I'm working on gluing down plywood on concrete as a nailing base for 3/4" solid plank (4" so I'm not going to glue + nail the planks themselves, just nail).

Most of the floor is carpeted (to be removed and 3/4" plywood glued down), but one 200 sq ft area in the kitchen is glued down with 3/8" engineered. This area has a big area that is damaged (due to leaving a wet vac on it top of it with a plastic sheet inbetween), most likely cannot be repaired.

So I will need to install the new 3/4" solid plank in the kitchen as well.

I could just remove the engineered, scrape out the glue (previous adhesives must be removed) and install the 3/4" plywood just like everywhere else, but this seems like a lot of difficult work which might not even be necessary.

Instead I'm considering leaving the existing glued down 3/8" engineered as is and attaching a 3/8" plywood on top of it to bring it to the same level.

Question:
Is the engineered + plywood combo considered a sufficient nailing base, or could the engineered "crumble" or something?

Is separation an issue at all, when the subfloor is glued down? At one edge between the engineered + ply and the full ply, the planks will be parallel to the edge. Should I do glue those two subfloor edges too, to avoid separation? Or glue + nail a couple of rows on either side?

Personally I wouldn't feel comfortable sandwiching the engineered between the slab, plywood, and the solid. I would rather start clean, and remove the existing engineered. It's possible that using plastic vapor barrier will result in injecting moisture into the engineered, cupping, mold, etc. Among other issues of going over the engineered, over concrete.

And how are you protecting the plywood base from vapor emissions? Adhesive alone will not do that. Besides that IMHO the plywood should also be mechanically anchored to the concrete unless your going to do a floating subfloor.

Thanks. Will be putting down moisture barrier or moisture reducer and adhesive system (Titebond). Not sure if barrier is required (or reducer is enough), but will probably do it anyway because it doesn't make sense to lose the whole floor on that consideration.

Remove the hardwood. No sense taking a chance here when it is only a portion of a larger job.

Just my two cents, I agree......take out the existing. For one, it's already suffered some degree of moisture damage, so you don't know what type of adhesion or other issues may come up. Also, you still have to have a vapor barrier below the solid before it is installed, and you don't want to sandwich that engineered between the vapor barrier and slab...trap moisture in the wood, possible delam, adhesive failure, mold/mildew.....lots of possible problems.

Thanks to all. I will remove the existing engineered completely as you suggested.

There was a lot of dust when I removed just 25 sqft of thinset from the marble entryway with the angle grinder (luckily the wind was blowing most of the dust out). If I'm going to remove the engineered wood, I am worried it might be a lot more dust to grind the adhesive used to glue down the engineered (200 sqft)?

It's right in the kitchen with family living here, so trying to see if I can do this without removing the engineered.

As an update, the situation currently is, I decided to go with a floating floor. Got about 25% of the floor done. 15 lb felt as a bed, 6mil poly over that, then insulayment, 23/32" plywood and finally another sheet of 15 lb felt stapled to the plywood so it doesn't move around. Then nail down hardwood.

The engineered was glued down by the builder, I'm guessing with Bostik and a sealant underneath. It's a thick rubbery type of adhesive. There are no moisture issues with it, from beneath (just the top has worn out a little in some places).

So assuming there is moisture control already via the sealant+glue, I'm thinking, skip the poly for this area avoiding rotting issues. Example on top of the engineered, lay out insulayment (so this floor doesn't feel colder), lay down (nail?) 3/8" plywood, 15lb felt stapled to it so it doesn't move around.

Is that OK or do you feel the poly is a must?

Any issues with the 1.5" cleat not being entrenched in this double layer of 3/8" engineered + 3/8" ply? Already it is 1.5" and it will probably only go half way into the engineered. I think for 1.75" cleats, I would be cutting it too close, might hit the concrete.

If the engineered must come out, how do you remove the thick rubbery adhesive generally, and does it put out a lot of dust when you remove it?

Your assumptions are incorrect. First, do not "grind off" old adhesive. It will not work anyway. One needs to scrape it off with a razor scraper. And don't assume the builder had a MV barrier system. The flooring was/is engineered, so it can withstand more MV. A solid cannot. You are still best off cutting out the engineered in the kitchen. Plus, mixing various subfloor systems is not a great idea. It will be a bad nail down subfloor in the kitchen because the 1.5" fasteners will go through the 3/8" plywood into the engineered flooring, anchoring down the plywood somewhat. It seems risky at best.