#7 High Temp Roof Boot
#7 Dek Tite High Temp EPDM Square Shape in Red. For use in Chimney installs on flat to 8/12 Roof Pitch. Conforms over ribs on steel roofing. Measures 16" across. Also comes in this form in round and gray. Smaller in diameter at approximately 14" across. https://flameinnovation.com/product/7-dek-tite-high-temp-round-roof-boot/
When you are installing you want to pick the right part for the roof. If you have a metal roof these are the best choice. They conform over the roof ribs. If you have a flat roof, these are great too. Seal it down to the roof with silicone. Buy the best 100% silicone. It will be money well spent.
Seal under the edge. Start at one corner and work your way around. Don't screw down all the corners first. You will end up stretching it and it will pull back over time. Make it fit and mesh with the roof. Furthermore, use a rubber washer head sealing screw. Like a roofing screw for metal roofs.
Other types of roof boots are metal. https://www.youtube.com/watch?v=x3B3fBwN-F0 This is a video on installing a metal roof boot on a composition roof. You can install a flat cone metal flashing on a flat roof as well. Fit you rubber roof boot around the exhaust pipe when going through. Don't leave any gaps for water to penetrate through.
For example, if it looks all smashed up on one side. Then you probably have a steeper roof than most. Carefully cut on the uphill side of the boot. This will make a slight egg shaped hole. Move the boot around by hand sliding it up the pipe edge on the back side until you have it looking as good as it will. #7 High Temp Roof Boot is good on 4/12 to 8/12 pitched roofs. I don't agree. 4,5,6 is about as steep as I would go or they just don't look good.