From a quick google scholar search, it seems that there is a trade-off between heating/cooling costs between summer and winter:

> The results indicate that the energy savings ratios of the rooms with the sedum-tray garden roof and with the white roof were 25.0% and 20.5%, respectively, as compared with the black-roofed room, in the summer; by contrast, the energy savings ratios were −9.9% and −2.7%, respectively, in the winter.

It seems that covering the building with a green garden is better than painting the roof white, but the heating costs were higher in the winter for both designs.

I guess that changing the roof could work in climates where the winter is not too cold.

On a side note, I wonder if it would be better to work on better insulation (both roof and sidings) rather than re-roofing a house.

Note: I am not a civil engineer so I have no clue if the paper is good or not.

[1]: https://doi.org/10.1016/j.enbuild.2017.09.091

Depending on your climate, once you get sufficiently cold, many of your buildings will accumulate their own layer of white stuff (snow) anyways in the winter. This study was conducted in Chongqing which basically doesn't have snow.

I suspect that the effect of this is highly location specific, both in terms of climate, as well as average insulation.

I wouldn't really say this is true. Snow pooling up on roofs is a bad thing -- that's a lot of moisture and weight. Homes in snowy places have sloped roofs to avoid it. When I bought my house in Massachusetts, the previous homeowner even gave me a tool to push snow off the roof, if needed.

You're right, you definitely don't want giant snow piles on your roof of your house, and yes, sloped roofs will help you avoid giant snow piles.

But you still often end up with some covering. Not 100%, but I think high enough to meaningfully affect the cost-benefit calculations. Not to mention large commercial buildings with flat roots.

It has been a requirement in Montreal for all new roofing for a few years now. Either green (vegetated) or light (with a solar reflection material). So most people have white elastomer roofs installed.

Someone correct me if I’m wrong but I believe it’s the only Canadian city with that requirement.

Isn't that only on commercial roofs, especially flat or low-pitch ones?

It is for flat roofs only I believe. However, Montreal does have a lot of "plexes" and small apartment buildings with flat roofs.

Wouldn't this be a net negative with more heating days than cooling days to worry about?

Many of the "heating days" would have snowy (ie white) roofs anyway.

Montreal has more days where heat is a problem (ex: for elderly people), than when cold is a problem. There are a lot of efforts put into reducing heat zones, from too much asphalt, not enough trees, etc. (I once ran for borough council :-)

Since 2010, most Quebec building standards have also been updated pretty massively. I don't have references, but where I live, we had 1" insulation in the walls, and now the minimum requirement for new buildings is 1.5". Doors and windows tech have improved a lot too in the past 10-15 years. Overall, as I went through upgrades, our heating bills went down pretty significantly, but AC usage keeps going up.

Has this had a measurable impact? (honest question, I like the idea very much.)

So when I moved to Montreal in 2016 I bought a 2 floors condo built before the new law was passed (2005 I believe).

Above the second floor it had gravel roof I think, I’m not sure what it was but it was very hot in the summer (not so much on the first floor).

After replacing it with elastomer, temperature definitely went down a little bit on the thermostat but I personally don’t feel much of a difference.

This could be because, in the morning, the sun is facing the large windows which warms the inside even more.

Anecdotal, of course. Regardless, roof had to be changed. But the AC is definitely ON when it’s 25C and above regardless of the roof type.

Where do solar panels fit into this if you have complete coverage of your roof?

An incredibly simple and effective measure that I've personally seen people rail against in tropical regions because it's ugly.

It’s not always a net energy saver. You need to compare how much you want heating vs cooling around the year.

Right, but in view of the ever increasing temperatures it seems like a good idea.

I've done that to my old workshop, just literally painted the roof white. Dropped ~10 degC during summer, no change in winter.

Seems to me that light roofs would help more during summer when there are lots of sunny days than dark roofs during winter when it's mostly cloudy.

None of this is counting infrared/UV, but reducing visible radiation is still much better than nothing.

Black radiates heat faster than white too...