Here's a weird bit of trivia I stumbled across while researching options for my old flat roof: apparently, putting the wrong kind of coating over modified bitumen can actually trap moisture and speed up deterioration. I always thought any coating would be better than none, but turns out, some acrylics or elastomerics don’t play nice with certain bitumen types, especially if there’s already a little water sneaking in. The coating can seal it in, and then you get blisters or even mold underneath.
I guess it makes sense, but I never would’ve thought about it until a contractor mentioned it (after I’d already bought a 5-gallon pail of the stuff, naturally). Anyone else run into this? Or maybe had a coating job go sideways? Curious if there are coatings that actually help with minor leaks, or if that’s just wishful thinking.
I get what you’re saying, but I’m not totally convinced every coating is a disaster waiting to happen. I used a silicone-based one on my old torch-down roof after patching a couple spots, and it actually held up fine for a few years. Maybe it depends on how dry you get it before coating? I do think if there’s already a leak, no coating’s gonna magically fix it long-term, though.
Silicone coatings can definitely buy you some time, especially if you catch the issues early and prep the surface right. I’ve seen them hold up surprisingly well after hail, but yeah—if there’s moisture trapped underneath or active leaks, it’s just a bandaid. Prep is everything, but even then, sometimes hidden rot or old seams come back to haunt you. The “magic fix” idea is what gets folks in trouble... coatings aren’t a cure-all, just a tool.
- Seen this a bunch—folks slap silicone on thinking it’ll fix everything, but if there’s already moisture in the deck or seams are failing underneath, you’re just sealing in the problem.
- Did a job last fall where the owner used coating right over wet plywood... six months later we had to tear up half the roof for rot repair. Not pretty.
- Surface prep is huge, but sometimes you can’t see what’s lurking under old layers—especially on older buildings.
- Coatings help, but if the bones are bad, it’s just delaying the inevitable. I’d rather fix leaks at the source than mask ‘em with a layer of silicone.
I’d rather fix leaks at the source than mask ‘em with a layer of silicone.
Yeah, that’s exactly the kind of thing I was worried about. My place is in a pretty humid area, and the roof’s got a couple decades on it—layers on layers, so who knows what’s trapped underneath. I’ve read that some coatings are “breathable,” but honestly, I’m skeptical. If there’s already moisture in there, seems like you’re just asking for trouble by locking it in.
I tried patching a leak with an acrylic coating a few years back (before I knew better), and it looked fine at first. But after one summer, I started noticing these weird soft spots and a musty smell. Ended up having to pull up part of the membrane and, sure enough, there was mold. Not fun.
I get the appeal of a quick fix, but now I’m leaning more toward actually addressing the leaks before even thinking about coatings. Wish there was a magic eco-friendly option that could handle minor leaks without making things worse, but I haven’t found one yet. Maybe some of those newer “liquid rubber” types are better? Still seems risky if you don’t know what’s going on underneath.
