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Is old-school tar paper hardwood flooring underlayment and old-school tar paper roofing underlayment the same thing? (lemmy.world)

submitted 1 month ago by grue@lemmy.world to c/homeimprovement@lemmy.world

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I fixed my roof a few years ago, and have part of a roll of tar paper sitting in the basement. I’m now re-laying some hardwood flooring. Can I use that leftover stuff as underlayment, or is there some subtle but important difference between “flooring” tar paper and “roofing” tar paper that means I ought to go buy some other product?

(If it matters, the house was built in the 1940s and uses materials typical of that era. I’m just doing repairs/small modifications, so I’m patching in like-for-like stuff.)

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[–] evasive_chimpanzee@lemmy.world 4 points 1 month ago (1 children)

I haven't heard of tar paper being used as underlayment, but I have used big rolls of paper for that. In my understanding, the point for the paper is to allow the wood to be able to move seasonally, and to prevent squeaking.

I don't think I'd want to use tar paper, though, cause it's hydrophobic. Whatever finish is going on the wood is going to also be a water barrier, and it's a general rule of thumb to never have 2 water barriers next to each other in a house. That's how you get trapped water that can damage stuff. If you spill water, and it makes it in a little gap between floorboards, it could just sit there indefinitely.

I could be wrong, though.

[–] grue@lemmy.world 1 points 1 month ago (2 children)

The existing floor has had tar paper (or whatever this stuff is -- it's black and looks/feels/bends/tears a lot like an asphalt shingle, but much thinner and without grit) for 75 years without any problems, so whatever it's doing to the moisture levels is apparently working. Also, because this part of the floor is in a bedroom closet, I'm not overly worried about liquid spills.

The reason I'm re-laying part of it isn't because it failed, by the way. It's because it was installed in the closet going perpendicular to the rest of the room, and then a subsequent renovation a couple decades ago expanded the closet and left it part perpendicular, part parallel, and part construction lumber filler strip because they were covering it with carpet anyway and didn't care. I do care -- and I expanded the closet some more -- so I'm pulling up the perpendicular section in order to splice it in properly.

In consideration of the moisture argument, I think I will just patch in some of my roofing tar paper only in the areas that are missing it (i.e. the strip where the wall used to be) instead of pulling up the existing and putting in a contiguous sheet. Even if I'm still going to be overlapping the seams by a few inches, at least there will be a few extra ones for any potential water to drain through.

[–] evasive_chimpanzee@lemmy.world 3 points 1 month ago

I just read into it a little bit. Seems that I was wrong, and "tar paper" is typically not completely waterproof, it's just water resistant. Most stuff these days isn't technically tar paper, either, it's roofing felt. I guess it's only called paper if it's made from cellulose. Apparently they did make the felt from asbestos back in the day.

Seems like it's definitely common enough to use it under flooring, especially if it's a thinner grade.

[–] ValenThyme@reddthat.com 1 points 1 month ago* (last edited 1 month ago) (1 children)

if it's old it might be flooring 'mastic' and it's so hard to remove! You have to get it tested for asbestos if it was old and you're gonna mess with it so be careful.

[–] grue@lemmy.world 1 points 1 month ago (1 children)

No, it's definitely a sheet of paper, not mastic. It's not even attached to the subfloor, let alone sticky; it was just held in place by the flooring being nailed through it. You can pick up the corner and bend it, although it'll easily tear if you crease it. For comparison, my roll of roofing felt is nearly identical except it's about half as thick and can be creased back and forth a couple of times before tearing. In other words, they're both tar paper but the new stuff is #15 weight and the old stuff is apparently #30 weight.

Anyway, in my search for an answer I've now run across exactly the same tar paper product being sold online as being "for flooring" and as being "for roofing", so the answer to my original question is a definitive "yes, they're the same."

[–] ValenThyme@reddthat.com 2 points 1 month ago

count your blessings then I have had to remove mastic and it's fucking hell!

Glad you got it sorted!

[–] OccamsRazer@lemmy.world 3 points 1 month ago

Should be fine. I've even had it recommended from pros. Not sure if that's because it's cheaper or more available or better for some reason, but anyway it should be fine.