The Cost of a Flat Roof

Want to know how much you’ll need to invest in your flat roof project to get the desired result?

Our roofers present the variables to take into account when estimating the cost of a flat roof in 2021.

Minor or major renovation?

Flat roofs are very common in urban areas like Greater Montreal. Many residential and commercial buildings have had theirs for decades, but unfortunately, the lifespan of a flat roof is not infinite. They need to be renovated periodically to make sure that they continue to function as an effective barrier against the elements.

Of course, a minor renovation to replace the covering will cost significantly less than a major renovation that involves replacing the entire roof system. The more components need to be renovated (insulation, ventilation, adding skylights), the more expensive it will be.

The size of the roof

It goes without saying that the larger the roof is, the more expensive it will be to construct or renovate. Most roofing contractors provide price estimates by square foot.

A very large flat roof can also make the project more complicated. The larger and higher up the roof is, the more challenging it will be to set up installations such as scaffolding and handle materials and debris. This generates additional costs in terms of the labour and tools (e.g. lifting equipment) required.

Roof structure and decking

A flat roof requires a structure and decking that will support the system and mechanical components.

For flat roof projects, prefabricated structural components such as roof trusses with a slight slope can sometimes be used to cut costs, because completely flat structures require tapered insulation.

The type of roof covering selected

A proper roof covering is waterproof and protects the building against inclement weather.

When it comes to membranes for flat roofs, owners have a variety of options to choose from. Of course, the price can change significantly from one type of membrane to another.

TPO membrane

TPO (thermoplastic polyolefin) flat roof systems are made up of one layer of synthetic material and reinforcing scrim. TPO membrane is one of the most common types of roofing for flat roofs on commercial buildings.

If you see a flat roof that’s bright white, it’s probably covered with a TPO membrane. This type of roofing saves energy by limiting how much heat is absorbed from UV rays.

EPDM membrane

EPDM is an acronym for ethylene propylene diene monomer, a popular covering for flat roofs. Picture this material as a flat car tire that is resistant to scratching, tearing and sun damage. EPDM membrane is generally considered the most durable and eco-friendly type of roof covering. It is non-toxic, often made with recycled materials, and has a very long lifespan.

Elastomeric membrane 

Elastomeric membranes are usually a double layer of modified bitumen membrane that can be installed at any time of year. Installing them on a flat roof can generally be considered a good investment, because they require very little maintenance and have an excellent lifespan.

However, this is a less eco-friendly option, because elastomeric membranes are not recycled in Quebec.

Multi-layer asphalt and gravel membrane

Multi-layer membranes are composed of a layer of asphalt covered in gravel. The gravel helps prevent the asphalt from expanding and contracting too fast during extreme temperature fluctuations.

Flat roofs with this type of membrane require regular maintenance involving caulking, gravel, sheet metal flashing and drainage.

Green roof

A green roof, also known as a living roof, is made up of a waterproofing membrane, a growing medium and plant life that covers the roof of a building. In addition to being eco-friendly, the vegetation helps regulate the temperature in the building.

This is the most expensive type of roof covering, because it requires reinforced structural support, effective underlayment and an advanced drainage system.


Proper flat roof ventilation is vital in order to preserve the integrity of the building and prevent problems. The National Building Code of Canada lays out ventilation requirements that owners and roofing contractors must follow.

When renovating or constructing a flat roof, part of the budget must be allocated to ventilation.


A flat roof must be properly insulated to keep the building at a reasonable temperature. Part of the renovation or construction budget must go to insulation.

Vented flat roofs must be paired with insulation that supports the ventilation system. Generally, the system will require fans placed in strategic locations, openings and insulation with a high R-value.

Unvented flat roofs, on the other hand, have no ventilation system in the attic and must therefore be insulated from above using rigid insulation panels placed directly on the vapour barrier.

There are many types of insulation products available at a wide variety of prices. Selecting the right one for the project requires an understanding of the properties and characteristics specific to each product.

The location of the worksite

When a roofing contractor prepares a quote, they always take the location of the worksite into account.

For example, building a flat roof in a remote area may involve additional transportation expenses for equipment, materials and labour that will need to be included in the bill.

Also, replacing a flat roof at the top of a 10-storey building can prove more expensive than replacing the roof of a duplex, because getting the necessary materials to that height is a complex task that may require additional equipment.


While flat roof maintenance costs aren’t incurred immediately, it’s still important to remember that keeping a roof in good condition requires some investment.

Flat roof maintenance includes regular inspections and snow removal after heavy snowfall.

The roofing contractor

Generally, the cost of materials stays relatively consistent from one contractor to another. However, the hourly rate companies charge their clients can vary considerably. This can be explained by the fact some companies provide warranties, use certified labour and have an excellent reputation, while others don’t. It’s generally worth paying a bit more for roofing work to get better results. Beware of prices that are too good to be true!

Trust Les Couvreurs Duro-Toit with your flat roof project!

Are you considering having a flat roof built or renovated? There are probably more variables to consider than you expect! Take the time to discuss your project with our flat roof experts—it’s the best way to get an accurate picture of the overall cost.

Don’t hesitate to contact our team to discuss your needs or request a quote for your roofing work.


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