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.)
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.