The problem of heat islands
In summer, dense neighbourhoods are noticeably hotter than wooded areas or the riverside. Street asphalt, parking lots and dark roofs store heat during the day and release it in the evening. This is called a heat island.
Roofs weigh heavily in this balance. Seen from the sky, plex neighbourhoods are a mosaic of flat roofs side by side. A black roof in full sun becomes too hot to touch. A pale roof stays much cooler.
What the boroughs require
According to the City, when a flat roof in Montreal is replaced, most boroughs require one of these solutions:
- a white roof
- a reflective material
- a green roof
A material's performance is measured by its solar reflectance index, or SRI. The required threshold ranges from 56 to 82 depending on the borough. The details by borough are in the pages below this one.
The rules vary by borough, type of building and type of work. Heritage buildings and heritage areas have additional standards. The only reliable answer for your address comes from your borough.
When the requirement applies
The requirement targets replacement, meaning the roofing is replaced. No one asks you to tear off a dark roof in good condition.
Ways to get a white roof
"White" is a shortcut. What counts is the measured reflectance of the material, not its shade to the eye.
- Elastomeric membrane with white granules. The cap sheet is covered in very pale granules. This is the most common solution on plexes.
- White single-ply membrane. This is a smooth TPO sheet, naturally highly reflective.
- Green roof. This is a roof covered with vegetation, on a membrane designed for the purpose. A green roof needs a structure able to carry its weight, to be confirmed by an engineer.
Before settling on a solution, check that your borough accepts it and ask us for the data sheet that gives its SRI.
What you gain
The requirement serves the neighbourhood first, but the owner benefits too.
- A cooler top floor. Less heat passes through the roof in summer. Top floor tenants feel it, especially without air conditioning.
- Less stress on the membrane. A surface that heats up less goes through smaller temperature swings between day and night.
Limits to know
A white roof does not stay white. Dust, pollution and leaves dull it over the years. Data sheets often give one value for the new product and another after ageing.
You sometimes hear that a white roof raises the heating bill. In winter the sun is low, days are short and the roof spends long weeks under snow. The colour of the membrane matters very little then. The roof insulation matters much more.
Frequently asked questions
Do I have to redo my roof in white?
You have to redo your roof in white only if your borough requires it, and only when the roof is replaced. As long as your current roof holds, nothing changes. The day you redo it, the rule in force in your borough applies.
Is a light grey roof accepted?
A light grey roof is accepted or refused based on its reflectance index and your borough's threshold, which ranges from 56 to 82 depending on the location. Some light greys pass in one borough and not in another. The product data sheet settles it.
Does a white roof cost more?
The price depends on the product and system chosen, and the gap with a dark finish varies from one manufacturer to another. Ask us to name the product in the quote so you can compare equivalent offers.
What are the benefits of a white roof?
A white roof sends the sun's heat back instead of absorbing it and keeps the top floor cooler in summer. The membrane heats up less and goes through smaller temperature swings between day and night. Across a neighbourhood, pale roofs help reduce heat islands. Most boroughs require a white, reflective or green roof when a roof is replaced.
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 black roof in full sun gets scorching and passes that heat to the top floor unit, while a pale roof stays much cooler. Roof insulation also plays its part.
Is it allowed to paint your roof white?
Yes, in several boroughs, where the City of Montreal counts a material painted white or covered with a reflective coating among the accepted roofings on a flat roof. Each borough has its own list, though, and some set a reflectance index to reach instead. Before having a coating applied, ask the permit counter whether a permit is required, and ask us whether the product suits your membrane.