Reading r/Flooring and r/HomeImprovement, this one comes up most weeks. Somebody pulls up carpet in a house built before the mid-eighties, finds nine-inch tiles bedded in a tar-black mastic, and wants to know whether they can just put a new floor on top of it.
The top-voted answer is usually right: leave it alone. That material is stable while it's intact and becomes a problem when someone goes at it with a grinder or a scraper. Encapsulate it, float a rigid plank floor over the top, and you've disturbed nothing. It's safer than abatement and it's usually cheaper.
Two things the threads consistently skip.
You can't bond to it. Cutback is asphalt-based and it never fully hardens — it stays slightly plastic for decades. Modern flooring adhesives don't form a durable bond with it, and some of the solvents in them soften it further, so the bond degrades instead of failing outright. That's the cruel version: the floor goes down beautifully, everyone's happy, and eighteen months later it starts sounding hollow underfoot. Anything glued is off the table unless you first bond an isolation membrane over the top to give the adhesive something legitimate to grab. Floating sidesteps the whole problem, which is most of why it's the right call here.
The height will find something. Old tile, plus underlayment, plus new plank adds up to more than people picture. In a 1970s kitchen that usually means the dishwasher no longer clears the countertop when you try to pull it out for service — which turns a future repair into a countertop job. Exterior doors that cleared by a hair start dragging. Measure the appliance clearances and swing every door before you order material, not after it's stacked in the garage.
None of this makes the encapsulate-and-float approach wrong. It's still what I'd do in my own house. It just has two consequences that the thread you're reading probably didn't mention, and both are much cheaper to plan for than to discover.
If you're weighing what to float over it, I wrote up how the material actually behaves on a slab in waterproof luxury vinyl plank.
