Those coatings can definitely trap moisture if there’s any already in the wood.
Had a similar thing happen with my porch roof. Thought I was being smart slapping on a rubberized sealant, but turns out the wood underneath was still a bit damp from a leak I hadn’t noticed. Couple months later, the boards felt spongy and smelled off. Lesson learned—if there’s any doubt about moisture, it’s better to wait or pull up a section and check. Those coatings are unforgiving if you seal in even a little dampness.
That’s a tough lesson, but honestly, it’s more common than people think. Those rubberized and elastomeric coatings are marketed like miracle fixes, but they’re basically a vapor barrier. If you’ve got even a little trapped moisture under there, it’s not going anywhere—just gets cooked in when the sun hits. I ran into something similar with a flat roof over my garage. Looked fine on the surface for about a year, then I started noticing a musty odor every time I walked in. Pulled up one of the boards and sure enough, black mold and soft spots.
A lot of folks don’t realize wood can hold onto moisture for quite a while, especially if it’s thick or the weather’s been humid. Even with a few dry days, you can still have dampness deep in the boards. I’ve started using a moisture meter before any sealing job now—one of those cheap pin-type ones from the hardware store. Not perfect, but gives you an idea.
Honestly, if there’s any doubt at all about whether something’s dry enough to seal, it’s worth waiting or even running a fan for a few days. In my area (southeast), humidity is brutal and things just don’t dry out as fast as you’d expect.
One thing I’ll add—if you’re dealing with older roofs or porches that have had leaks before, sometimes it pays to pull up a couple boards and check underneath before doing any new coating. It’s extra work up front but saves way more hassle down the line. Once rot sets in under a sealed layer, you’re looking at replacing whole sections instead of just resealing.
Not saying these coatings are bad across the board—they do have their place—but they’re definitely not “set it and forget it” solutions. The prep work makes all the difference... and unfortunately, skipping steps usually comes back to bite you later.
Had a similar mess with my back porch last year—thought I was being clever slapping on one of those “flexible” coatings. Fast forward to summer, and the boards underneath felt like wet bread. Is it just me or do those miracle products always leave out the fine print about moisture? Anyone else end up running fans for a week just to be safe, or am I just paranoid?
Is it just me or do those miracle products always leave out the fine print about moisture?
You’re not alone. I slapped one of those “waterproof” coatings on my deck, thinking I was a genius. Next rainstorm, the water just pooled up and sat there like it was waiting for an invitation inside. Ended up with soggy boards and a weird musty smell that wouldn’t quit. Ran two box fans for days—felt like I was trying to dry out a swamp. Lesson learned: those coatings are only as good as what’s underneath, and if there’s any moisture trapped, you’re basically sealing in the problem. Now I’m paranoid about every little creak underfoot...
That’s a tough one. I’ve seen similar issues on flat roofs—people slap on a coating thinking it’ll fix leaks, but if the substrate’s damp or there’s poor drainage, it just traps water underneath. Did you check for ventilation under your deck before sealing it? Sometimes airflow makes all the difference.