OK... here is the deal. My basement utility room floor (tiled concrete) is far from being flat... in its 12 foot width the center is depressed by nearly 1.5 inches. There are no floor drains and it is above grade. The floors are also tiled with what is believed to be asbestos containing tile that has been there since 1963. I wish to do something to first encapsulate that time... and provide a reasonable level surface that I can lay ceramic tile over. This room is roughly 24 feet long so the typical self leveling products would be absurdly expensive. This is a fully finished basement so there are fully framed and sheet rocked walls on every side as well.

Hiring a contractor isn't an option as the money isn't there for it. I am fully capable to do the work with basic instructions, know how to do framing, wiring, plumbing... and tile work. And want a cost effective and durable means to do this by a very experienced do-it-yourselfer.

There are no indications of cracks that have telegraphed through the tile and I don't know if it was this way from day one because idiots did the work... or it it somehow got this was over the years since it was built in 1963 via subsidence.

..

I would go with a different approach. I would:
1) use string to determine level on about a 6x6 grid.
2) glue blocking to the floor that JUST comes under the string. I would probably use stacks of cheap tile to build up the blocking.
3) Remove the string and bring in concrete to build up the floor. Use a straight 2x4 on the guides to keep things level.
4) Finish off the last bit with self-leveling compound.

You will have a great finished floor to work on, You will have strengthened your slab, and it is probably the cheapest method that will work well. It will take about 35 bags of quickcrete at $3.70 a bag, and perhaps 5 bags of self-leveling compound at $30 / bag. So under $350 all told.

Great... that will fall within my budget. I never really considered how far out of flat it was until I had a water spill from the washing machine, then it really got me looking when I saw how deep it was pooling in the center of the room. The existing tile is butt ugly and was wanting to cover it up with something far more attractive... but also to correct that major defect as well. I'm a firm believer in any job worth doing is worth doing right.

Just reread your post and noted that you said you wish to install ceramic tile. Not sure which method you are considering but if it's the plywood, that top layer should be 1/2" cement board (Hardie board, Durarock or Wonderboard).

Not sure that I agree with Leifweaver. Minimum thickness of concrete is 2". Leveling or striking off concrete in this situation is not for the faint of heart.

No way could I maintain a 2" min thickness on concrete particularly on the outer half of the floor. Issues with entry to the other rooms plus two exterior doors would nix that.

And I do fathom the mess having it start to break up down the road would create. And why I wanted to make no assumptions being this is not my area of expertise.

This is part of a fully finished basement (technically a split foyer home) so completely gutting half the house to redo a slab is NOT an option for a number of reasons. Even doing this room alone like that really isn't an option because it has the furnace A/C, water heater and the Washer and dryer. That would get way beyond my abilities to either do or pay to have done. Besides there is no indication the floor slab isn't intact and stable as it is now.

Thus you see my reasoning for leveling the existing floor.

I have concerns about plywood and long term durability... being this is a basement floor... and encapsulated the long term effects of moisture even though as far as I can tell for the decade plus I've been in this house water infiltration isn't an issue. I only need to run a dehumidifier between the heating and cooling seasons. And that is only based on a humidity meter reading, not for condensation or mildew..

Moisture through the existing concrete floor should not be a concern. The existing tile as well as the construction adhesive would serve as an effective moisture barrier.

I also would have concerns about the thinner areas of concrete breaking up if you choose that approch.

If you really have concerns about using plywood you could consider using the cement board instead. Be more costly and have to rethink whether to use thinset or construction adhesive to install. I believe 1/4" Hardiboard is available for the top layer to minimize the increase in floor height. You would have use the thinset to fill the voids between it and the second layer.

I did say concrete, but probably you will want to mostly use cement. Strength is not the real issue here, you are basically using it for fill. The minimum thickness is if you are using it for structure. The leveler will keep it from being friable, and since one part can't move in relation to another part, because of the slab below it, it will not break into pieces.

I guess the key is the right bonding agent that will allow it to "stick" as well as possible to the existing floor too.

Agree. If you chose the plywood approach use the trowable construction adhesive. When cured, rest assured that it will not separate. The screws are only used to keep in place until the adhesive has cured.

If you chose the cement board, use thin set mortar. The thin set is used to bond the tile to the cement board so it should give maximum adhesion between the cement board and some adhesion to the tile. You could use the construction adhesive between the cement board and the tile but the thin set will fill any voids better.

Think about it. If you chose the cement board, which after giving is some thought I tend to favor, you will have a continuous, solid cement product floor surface. Due to the weight, gravity alone will hold it in place.

Search for "slabjacking", "mudjacking" or "concrete leveling" and hire someone to correct the problem. The cause of the problem lies under the bottom of the slab, and putting a ton (quite literally, possibly) of tile on top is only going to make the problem worse. Tile adhesive does not create a moisture barrier, and if you just throw a bunch of material on top until it's level, you're bound to experience further problems, unless you are unbelievably lucky. A concrete leveling contractor can quickly correct this type of problem and help to eliminate the chances of further problems and save you tons of headaches. Check out http://www.concretejack.com/slab-floor-leveling for a few pictures of the results from the concrete leveling process, and a couple of diagrams of what causes the type of problems that you're experiencing. From someone who spends everyday getting paid to correct this type of problem for people, it's very difficult to tackle on your own and properly fix.

Thanks... need to look into that option. Never got started on anything yet because of too many projects and too little time left for them.