Solid Hardwood, Engineered and Laminate Flooring - Subfloor Prep Quesitions

First let me say how impressed I am by this website! There's a tremendous amount of quality info here.

I am getting ready to install a Pergo Presto floor on the 2nd story of my 28 year old, colonial house located LI, NY. I ripped up the carpeting and found a thin 1/8 inch tile (like the stuff you'd put on a basement floor) glued to .25" Luan underlayment on top of .5" plywood subfloor. To my surprise, the floor reads level in all directions using a 6' level. My problem is that there is moderate sized depression in the middle of the floor. (It kind of looks like a dried riverbed) The depressed area is about 4' long, 3-4" wide and almost a .25" deep in the middle. The floor reads level even with the center of the level directly across the depression.

My questions:
1) Any idea what could have caused this sag? Extreme weight? Settling? Plywood expansion? Rot? (The wood appears sound).

2) Do I need to replace the part of the floor that sags, or can I get away with using a leveling compound?

2) Do I have to rip up the tile and clean up the black glue before attempting to do the repair, or can I just pour leveling compound directly onto the tiles in the depression?

3) There is no crawl space under the floor, but there is a space between the lower room ceiling and the 2nd story subfloor. Does this pose a moisture threat and should I put down moister barrier? The Pergo instructions only mention the need for a barrier when laying over a concrete subfloor, but the advice on this site seems to almost always indicate that its necessary.

4) Any way to tell if the tiles contain asbestos? I dont think they do, but I want to be certain.

Anyway that's it. Thanks in advance for any answers.

DIY:

One quarter of an inch isn't unusual at all.

1- It could be a number of things w/o seeing it.

2- Instead of a levelling compound you could just as easily use some 30# roofing felt to "fill" in the low area. Taper 2-3 layers back to provide some even bridging with the new flooring.

3- Pergo makes two types of foam underlayments. One that doesn't have the film on the bottom side of the foam which is used in situations like yours.


4- Asbestos? Quite possibly so. But in not removing it you shouldn't have any concern.

Chances are that the tile could be asbestos. It should be confirmed and if it is asbestos it should be at least encapsulated. Flat is important, level not so much so and 1/4" is a little more than acceptable.

Thanks Fred:

I gotta admit, I rarely go near asbestos. Most of the work I involve myself are home or condos less than 20 years old here in Florida.