I’m about to redo the roof (asphalt shingles, pretty standard, house is in the Midwest so lots of freeze/thaw) and trying to figure out how picky I need to be with the deck before rolling out the underlayment. The plywood seems mostly solid but there are a couple spots with old nail holes and a few questionable soft patches near the eaves. I’ve seen some folks say you should replace any questionable wood, others just patch or use some kind of sealer.
Also, do y’all sweep it clean or actually wash it down? Is there a specific type of primer or treatment you’d recommend before the underlayment goes on? I don’t wanna overdo it but also don’t want to cut corners and regret it later. What’s worked (or not worked) for you?
I know a lot of folks say to just patch or seal soft spots, but honestly, in the Midwest with all that freeze/thaw, I wouldn’t risk it. If you’ve got questionable patches near the eaves, those are the first places to rot out or let water in down the line. I’d cut out and replace any soft or punky wood, even if it’s a hassle now—it’s way less trouble than dealing with leaks later. As for prep, I just sweep it clean—never bothered washing unless there’s a ton of dust or bird mess. Haven’t used primer or sealer under underlayment, and never had issues. Just make sure it’s dry before you start rolling anything out.
That’s a solid approach, especially with Midwest weather—freeze/thaw cycles are brutal on any weak spots. I’ve always leaned toward replacing questionable wood too, even if it feels like overkill at the time. One thing I sometimes wonder about is whether skipping primer or sealer under the underlayment could come back to bite me in a really wet year, but honestly, I haven’t seen any issues either. Just making sure everything’s bone dry before rolling out underlayment seems to be the key. It’s amazing how much trouble you can avoid just by not cutting corners early on.
I get the logic behind just making sure everything’s dry, but I’m not totally convinced skipping primer or sealer is always safe, especially in the Midwest.
Maybe you’ve been lucky so far? I had a spot near the edge where moisture crept in after a wet spring, and it definitely would’ve helped if I’d sealed it first. The upfront cost isn’t huge compared to replacing boards down the line. Just my two cents—sometimes those “extra” steps pay off.“One thing I sometimes wonder about is whether skipping primer or sealer under the underlayment could come back to bite me in a really wet year, but honestly, I haven’t seen any issues either.”
I get where you’re coming from about the “extra” steps sometimes being worth it. I used to be in the “just make sure it’s dry and solid” camp, but after dealing with a couple of nasty surprises (one was a soft spot that turned into a full-on rot issue after a rough winter), I’m a lot more cautious now. Midwest weather is no joke—ice dams, wind-driven rain, all that.
For me, if there’s any softness or punky wood, I just bite the bullet and replace the section. Patching or sealing over questionable spots never really held up long-term, at least not for me. As for cleaning, I sweep and vacuum the deck, but I don’t wash it unless there’s something greasy or sticky—moisture trapped under the underlayment is just asking for trouble.
I’ve tried a wood preservative on the eaves and valleys, especially where ice dams like to form. Not a full-on primer, but something to slow down any future rot. It’s a little extra work, but honestly, it’s cheaper than replacing decking again in five years.
