Yep, Tim, that's what I'm thinking. The subfloor is 5/8" t&g plywood. The previous ceramic tiles survived 48 years without significant issues, but they were small, maybe 5.5 - 6 inches maximum (the 4" in my original post was wrong). I'm going with stronger tile (Daltile Ayers Rock), but at least 13", so a stiffer subfloor would certainly be better. Not removing that existing mastic would add a bit of height under the plywood. Looks like the stackup (mastic, 3/8" plywood, Ditra, 3/8" tile, maybe some leveling) would hit 1", which means I'll have to add ~1/4" plywood under the rest of the flooring in the house (solid hardwood for some, cork for the rest so that needed more height anyway).

You can cut off a chunk and send it off to a lab to test for asbestos content-- obviously, don't do that in an obvious place if you're not immediately planning on replacing it or covering it. If you can lift a baseboard and get material from the edge, then cover the missing section back over with the baseboard, that's often a good location. For the mastic, you can get it wet and scrape up some and send it off for testing. The lab will almost always have specific instructions on safe ways to collect material for testing and how much you need. Similar appearing materials are all over the place on actual asbestos content. Vinyl asbestos tiles tend to be square and have kind of a marbled look. They are often, but not always, 9x9. The vinyl asbestos tiles are generally quite low risk compared to the asbestos-backed sheet vinyl, because the asbestos fibers are bound in a matrix with the vinyl. Unless you pulverize them in removal, you're not going to get airborne fibers taking them out. If you contrast whatever your local recommendations are for VAT removal with sheet vinyl removal, they're usually pretty different-- the main thing people are concerned about with VAT is how you dispose of it. For sheet vinyl, though, they're really worried about how you take it up. Mastics can be pretty difficult to get up. For example, the black mastic was really easy to get up with Bean-E-Doo (whatever they call it now)-- anything else, though, forget it. The remnants of the Bean-E-Doo itself were challenging (lots of soap, water, scrubbing, etc.) However, though the same chemical said it works on old carpet adhesives, it didn't work at all for me. If you have things tested and the tile's asbestos and the mastic's not, the tiles are pretty easy to remove nondestructively with steam (e.g., a steam iron or a wallpaper steamer, or you can rent a big machine that heats the tiles for you). Then you can rent a big commercial scraper to get the mastic up, which is the most efficient. If the mastic does contain asbestos, either it shifts easily with heat or it doesn't, it's water soluble or oil soluble, etc. Use whatever people think is the best method for the specific type of mastic you have. Or paint over the mastic with one of the sealing compounds if the standard removal methods don't work. Since your tiles are loose, though, I'd take them out before putting any new flooring down.

The discussions on this subject are voluminous in these forums. A simple google search for asbestos containing linoleum or tile will lead you to "cutback" adhesive. Here is the story; An interesting discussion, linoleum, and possibly cut-back adhesive. This product, as with all products containing asbestos as the binding agent only atain a "hazardos" status when they become "friable", iE airborne. Since there was a lack of consistency, state to state, for the remediation of these products, encapsulation became the method of choice and so, many have been simply covered over with "safe" products, such as Hardie-backer. The simple test that determines whether or not cut-back adhesive is present is to spill boiling water on an exposed area. If the water puddles, you have cut-back adhesive, if the adhesive melts, you do not. The solution in the removal process is to keep the area moistened which will serve to prevent the binder from becoming friable. There are also several products which work quite well for adhesive removal.

I paid about $2K to have an asbestos removal company take up my kitchen and basement tile floors. They did it correctly, hazmat suits, air blowing outside the house, rooms sealed off, etc. But this powder room was forgotten! Someone shut the door and they didn't see it! So, now I've got 16 square feet of presumed asbestos-containing floor tile (don't know since this particular tile wasn't tested) either to cover up or remove. My carpenter doesn't think it's worth calling in the asbestos removal people, and in my state, a homeowner can remove non-friable suspected asbestos-containing flooring up to 100 square feet. I'd like to cover it up. All the EPA websites say cover up asbestos tile. It's not dangerous if it's not friable (crumbling) and my tile is not. The underlayment for Click will add more height to the powder room floor. There's a silvery foam type sold by Farbo (it's 1/8" at least), and whisper wool underlayment that's 1/8" thick. So my powder room floors would be 3/8" higher than my kitchen floor. Will this look bad? Will the underlayment be enough to "hide" the uneven floor caused by the missing pieces of white tile? Or will I need to fill them in/even out the floor with some leveling or patching product? I looked up t-moldings and reducer moldings -- not the look I was hoping for, but I can live with it if I have to. I understand Farbo no longer sells transition moldings. I was hoping I could get the Marmoleum and the wood floor to abut each other at the same height, but I'm assuming this isn't possible with Click? Does anyone know? Does anyone have Click installed adjacent to a wood floor without an expansion gap?