In Montreal, the idea of building a “mezzanine” is a recurrent one. In fact, a “mezzanine” is a roof-mounted structure whose setbacks and surface area do not count towards the building's height. It is therefore a type of construction often explored by families occupying the top floor of an apartment building in order to enlarge their space and avoid having to move. This type of project enables light urban densification and can help reduce the exodus of families to the suburbs.
How much does it cost? What's at stake? Is it really a viable alternative?
While this type of project may seem like an easy solution to a space problem, it's actually quite a complex one. In addition to structural and budgetary considerations, there's the whole regulatory aspect to consider.
In the course of my career, I've worked with several families on out-of-roof construction projects. Some projects have been successful, while others have unfortunately had to be abandoned.
In all cases, the road was strewn with pitfalls. The purpose of this article is to inform you about the various stages involved in planning such a project, while warning you of the potential difficulties.
The budget
First of all, adding a storey to a building is a fairly major project, regardless of the size of the addition. This is because it involves, in most cases, not only major structural reinforcement, but also the removal of the existing roof to allow construction of the addition. In addition to the substantial construction costs, you'll need to factor in the cost of all the protective measures required to prevent water infiltration into the floor below during the work.
For this type of project, a minimum budget of $300,000 is required, regardless of the size of the construction. For additions of 30 to 60 m2 (300 to 600 ft2), it's best to do your calculations and assess the real estate market to determine whether the investment is worthwhile.
Navigating urban planning constraints
First of all, it's worth evaluating the possibility of building an entire floor, because, let's face it, in terms of cost per square foot, it's a much more interesting project.
First of all, you need to find out the precise zoning for your building, so you know the maximum height and number of storeys allowed. Sometimes, these criteria theoretically allow you to add a storey to your building, but if all your neighbors are lower, the application may still be refused by the city, since your addition would create an anomaly in the urban ensemble. So the first step is to check with your borough's urban planning division to find out what's possible.
If the bylaws do not allow you to add a storey, this is when the option of an above-roof construction, also known as a “mezzanine”, can be considered. For this extension to be permitted, it must meet strict criteria in order to benefit from city exemptions and not count towards the building's floor and meter heights.
Please note that in some boroughs, such as Plateau Mont-Royal, a derogation must be applied for in order to build outside the roof. The process is longer and more costly, and must be submitted to the urban planning advisory committee for approval.
Mezzanine construction criteria
Firstly, the surface area of the construction must be less than 40% of the surface area of the floor below. Secondly, the construction must be set back from the front and rear facades of the building. At the front, a setback of 2 times the height is required to ensure that the non-roof construction is barely visible from the street. At the rear, the setback must be 1 times the height to limit the shading of the addition on neighboring constructions.
Dimensions et proportions of a mezzanine
With these constraints in mind, you can already assess whether your building is suitable for this type of construction.
For example, if your building has a gross surface area of 80 m2 (860 ft2), the roof construction can be no larger than 32 m2 (340 ft2). Once the surface area of the exterior walls has been removed, only about 25m2 (270 ft2) of living space will remain, from which the space required for the staircase will have to be subtracted.
If you find the surface area allowed interesting, consider the proportions of your building. A narrow but deep building will offer more flexibility for the addition of a roof-mounted structure, as opposed to a wide but shallow building, due to the setbacks required.
As an example, let's consider a fairly standard 3m-high off-roof construction. A building with a depth of 15 m (50 ft.) will, once the setbacks have been calculated, have a depth of around 6 m (20 ft.). On the other hand, a building with a depth of 10.5m (35 ft.) would only allow an off-roof construction 1.5m (5 ft.) wide, which is not viable unless you only wish to build a roof access.
So, following this example, if your building is less than 12.5m (40 feet) deep, the off-roof construction option is unattractive.
The most common structural strategies
Now that we've assessed the feasibility of your project from a regulatory point of view, it's time to evaluate the constructability of your off-roof construction. A meeting with a structural engineer very early in the project is advisable to assess the structural possibilities of the extension.
The structural strategy employed will vary according to the building's age and type of construction. In a more recent building, the existing structure may be strong enough to accommodate the new storey through the addition of simple reinforcements. In some cases, engineers will design a platform to rest on the existing party walls.
However, the most common structural strategy, and the least risky for engineers, is to install steel columns running through all floors of the building down to the basement. These new columns are supported by foundation footings, ensuring that the new floor is properly supported. This work has the potential to be highly invasive, especially if the lower floors are occupied by others. Basically, if this structural strategy is chosen, you'll have to plan work on every floor of your building.
Mezzanine Basile-Routhier
This project involved enlarging the living space of a family with 3 children living on the 2nd floor of a duplex in Ahuntsic. Having assessed the real estate market, the parents had gambled that this extension would enable them, at a lower cost, to remain in their neighborhood.
Unfortunately, the project had to be abandoned after the ground-floor neighbors withdrew their support just after the permit application was submitted. The scope of the work in their unit seemed to increase with each new visit from the contractor, and their fears became too great.
Condominium owners, be aware!
Do you live in a condominium? A discussion with your neighbors is essential. While the work may be limited to your unit, as we've just seen, it could also have a major impact on your neighbors. They may even need to be relocated during part of the work. Their approval is also a prerequisite to the issue of your building permit.
What's more, some structural engineers will even refuse to work on this kind of project at the risk of being sued by the co-owners for the slightest crack. That's why it's so important to be well informed before you start planning your project.
Conclusion
Deciding to expand from above with an off-roof construction (or “mezzanine”) can make the difference between staying in town or moving to the outskirts. For the city, this type of project allows for gentle densification of the built environment and helps retain families. For occupants, it not only saves space, but also provides access to more natural light and a roof that can be planted and cultivated, helping to reduce the effects of heat islands.
However, it is essential to understand the challenges and regulations specific to this type of project. Structural requirements, zoning restrictions, aesthetic considerations, views and sunlight from neighbors all need to be carefully assessed to ensure the success of your project. For those ready to explore the possibilities of top-roof extensions, a first step is to assess your building's potential by calculating the potential surface area and depth of this top-roof extension based on the dimensions of your dwelling.
To learn more about the subject of “mezzanines” and the regulatory debate that took place in the Plateau Mont-Royal borough a few years ago, I invite you to read this document published by the Vivre en ville organization. It will help you better understand the issues involved in this type of construction from an urban planning point of view, and help you assess your project's potential for acceptance.
The next step is to meet with an architect to assess the specific constraints of your project. If, after reading this article, you believe in the potential of your project, contact me so that together we can begin to give it shape.
About the author:
Kim Cloutier is an experienced architect with a career marked by projects focused on simplicity and sustainability. She is a certified LEED Passive House Designer and Green Associate, demonstrating her commitment to high standards of energy performance and environmental sustainability. Kim aspires to promote healthier living through her projects, with a focus on creating healthy, sustainable environments.