Many come from hot climates, so working in most places in the US isn't a problem- they know how to deal with the heat. One way, as they do in Arizona, is to start before sunrise. It's still hot, but the Sun isn't baking them. Street temperature can be >110°F and they're out there but roofing is a bad idea because it will be >120°F, midday.
Most people don't want to work in roofing. Have you ever done it?