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Redoing a flat roof in Montreal is no longer a colour choice. The list of permitted coverings is closed, and the figure required changes from one borough to the next. Here is what the city imposes, what the Régie du bâtiment du Québec demands before soil goes onto an existing frame, and what your insurer has to hear from you.
The Montreal by-law closes the list of coverings
The city’s page “Rénover un toit plat ou à faible pente,” updated on February 2, 2026, starts with the threshold: a slope below 2:12, that is, 16.7%. Below that threshold, the only authorized coverings are a material that is white, painted white or covered with a reflective coating or white gravel, a material whose solar reflectance index reaches your borough’s threshold, a green roof, or a combination of those coverings.
Note the logic: the by-law does not order you to plant anything, it orders you to reach an index and accepts the green roof as an equivalent. The vegetated roof is the way out, not the goal.
Three nuances change everything depending on your address. In Anjou, no permit is needed to replace a flat roof, but the list of materials applies all the same. In Plateau-Mont-Royal, it covers only new buildings, extensions and a change of volume or slope. In Saint-Léonard, for a single family home, the authorized materials are those permitted by the building code. Elsewhere, the permit is required, and the manufacturer’s technical sheet stating the index is one of the documents demanded. Analysis fee: $9.80 per $1,000, minimum $167.40 in residential. For a vegetated roof, several boroughs require an application in person at the counter.
In Laval, the logic is reversed. The exterior residential renovation permit sheet is explicit: replacing the roof covering with the same material needs no permit, but replacing it with a new material requires one, issued in 30 days, for $92 up to $20,000 of work then $2.50 per $1,000. Going from black to white, or to vegetation, is precisely the case that triggers the permit. No Ville de Laval page I was able to open imposes a minimum reflectance index on a residential roof.
The index required changes from one borough to the next
Two neighbouring streets in two different boroughs do not have the same list. Here is what the city’s page showed on September 25, 2026.
| Solar reflectance index required | Boroughs |
|---|---|
| 56 | Côte-des-Neiges and Notre-Dame-de-Grâce, LaSalle, Plateau-Mont-Royal, Mercier and Hochelaga-Maisonneuve |
| 66 | Ahuntsic-Cartierville, Pierrefonds-Roxboro, Villeray, Saint-Michel and Parc-Extension |
| 78 | Anjou, Lachine, Sud-Ouest, Île-Bizard and Sainte-Geneviève, Montréal-Nord, Rivière-des-Prairies and Pointe-aux-Trembles, Rosemont and La Petite-Patrie, Saint-Laurent, Verdun, Ville-Marie |
| 82 | Saint-Léonard |
| 78 over 75% of the surface, or 66 over the whole | Outremont |
Several boroughs exclude the part of the roof taken up by mechanical equipment or a terrace. The by-laws involved are the Règlement d’urbanisme 01-274 and the Règlement sur la construction et la transformation de bâtiments 11-018. For the grant and the rest of the work, see climate resilient renovations.
White roof, green roof, blue roof: three techniques, not three shades
The white roof is a covering. It changes nothing structurally and satisfies the by-law through its index.
The green roof is a system laid over the waterproofing. According to the environment ministry’s Guide de gestion des eaux pluviales, the studies reviewed give an average retention rate of 60%, in a range of 39 to 72%, a delay in peak flow of 20 to 40 minutes, and a reduction in runoff volume of 45 to 60%.
The blue roof is temporary water storage. The guide is blunt about where it belongs: it applies above all to large flat industrial or commercial roofs, residential roofs usually being sloped. And about its effect: retention reduces the peak flow downstream, but the volume discharged to the sewer does not go down, since the discharge is spread over a longer period.
| White roof | Green roof | Blue roof | |
|---|---|---|---|
| Nature | a covering | a system on the membrane | water retention |
| Volume discharged to the sewer | unchanged | reduced | unchanged, spread out |
| Permanent load added | no | yes | the retained water |
| On Montreal’s list | yes, if the index is met | yes | not mentioned |
That last line counts: a blue roof does not exempt you from the index, retention not being a covering.
The load a green roof adds to an existing frame
The reference document is the guide from the Régie du bâtiment du Québec, Critères techniques visant la construction de toits végétalisés, updated in June 2015. Its basic conditions already rule out many buildings: noncombustible construction, a roof no more than 46 m above grade, a slope of no more than 17%, and a covering that is not of the multilayer, asphalt and gravel type. The fire department must be consulted from the planning stage on.
The weight. The guide requires the structure to be designed under part 4 of the Code taking into account the weight of a water saturated vegetated roof for every calculation, including seismic loads. Substrate density is determined under standard ASTM E2399-12 or, failing that, with an unfactored value of 2,000 kg/m³ for saturated density. The substrate must be at least 100 mm. Do the arithmetic yourself: 100 mm at that density is 200 kg per square metre, before the vegetation, before the snow and before the water held in the drainage layer. The real figure for your roof comes from the engineer, under standard ASTM E2397-11.
The calculation must also assume the worst: blocked drains, flooded substrate, water rising to the scuppers or overflows, whose accumulation the guide caps at 150 mm. To the live loads you add the snow that vegetation holds differently from a wind swept roof, and the substrate stored on site the day the membrane gets redone. The environment ministry’s guide says it among its limitations: the weight of the snow can restrict the application to roofs with sufficient load bearing capacity.
Permit, sealed plans and attestation: what the owner keeps
Two requirements in the guide answer the question everybody asks. All loads must be shown on plans and specifications bearing the seal and signature of an engineer. And for an existing building, structural resistance must be analyzed by an engineer, taking into account the new permanent load, the increased snow loads, rainwater accumulation and seismic behaviour. Part 10 of the Code, the one on alterations, applies.
The waterproofing must be verified before the plant cover goes down, by one of the six tests the guide lists, and the report is given to the owner, who must also obtain from the designer an attestation that the guide’s conditions have been met. Sealed plans, waterproofing report and attestation: exactly what a serious buyer will ask for, and the seller’s guide describes the rest of the file.
An honest warning: this guide governs buildings subject to the Code, and its first condition is noncombustible construction. A Montreal plex with a wood frame is not. What applies to your building gets verified with the Régie du bâtiment and an engineer, not in an article.
Mandatory maintenance, and what the insurer has to know
Maintaining a green roof is not a good practice, it is a condition. The guide requires a program drawn up by a landscape architect or another specialist, given to the owner and recorded in a register kept on site, covering irrigation, fertilization, weeding, replanting and inspection of the edges, the drains, the flashing and the membrane. The mature vegetation cover must be maintained at 80% or more.
On the insurance side, the rule is in the Code civil du Québec. Article 2466 requires the insured to promptly declare circumstances that aggravate the risk and result from the insured’s own acts, where they are of a nature to materially influence the premium, the assessment of the risk or the decision to continue the insurance. Article 2467 gives what follows: the informed insurer may cancel the policy or propose a new rate in writing, which the insured must accept and pay within 30 days, failing which the policy ceases to be in force.
The Bureau d’assurance du Canada puts “had the roof redone” on its list of changes to review at renewal. Two of its rules touch the green roof directly. The indemnity for the building is the cost of rebuilding in the same place with materials of the same kind and quality: a vegetated system must therefore sit inside the insured amount. And the insurer must have the damaged part repaired, not the whole roof redone. On a green roof, that means finding the leak under the substrate, then lifting it. The ministry’s guide notes it as a limitation: since water is encouraged to stay longer on the roof, faulty waterproofing becomes damaging.
No Bureau d’assurance du Canada publication I was able to consult puts a figure on what a vegetated roof does to a premium. The files that become hard to place are described in a home that is hard to insure in Quebec.
What can be demonstrated, and what cannot
No official source lets me put a figure on what a green roof or a white roof adds to the resale price of a Montreal plex. If a supplier announces a percentage of added value, ask for the study.
What can be documented is more modest. The National Research Council of Canada instrumented two large green roofs in Toronto and published its observations in 2005: green roofs reduce heat flow through the roof, which lowers energy demand for heating and cooling, whatever the roofing system, with greater efficiency in summer than in winter. Neither the NRC nor the ministry’s guide translates that into dollars on a Montreal electricity bill: do not budget a saving. For the membrane, the guide states a longer useful service life, thanks to protection against ultraviolet light and the impact of rain, without advancing a number of years. The particularities of a duplex flat roof are in the features of the Quebec house.
Frequently asked questions
My borough requires 78 and my roofer is proposing a grey membrane. Is that allowed? Only if the manufacturer’s technical sheet attests to the index, or, in certain boroughs, a professional’s opinion or specification. Pale grey is named as an acceptable example only in Plateau-Mont-Royal, where the threshold is 56.
Can I put a green roof on my wood frame duplex? The first condition in the Régie du bâtiment guide is that the building be of noncombustible construction. A wood frame is not. Put the question to the Régie and to an engineer before you spend a dollar on design.
How much does a green roof cost per square metre? No official source I was able to open publishes an installation cost per square metre for Quebec. What is published is the permit fees and the Montreal grant per square metre. The rest comes from your quotes.
Do I notify my insurer before or after the work? Before. Article 2466 of the Code civil speaks of a prompt declaration, and article 2467 allows the insurer to cancel or require a new premium within 30 days. Better to know the answer before the soil is on the roof.
To go further, read the service life of a roof in Quebec and how to choose a renovation contractor.
Torn between redoing a flat roof with a white membrane and planting it before you sell? Write to me. We look at the index required in your borough and at the documents the buyer will ask you for.
Georges Matar
Residential Real Estate Broker · RE/MAX DU CARTIER INC.
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