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This was found under two layers of sheet vinyl flooring in a late 1940s house. Subfloor is fir and there appears to be a black tar-like mastic between it and the wood. The green underlayment feels sort of like construction paper that kids cut shapes out of.

I'm going to install a new floor and want to know what it is that I'm sitting on top of.

UPDATE

In response to the comments below, yes, I am aware of asbestos concerns. And no, this is not the original linoleum. It is a paper like substance that is clearly being used as an underlayment.

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  • why do you not include a closeup picture of the material in question?
    – jsotola
    May 28, 2020 at 5:00
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    Tar like adhesive on the floor of a 1940's house? It looks like you are sanding it to remove it....... Did you test for asbestos first? Asbestos was VERY common in "tar like" substances back in the day.
    – Gunner
    May 28, 2020 at 9:51
  • Agreed with Gunner. It's quite possible you do not want to know what this is, nor remove it. Encapsulate it with your new flooring, disturbing it as little as possible, unless you're going to send off a sample for testing, to confirm an assumption that you should be working under anyway. A confirmation that I think you would have to legally disclose when you sell the house.
    – Mazura
    May 28, 2020 at 12:04
  • ... or w/e. My grandpa was a pipe fitter his entire life. He died from emphysema at the age of 95 having smoked cigarettes for 40y. He did have part of his finger and one of his 'goodies' removed due to cancer though. If you've got kids in the house however, that's a different story.
    – Mazura
    May 28, 2020 at 12:14
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    Yours is probably hardboard if it's noticeably thick. IDK what that stuff's made from. As with anything +80yo, 'work under the assumption' that it's something you shouldn't be breathing and try to handle it carefully to avoid it becoming friable (make dust).
    – Mazura
    May 28, 2020 at 22:10

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Underneath vinyl tile (which if old enough may contain asbestos) was usually either rosin paper, hardboard, or cutback.

Rosin paper is really thin and tears up into annoying little pieces. It's more or less readily identifiable; certainly not easily mistaken as either of the other two.

Hardboard is "often used as underlayment. Likely 1/8" thick. If so, you can remove or go over if in decent shape" with basically any type of flooring besides real tile.

Cutback... gah, is "that black tar stuff" which may or may not also contain asbestos.

Asphaltic cutback adhesive is an older type of mastic made with asphalt-based cement. As a petroleum-based material, the cutback is not softened by water. Some cutback adhesives contained asbestos. source

can be very difficult to remove unless you use shot-blasting equipment. Fortunately, most grout and mortar manufacturers make a thinset which will bond to "cut back" adhesive.

Typically, the "cut back" should be scraped smooth so the tile is not sitting on ridges of adhesive - although it does not have to be absolutely flat. Note, not all thinsets will bond to cutback so it is important to make sure the thinset used is made for that purpose.

Also, the adhesive must be true "cut back." That means it was made from asphaltic material, is not water sensitive, and will not soften when exposed to water. There are also black-colored latex-based vinyl adhesives that will not hold up under ceramic tile if they get wet. source

[If laying down new vinyl, the] plasticizers in the vinyl migrate down through and attack the Cutback Adhesive which softens it and then the cutback starts migrating up, causing staining in the vinyl flooring. Cutback Adhesive residue will also eventually bleed up through a felt-backed flooring product as well. source

It's easier to just rip up the subfloor if it's cutback. When I had to remove it from concrete, it required a scraper blade in an SDS (rotary) chipping hammer drill, a commercial floor machine with an abrasive pad, and several days of labor for a single room. Mineral spirits just made it gooey and even harder to scrape up, aside from being noxious and a fire hazard.

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