Pale or dark under the July sun
A dark surface absorbs a large share of the radiation it receives and heats up. A pale surface sends a large share back and heats up much less.
In built-up areas, streets, parking lots and dark roofs store this heat during the day and release it in the evening. The phenomenon has a name: the heat island.
The City, for its part, measures tree cover, the canopy. Its Plan de foresterie urbaine (urban forestry plan) put it at 23.6% for Laval in 2017.
The four choices in section 380
For a flat roof in Laval, section 380 of the Code de l'urbanisme requires main and accessory buildings to have one of the following coverings.
- A green roof.
- A white material, a material painted white, or a material 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.
The Code treats as flat a roof with a slope of 16.7% or less, or 2:12. The section applies to the whole city. It excludes three cases: buildings and equipment authorized on a roof, residential accessory buildings with a footprint of less than 40 m², and agricultural buildings.
The caveat to keep in mind
The section does not say whether it applies to the replacement of a roof already in place, in particular when the same covering is put back. The question goes to the City's Service de l'urbanisme (urban planning department) before the materials are ordered.
How a roofer gets a pale surface
The word white describes a result, not a product.
- Elastomeric with a light finish. The top membrane carries very pale granules rather than dark ones.
- White single-ply. A smooth sheet of TPO, highly reflective by nature.
- Green roof. Plants grow in a growing medium, on a membrane designed for that use. The whole assembly is heavy, and an engineer must confirm that the structure can carry it.
Two surfaces that look alike from the street can behave differently in the sun. Have the exact name of the finish product and its colour written into the quote, then ask us for the manufacturer's data sheet.
What the owner gets out of it
- A cooler top floor in summer. Less heat comes in through the roof. The effect is most noticeable in a dwelling without air conditioning.
- Less stress on the membrane. The membrane heats up less during the day, and the gap with the cool of the night narrows.
Dirt and heating
A white surface gets dirty. Dust, pollution and dead leaves dull it from one year to the next. Several manufacturers' technical data sheets give one value for the new product and another after aging.
There remains the fear of a higher heating bill. In winter, the sun is low, the days are short and snow covers the roof for weeks. The shade of the membrane then matters very little. The roof's insulation matters much more.
Frequently asked questions
My dark flat roof is reaching the end of its life. Do I have to redo it in white?
Laval's Code de l'urbanisme does not say. Section 380 lists the coverings a flat roof must have, without saying whether it covers an existing roof redone identically. Ask the Service de l'urbanisme before choosing the finish.
Will a pale grey do?
Section 380 names a white material, or a material with a solar reflectance index of 78. The Code does not say where white ends. For a grey, the figure on the manufacturer's data sheet is the reference to present to the City. For an existing roof redone identically, the Code does not say what applies.
Can a roof be coated instead of replaced?
Section 380 includes among its choices a material painted white or covered with a reflective coating. It does not say what applies to an existing roof redone identically. We can tell you whether the product suits your membrane. The Service de l'urbanisme can say whether a permit is required.
What are the advantages of a white roof?
A white roof sends back a large share of the sun's heat instead of absorbing it, which helps keep the top floor cooler in summer. The membrane heats up less and goes through smaller temperature swings between day and night. In Laval, a white covering is also one of the choices that section 380 of the Code de l'urbanisme names for a flat roof, without addressing the case of an existing roof redone identically.
Is a flat roof effective against heat?
A flat roof is effective against heat when its surface is white or reflective, and much less so when it is dark. A dark roof in full sun gets very hot and passes some of that heat to the dwelling below. The roof's insulation counts too, and we can tell you what yours contains.
Is it allowed to paint a roof white?
Yes for a flat roof. Section 380 of Laval's Code de l'urbanisme counts a material painted white or covered with a reflective coating among the coverings it names. It does not say what applies to an existing roof, and the City does not define what the same material means for the permit. Before having a coating applied, ask the Service de l'urbanisme whether a permit is required, and ask a roofer whether the product suits your membrane.
Official sources, consulted on October 8, 2026: