Via my DH: first check to be sure this wasn't an adhesive with asbestos. If it's black, don't sand! Otherwise, he thinks you can use (carefully!) one of those floor sanders like they use to refinish wood floors.

When we took up our vinyl tile we had leftover adhesive, but it wasn't thick or lumpy so we just went over it with the next floor. Can you take a photo of what yours actually looks like?

I believe that not all asbestos-containing mastic is, in fact, black. That is, if it is tan or grey, it still can contain asbestos. Proceed with caution.

What kind of tile was it? What substrate is left behind? If, for example, it was vinyl tile stuck down on thin underlayment with mastic, you may just want to pull up the underlayment. If, on the other hand, it was ceramic tile cemented to plywood using thinset mortar, you would probably want to chip/sand the old thinset off and keep the plywood.

BTW, I should not have said "I believe." I should have said "I know for a fact." Before our renovation, I moved the fridge out and took a "core sample" through the many layers of vinyl tile and sheet vinyl. None of it was black. However, the local testing laboratory confirmed lots of asbestos in one of the mastics, and a fair amount in one of the vinyl tiles.

I would also see if this is the subfloor, or in fact a thin underlayment like lauan plywood that was put over the subfloor. If so, it may be easier to take up the lauan than to remove the adhesive.

Good to know, Angie. For those of us with asbestos era houses, it's always a dance figuring out what's what, and then what to do about it.

Will it add that much to the cost to replace the subfloor? I'd just do that and be done with it. As long as you don't sand it, the asbestos in tile and mastic isn't really all that dangerous. Now, if you SAND it...that's a whole 'nother story...

Here's a shot of the floor. We tried to lift the subfloor last night, but wasn't prying up easily. (of course!) Wonder if we could spread leveling solution over the entire floor?

Age of the floor you tore up is important. Late 1970's and beyond are most likely asbestos free.

What kind of tile are you putting down? What is your planned underlayment? For most options you can just knock down the high spots with a sander (if you are past the asbestos era) and then go over with your underlayment.

To me, that does look like a proper plywood subfloor (i.e., not an underlayment) with remaining thinset. Just to be sure, does the remaining adhesive stuff feel like cement to you, or is it at all "giving"? If you take a knife and press it down into one of the ridges, does it yield at all, or is it hard as a rock. (Yield=mastic, rock-hard=thinset mortar.)

If it is mortar/cement, you can try to chip it off or grind it down. You can also use self-leveling cement to cover it, as you suggest. You can also get it reasonably flat by chipping, then just cover it with cement-board underlayment cushioned with thinset.

For chipping, I just found out that they sell wide "blades" (like a spatula) made to go into a reciprocating saw (aka Sawzall). That might make quick work of it. Otherwise, I would get a tool made for chipping icy sidewalks (if you live in a cold climate!).

Probably would be worth posting your query in the johnbridge . com forums. Those people would tell you what your best plan is in a red-hot minute!