When it comes to building or renovating a building with a flat roof, one of the first questions that comes up is: how much will it cost? By understanding all costs associated with installing a flat roof, you can plan a realistic budget and make good decisions.

In this article, our roofers present the different factors that can affect the price of constructing a new flat roof.

What does a flat roof cost per square foot?

The cost-per-square-foot approach is widely used in the construction industry to determine the price of a flat roof. This method offers a convenient way to compare prices and obtain a general budget estimate.

Based on the experience of our roofing contractors, the average cost per square foot for a flat roof in Quebec can vary from $10 to $40. This estimate is based on several factors, which we explain below.

The price of a flat roof, depending on the type of covering

The right flat roof membrane system guarantees that your roof will be waterproof and your building will be protected from the elements.

Homeowners have many choices when it comes to choosing the right flat roof membrane. Of course, there can be significant price differences between different types of roofing.

TPO membrane

TPO (thermoplastic polyolefin) flat roof systems consist of a single layer of synthetic material with a reinforcing grid. The TPO membrane is usually tacked down or fully adhered to the roof substrate.

This type of membrane stands out for its exceptional durability, and is usually guaranteed for 20 years. However, the quality of this membrane is greatly influenced by ambient temperature. In areas like Quebec where cold temperatures are common, the joints between membranes can become brittle. Even so, TPO membranes remain one of the best-performing membranes on the market, as long as it is installed correctly and used under the right conditions.

The average price per square foot for a TPO-coated roof is estimated at $15 to $25.

EPDM membrane

EPDM is an acronym for ethylene propylene diene monomer, a type of flat roof membrane used mainly for industrial buildings. The best way to imagine this material is to compare it to the inner tube of a bicycle tire.

Waterproofing is achieved with a single layer of membrane. Although it is durable, this roof coating is not recommended if there will be a lot of traffic on the roof. It can be installed in several ways: mechanically attached, fully adhered or held in place solely by the weight of the gravel.

Like TPO membrane, the average cost per square foot is between $15 and $25.

Multilayer asphalt and gravel membrane

This type of roof covering is composed of several layers of superimposed felts saturated with hot asphalt, which is then covered with gravel to slow the contraction or expansion of the asphalt during extreme temperature changes.

This inexpensive waterproofing membrane does, however, require recurrent long-term maintenance and the lifespan of this type of flat roof is generally shorter. The use of large quantities of asphalt in multi-layer roofing is also less environmentally friendly, as the extraction and processing of bitumen is energy-intensive and generates pollutant emissions. What’s more, asphalt is not biodegradable and contributes to the longevity of landfill waste once it has reached the end of its life.

The estimated price for this membrane is $12 to $18 per square foot.

Elastomeric membrane

Elastomeric membrane coating is considered one of the most durable roofing membranes, and the best for Quebec temperatures. This waterproofing system is composed of two plies of heat-welded modified bitumen membrane which can be installed at any time of year. Installing elastomeric membrane on a flat roof is often considered a good investment, as it requires little maintenance and has an excellent lifespan.

Although elastomeric membrane contains bitumen, it is a highly environmentally-friendly solution in the long term, thanks to its remarkable durability and resurfacing capacity. It’s both an economical and environmentally friendly choice. The cost per square foot is between $20 and $25.

Flat green roof

A vegetated roof or “green roof” makes it possible to grow plants on the roof of a building, which also regulates its temperature.

The cost includes the expense of the waterproofing membrane that must be installed underneath, and can vary from $10 to $30 per square foot.

Additional costs and fees associated with flat roofs

Beyond the cost per square foot and the choice of cladding, various elements can influence the overall budget of a flat roof project. Keep reading to learn more!

Minor or major roof renovation?

Flat roofs are very common in urban areas like Greater Montreal. Many residential and commercial buildings have flat roofs that were installed decades ago, but unfortunately, flat roofs do not last forever. They need to be renovated periodically so 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 that need to be renovated (insulation, ventilation, adding skylights), the more expensive it will be.

The size of the flat roof

It goes without saying that the larger the roof is, the more expensive it will be to construct or renovate.

On the other hand, keep in mind that the surface area that requires covering does not include any obstacles on the surface. Also, the larger and taller the roof, the greater the challenge of installation and handling materials and debris. It’s important to consider costs related to labor and tools/equipment (e.g. hoists) required to carry out the work, in addition to permits to block streets, etc.

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.

Flat roof ventilation

Proper flat roof ventilation is vital 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 are obliged to follow.

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

Flat roof insulation

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. To choose the right one, it is necessary to thoroughly assess the properties and characteristics of each product.

The location of the worksite

When a roofing contractor prepares a quote, they always consider the location of the worksite in their estimate.

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 be 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.

Roof maintenance

While flat roof maintenance costs are not part of the installation price, it’s still important to remember that keeping a roof in good condition requires some investment.

Flat roof maintenance includes regular inspections and occasional snow removal. Neglecting to care for a flat roof will inevitably shorten its lifespan.

The roofing contractor

Generally, the cost of materials is relatively consistent from one contractor to another. However, hourly rates can vary considerably. Some companies provide warranties, use certified labour and have an excellent reputation, while others do not. It’s generally worth paying a bit more for high quality roofing work. Beware of prices that are too good to be true!

Get a quote for your flat roof replacement with Duro-Toit

Are you considering having a flat roof replaced or repaired? There may be more factors to consider than you thought! 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.

Contact our team to discuss your needs or request a quote for your roofing work.

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