I hear you on the mold—humidity here (Gulf Coast) is brutal, and tarps just don’t breathe. I tried doubling up with plastic sheeting once, but honestly, it trapped even more moisture. One thing that helped a bit was propping the tarp up off the roof with a few 2x4s to get some airflow underneath. Not perfect, but slowed down the funk. Still wish there was a magic cheap fix that didn’t bring a new headache...
I get the urge to double up on plastic, but honestly, every time I’ve tried that, it just made things worse. The moisture gets trapped and then you’re dealing with a whole new mess—mold, rot, you name it. I’m not convinced propping up the tarp is much better in the long run either. Maybe it slows things down a bit, but if you get a heavy rain or wind, those 2x4s can shift or even poke through the tarp.
What’s worked for me (at least for a few months) is using one of those woven blue tarps, stretched tight and secured with furring strips screwed right into the decking. Not pretty, but it keeps most of the water out and doesn’t seem to sweat as much as plastic sheeting. Still, nothing’s perfect in this humidity. I wish there was a cheap fix that didn’t end up costing more down the line... but I haven’t found it yet.
I’ve had similar luck with the blue tarps—definitely not pretty, but they seem to breathe a bit better than plastic. I tried using heavy-duty clear plastic once and it just turned into a greenhouse under there, which made the decking even worse. Have you ever tried any of those self-adhesive roof repair membranes? I’ve seen them at the hardware store, but I’m not sure if they’d hold up in our humidity or just peel right off after a few storms.