The Appendix C definition
According to Appendix C of the Code, a roof is flat when its slope is 16.7% or less, or 2:12. Beyond that, the Code speaks of a sloped roof. This definition tells which roofs the white roof rule and the sections that follow are addressed to.
Section 380: the covering
Main and accessory buildings with a flat roof must have one of these four coverings.
- A green roof.
- A material that is white, painted white, or covered with a reflective coating or white ballast.
- A material with a solar reflectance index (SRI) of at least 78, attested by the manufacturer's specifications or by a professional's specification.
- A combination of the first three.
Section 380 provides for exceptions.
- Buildings and equipment authorized on a roof.
- Residential accessory buildings with a footprint of less than 40 m².
- Agricultural buildings.
The section does not say whether it covers an existing roof redone identically.
No equivalent requirement was found in the Code for sloped roofs.
The official text contains an inconsistency, reported here without interpretation. Appendix C defines the index on a scale of 0 to 1, and section 380 sets a threshold of 78.
Section 379: permitted materials
Section 379 sets out a closed list of roof covering materials. It includes, among others, the following materials.
- TPO or EPDM membrane, reflective coating membrane or any other material certified as a white roofing material.
- Built-up tar membrane or elastomeric membrane (modified bitumen).
- Heat-welded or self-adhesive membrane.
- Gravel covering a membrane authorized in this section.
- A green roof with a waterproofing membrane.
Section 379 says which materials are permitted. Section 380 says what the covering of a flat roof must include.
Section 141: more than three storeys
Section 141, in the chapter on sustainable buildings, requires the roof structure to be designed to carry an extensive green roof, in addition to the loads prescribed by the CCQ (Quebec Construction Code), when three conditions are met.
- The building has more than 3 storeys.
- The roof area of the building, or of its addition, exceeds 250 m².
- The roof is flat.
The section excludes certain roofs through exceptions, to be read in the official text. Section 141 deals with the design of a structure. For an existing building, the question goes to the City's Service de l'urbanisme (urban planning department).
The same chapter adds that before a green roof is installed, the owner must have a watertightness test done at their own expense, using a method recognized by the industry.
The question the Code leaves open
Section 380 does not say whether it applies to the replacement of an existing flat roof, in particular when the same covering is put back. No acquired rights provision specific to roofs was found in the Code.
Before signing for a dark finish, ask the City's Service de l'urbanisme, which can be reached at 311 or, from outside Laval, at 450 978-8000, and ask for a written answer.
Frequently asked questions
Do the rules change from one part of Laval to another?
The sections described here do not vary by address. Areas of heritage interest have additional provisions, and the Service de l'urbanisme can tell you whether your address is affected.
Is gravel still permitted on a flat roof?
Section 379 permits gravel covering an authorized membrane, and section 380 counts white ballast among its choices. Section 380 does not say whether it covers an existing roof redone identically. For gravel of another shade, the question goes to the City.
Is my flat-roofed shed covered by section 380?
Section 380 excludes residential accessory buildings with a footprint of less than 40 m². Measure yours before drawing a conclusion.
Official sources, consulted on October 8, 2026: