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.

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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.”