What SRI measures
SRI is the number that decides whether a roofing counts as a white roof in a borough's eyes. SRI combines two properties of the material.
- Solar reflectance. This is the share of the sun's energy the surface sends back. A surface that sends back almost everything stays cool.
- Thermal emittance. This is how easily the surface releases the heat it has absorbed anyway.
How to read the scale
The scale is built on two reference surfaces. A standard black surface scores 0. A standard white surface scores 100.
An ordinary black roofing sits near zero. Smooth white membranes get the highest values, and some exceed 100. Pale granule finishes fall in between, with large gaps from one product to another.
Two roofs that look the same shade can have different indexes. The eye is not enough.
Where to find a product's SRI
The index appears on the manufacturer's data sheet, among the product properties. French documents call it IRS.
When you read the data sheet, check the product and the value given.
- The exact product. The data sheet must cover the cap sheet that will be installed, in the planned colour. The same membrane in grey and in white does not have the same index.
- Initial or aged. Many data sheets give two values, one for the new product and one after a few years of exposure. The surface gets dirty, and the second value is lower.
Ask your borough which of the two values it uses, the initial or the aged one.
Thresholds in Montreal
According to the City of Montreal, when a flat roof is replaced, most boroughs require a white roof, a reflective material or a green roof. For reflective materials, the required SRI threshold ranges from 56 to 82 depending on the borough.
The gap is wide. A product accepted in one place can be refused a few streets away, across a borough line. The details by borough are in the pages under "White roof".
What it changes in your project
SRI should come up at the quote stage, not after.
- Ask us to write the full name of the cap sheet product and its colour.
- Ask us for the data sheet and find the SRI line.
- Compare the number with your borough's threshold.
Frequently asked questions
What SRI does my roof need?
The threshold depends on your borough and ranges from 56 to 82. Call your borough's permit office or check its page on montreal.ca before choosing a product.
Are white granules enough?
White granules are not always enough. Depending on the product, a pale granule membrane can exceed a moderate threshold and stay under a high one. Only the data sheet of the exact product can tell.
What happens if the installed product does not meet the threshold?
Ask your borough what it requires in that case. It is better to settle the question before installation, by having the product approved when the permit application is filed.