Making sure the deck’s ready for underlayment - what’s your process?

Ever tried a cheap inspection camera? They’re not flawless, but they help if you hate crawling around.

I’ve messed around with one of those $30 USB snake cams from the hardware store. Honestly, it’s a bit of a mixed bag. The picture’s grainy and the cable’s always flopping around, but it did help me spot a patch of soft wood behind a joist last spring. Saved me from laying underlayment over a spot that would’ve rotted out by next year. I still end up on my hands and knees for the stubborn spots, but the camera’s good for those tight corners where even my mirror-on-a-stick won’t reach.

I’m with you on spending extra time up front. I learned that lesson the hard way - years ago, I rushed through an inspection on my old shed roof, skipped a couple boards that “looked fine,” and sure enough, two seasons later I had mushrooms growing under the eaves. Not my proudest moment.

One thing I started doing lately is running a garden hose over the deck before underlayment goes down. Not flooding it, just enough to see if any water sneaks through nail holes or cracks I missed. It’s not foolproof, but it’s caught a few sneaky leaks for me. Especially important if you’re using any kind of green or living roof system - moisture finds every weak spot.

I’ll admit, sometimes I get a little obsessive about it. But after seeing what water can do when it gets under there... yeah, I’d rather be paranoid now than rip everything up later.

Curious if anyone’s tried those thermal cameras for spotting moisture? I’ve only seen them used on big commercial jobs, but maybe there’s a budget version out there now.

Curious if anyone’s tried those thermal cameras for spotting moisture? I’ve only seen them used on big commercial jobs, but maybe there’s a budget version out there now.

I actually borrowed a buddy’s FLIR One (the phone attachment kind) last fall. Not exactly cheap-cheap, but not insane either. It did pick up a couple cold spots where water had wicked in - definitely not as dramatic as the pro gear, but enough to give me a heads-up. Still had to poke around with a screwdriver to be sure, but it was a step up from just guessing. Only catch is, you need a decent temp difference to see anything useful. If it’s all the same temp, you’re just staring at a psychedelic mess.

I’ve used the FLIR One too, and I’d say it’s decent for a quick check, but you’re right - it really depends on having a temp difference. If the attic’s warm and there’s a wet patch, you’ll see it, but on mild days it’s not much help. I still rely on a moisture meter and the old screwdriver test for confirmation. Thermal imaging’s a nice extra tool, but I wouldn’t skip the hands-on inspection, especially if you’re prepping for underlayment. Sometimes those subtle soft spots don’t show up at all on camera.

I hear you on the FLIR - great for a quick scan, but I’ve had it miss plenty of borderline spots, especially when the weather’s not cooperating. I’m still a big fan of the “tap and prod” method. There’s just no substitute for actually feeling that give under your hand or foot. Moisture meters are handy, but sometimes even those can be fooled by old stains that have dried out. For me, it’s always a combo: tech toys plus some good old-fashioned crawling around on my knees.

I’ve had similar issues with FLIR, especially when the sun’s been beating down on one side of the roof and you get those weird false positives. I still use it for a quick overview, but I don’t trust it alone. Like you said, nothing beats actually poking around - sometimes you just feel that soft spot before you see anything on a meter or camera.

One thing I’ve run into: older decks with multiple layers of underlayment or patch jobs can really throw off moisture readings. I’ve had meters light up like crazy, only to find out it’s just old adhesive or a stain from years ago. Curious if anyone’s tried using pinless meters versus pin-type for this? I’ve found pinless gives me a better sense of the overall area, but sometimes misses deep pockets.

Also, do you guys ever pull random fasteners to check for rust or rot underneath? I started doing that after getting burned by a “solid” deck that turned out to be mushy between joists.