand this factor before the project begins. In dense overgrown areas of trees, the stone ballast can collect algae, organic debris, and dirt, which can be worsened by ponding water. Avoiding such problems requires a strict maintenance program.    


2-3-Ply Hybrid BUR 

Overview: 3-ply hybrid BUR can be installed quickly, weighs less, and offers excellent fire protection. However, the material requires careful maintenance, especially in a wooded setting.

The Roof Assembly

3-ply hybrid BUR membranes combine either 2 plies of base sheet of  1-ply base sheet with two plies of fiberglass felt paper to enhance performance. These systems create a watertight seal that can perform well in both high and low temperatures.

Looking Into a 3-Ply Hybrid Built-Up Roofing System? Here's What You Need To Know


As with 4-5 ply BUR, multi-ply redundancy reduces risk of accidental penetrations, while putting less weight on the structure. Pea gravel as a ballast which yields a Class A fire rating. Combining either two plies of SBS base sheet or a single-ply SBS base sheet with two plies of glass enhances performance and allows for faster installation.     


Again, like 4-5 ply BUR, this roofing system is not ideal for wooded settings as the gravel ballast can trap vegetation and debris. Prolonged exposure to direct sunlight can also be a problem.


2-ply SBS Modified Bitumen Roof Assembly for Commercial Buildings 

Overview: 2-ply SBS modified bitumen is easy to maintain and performs well, even in Toronto’s harsh winter. However, there is an increased risk of seam failure if installed by a roofer without adequate experience in cold-weather techniques.

The Roof Assembly

2-ply SBS (Styrene Butadiene Styrene) modified bitumen membranes are pre-fabricated and reinforced with mats of polyester, fiberglass, or a combination of materials.

partial roofing replacement with 2-ply sbs modified bitumen membrane


The smooth surface of 2-ply SBS modified bitumen is easy to maintain. The membrane also performs well in harsh weather conditions, and its 8mm thickness makes it less likely to be punctured. 2-ply SBS modified bitumen also overcomes many installation constraints by having the flexibility to use various hot and cold application methods. Bright white membranes are available if achieving a cool roof designation is important.


Repairing or replacing this roofing system can be difficult in winter because it requires special cold weather application techniques. If you choose to use this material in colder weather, choose a reputable warranty certified contractor with experience successfully applying the appropriate techniques. A contractor lacking proper qualifications and welding expertise is more likely to make mistakes that result in seam voids or failure.


1-ply Membranes

Overview: PVC (polyvinyl chloride), TPO (Thermoplastic Polyolefin) or EPDM (Ethylene Propylene Diene Monomer) 1-ply membrane options are suitable for commercial, industrial, and manufacturing facilities looking for a sustainable, economically efficient option.

The Roof Assembly

Single-Ply Membranes are hard, wide-with sheets of rubber and other synthetics that can be ballasted or chemically adhered to insulation.

Flat Roof on a Self Storage Building


Single-ply roofs are easy to install, can withstand the elements, have far fewer seams than asphaltic rolled roof systems, and installation is flame-free. The membranes are suitable in many situations, with a choice of insulation materials, reflective or retentive surfaces, and membrane thicknesses. Lightweight and flexible, the material works for complex roof designs and stays intact as it expands and contracts. When kept clean, its roof-cooling ability can lower a building’s energy consumption. 


This lightweight material can be easily punctured by heavy roof traffic, trades working on rooftop equipment, tools dropped on roof, and other risks. Installation can take longer because roofers need to cut the material to build it up around protrusions. In addition, overlapping and fastening the rolled out sheets creates seam vulnerabilities. Other risks involve materials degrading from exposure to UV Rays, slippery-when-wet surfaces that create safety hazards, and susceptibility to wind blow-off.  Loose laid, mechanically fastened and ballasted systems offer up challenges with leak detection as the material acts like a tarp, with the concern that water can spread underneath the membrane, following a seam failure or puncture.


IRMA (Inverted Roof Membrane Assembly) or PRMA (Protected Roof Membrane Roof Assembly)

Overview: IRMA and PRMA roofs offer longevity, perform well in all weather, and are highly durable. However, they require high load-bearing capacity and are prone to water ponding and debris build-up.

The Roof Assembly

These assemblies are often called upside-down roofing systems, referring to reversed positions of the membrane and insulation compared to a conventional assembly.

IRMA or PRMA Explained.


IRMA and PRMA systems are largely unaffected by weather because of how they use insulation, which is resistant to the water absorption and condensation caused by Toronto’s freeze and thaw cycles. These roofing systems have superior longevity of up to 30+ years, with the ability to protect against high roof traffic, potential damage during restoration projects, and the sun’s UV rays. Plus, leaks and other roofing issues are more easily detected, leading to less frequent repairs.  


When the ballast is frozen, repairs become challenging and complex in wintertime. These systems are also prone to dirt build-up, damaging water ponding, algae growth, and insulation blow-off. An IRMA or PRMA roof assembly for commercial buildings may not be suitable for all structures because it requires a load capacity of 15-20 pounds per square foot.


A High-Quality Roofer Can Guide Roof Assembly for your Commercial Building

Each type of roofing system has its pros and cons when it comes to commercial buildings in Toronto. The experts at Elite Roofing are here to help you make the right decision given your goals, budget, and timeframe. For more than six decades, Elite Roofing has designed high-performing, complex roofing systems and partnered with building owners, architects, and developers to ensure their needs are met. To learn how we can help with your commercial construction projects, contact us. We’re here to help you get the Elite Advantage.