Went through something similar last year. Thought slope alone would do the trick, but after a couple storms, had those annoying puddles hanging around. Ended up adding mats later—wish I'd just done it upfront, would've saved some headaches (and cash). Lesson learned, I guess...
- Slope alone can actually work fine if drainage is properly planned from the start.
- Seen plenty of roofs without mats holding up well—usually issues pop up when slope angles or drain placements aren't optimal.
- Mats help, sure, but they're not always necessary if the initial design's solid.
Agree slope alone can work, but insulation-wise, I'd still lean toward adding something extra beneath the bitumen. I've seen roofs with just slope and good drainage hold up fine structurally, but insulation performance is another story. Modified bitumen itself doesn't offer much in terms of thermal resistance, so if you're looking to keep heating/cooling costs down, you'll probably want some rigid foam insulation underneath.
I redid my garage roof a couple years back—initially thought slope and drainage would be enough. Technically it was fine, no leaks or pooling water...but man, the temperature swings inside were brutal. Ended up pulling it apart again (lesson learned the hard way) and added a layer of polyiso foam board under the membrane. Huge difference in comfort and energy bills.
Mats are helpful for durability and puncture resistance, sure, but they're not really doing much for insulation anyway. If your main concern is thermal performance rather than just waterproofing or durability, I'd definitely recommend looking into rigid foam boards or spray foam beneath the bitumen layer. It's a bit more upfront cost and effort, but worth it in the long run.
Just my two cents from experience—good luck with your project!
Good points raised here. Modified bitumen is solid for waterproofing, but insulation-wise it's pretty minimal. I dealt with a similar scenario after storm damage repairs on a client's roof—initially they were hesitant about insulation due to upfront costs. Eventually, we went with closed-cell spray foam under the membrane, and their feedback was overwhelmingly positive in terms of comfort and reduced HVAC strain. Definitely worth the extra effort if thermal control matters to you.
"Eventually, we went with closed-cell spray foam under the membrane, and their feedback was overwhelmingly positive in terms of comfort and reduced HVAC strain."
Closed-cell foam is great for thermal control, but did you run into any moisture trapping issues? Curious how you handled ventilation or vapor barriers in that setup...
