Flat roof in Laval

Laval's definition

Appendix C of the Code de l'urbanisme settles the matter with a number. Any roof with a slope of 2:12 or less, or 16.7%, is flat. Above that threshold, the Code speaks of a sloped roof.

A flat roof keeps a slight incline, just enough to guide water toward its drain. That incline is too gentle for overlapping shingles to do the job. A flat roof stays watertight thanks to a continuous skin.

If your roof is leaking today, the page on roof repair describes the first steps.

What the City says about its buildings

The City publishes a portrait of its buildings, not of their roofs. Its land use plan describes an urbanized territory occupied mostly by single-family homes, businesses and industries of one or two storeys. According to its 2024 figures, 47% of dwellings are in a building with at least two units, and 42% are detached single-family houses.

Its Politique de l'habitation (housing policy) notes a higher share of plexes and mixed-use buildings in the older neighbourhoods: Saint-Vincent-de-Paul, Pont-Viau, Laval-des-Rapides, Chomedey and Sainte-Rose. Your roof is flat if its slope fits the Code's definition, whatever building it sits on.

From the rain down to the wood: five layers

The membrane is only the top of an assembly. Starting from the sky, you go through:

1. The membrane. It alone takes the rain, the sun and the snow.
2. The cover board. It gives the membrane a firm base.
3. The insulation. These are rigid boards. Some are tapered to create the slope toward the drain.
4. The vapour barrier. It holds back the moisture rising from the dwelling before it reaches the insulation.
5. The deck. Wood planks or panels nailed to the joists, which carry everything else.

A good quote names the layers kept and the layers replaced.

Three families of coverings

• Two-ply modified bitumen, also called elastomeric membrane. The top ply carries granules. Order of magnitude: 25 to 35 years, depending on the manufacturers and the quality of the installation.
• Built-up tar and gravel. Felts embedded in hot asphalt, then a bed of gravel that protects them. It is still found on older roofs. Order of magnitude: 15 to 25 years.
• Single-ply membranes, TPO and EPDM. A single sheet, of plastic for TPO, of synthetic rubber for EPDM. They are installed mostly on large, open surfaces. Order of magnitude: 20 to 30 years.

What Laval's Code permits on a flat roof

Section 379 of the Code de l'urbanisme sets out the closed list of roofing materials. For a flat roof, it includes:

• TPO or EPDM membrane, reflective coating membrane or another material certified as a white roof
• built-up tar membrane and elastomeric membrane, that is, modified bitumen
• heat-welded or self-adhesive membrane
• gravel, if it covers a membrane permitted by the section
• a green roof with its waterproofing membrane

The section names materials, not colours. Appearing in section 379 does not settle the question of colour. The next section deals with it.

Section 380 applies as well as soon as the roof is flat. It requires the building to have one of these coverings, or a combination of them:

• a green roof
• a white material, painted white, or covered with a reflective coating or white ballast
• a material with a solar reflectance index of at least 78, attested by the manufacturer's specifications or by a professional's specification

The section covers main and accessory buildings. It provides three exceptions: buildings and equipment authorized on a roof, residential accessory buildings with a footprint of less than 40 m², and agricultural buildings.

The Code does not say what applies to an existing flat roof redone identically. Before ordering a membrane, ask the City's Service de l'urbanisme (urban planning department) and keep the answer in writing.

The permit, for a residence

The City's page on exterior residential renovation sets out the rule. For a residence, replacing the roof covering with the same material requires no permit. Replacing it with a new material requires one. The criterion does not change whether the roof is flat or sloped, nor from one part of the city to another.

The City does not define what it means by the same material. Going from gravel to a membrane, or from a black membrane to a white one: only the City can decide these cases. It is best to ask before setting the date of the work.

In an area of heritage interest, the City may require a PIIA (site planning and architectural integration program) to change the roofing materials of a main building. Section 1616 of the Code exempts replacement with the same material of a similar colour, a material that keeps or restores the original or traditional features, and renovation work on a flat roof.

Where water finds its way

The open field of a sound membrane causes little trouble. Leaks often start where the membrane rises, stops or is pierced:

• at the base of parapets and under their coping
• all around the drain
• where a plumbing vent, a ventilation outlet or an electrical mast passes through
• at the frame of a skylight or a hatch
• at the anchor points of a rooftop terrace or a guardrail installed after the roof

Four neighbours of the membrane deserve a question to the roofer.

The parapet. This low wall borders the roof. Tired masonry should be examined before a new covering is laid against it.

The skylight. The membrane climbs onto its frame. If the skylight is old, replacing it while the roof is open makes the work simpler.

The rooftop terrace. It hides the membrane it rests on. It has to be taken apart before the roof replacement, then put back without piercing or crushing the new covering.

The equipment. Heat pump, air conditioner, fan and ducts sit on bases that the membrane goes around. Ask who disconnects, moves and restarts them.

A roof replacement, from the first visit to the final cleanup

1. The survey. We go up, take measurements, look at every detail and at how materials will be brought in.
2. The paperwork. You compare quotes. If the material changes, the permit application goes to the City.
3. Site setup. A container arrives, the facade and the yard are protected, tenants and neighbours are notified.
4. Tear-off. The old roof comes off in sections. Each opened section is closed again before evening.
5. The wood. Rotten deck planks are replaced.
6. The body of the roof. Vapour barrier, insulation and cover board go in, with the slope toward the drain.
7. The membrane. It covers the surface, then rises onto the parapets and wraps each obstacle.
8. Closing up. Metal flashing, drain and strainer, cleanup on the ground, then a last walk around the roof.

Why two quotes are not alike

For the same area, the gap between two prices comes from:

• the number of parapets, vents and skylights to deal with
• the height of the building and the space for a crane or a container
• the number of old layers to remove
• the condition of the deck, known only once the roof is open
• added insulation or a slope to correct
• the chosen system and the warranties wanted

To compare, insist on a document that states:

• the system, the full name of the finish product and its colour
• each layer removed and each layer kept
• how replacing the deck will be billed
• what is planned at the parapets, flashing, drain and skylights
• the written warranties from the manufacturer and for the labour
• who handles the permit
• the roofer's licence number

A contractor's licence is mandatory in Quebec for every construction contractor. The Régie du bâtiment du Québec (RBQ, Quebec's building authority) issues it and keeps a public register of licence holders.

Frequently asked questions