I recently purchased a ninety year old townhouse that was converted into a 3
unit building shortly after World War II. The 3rd floor bathroom originally
had a tile floor (approx. 1" hexagonal tiles) which has sagged probably due
to settlement. I recently discovered the tile after removing vinyl flooring
which had been glued on top of the tiles.

I would very much like to install ceramic tile, probably 12" square tiles,
however removal of the old tiles and a 2 1/2 concrete subfloor is too
costly.

Figuring that most of the settlement that was going to occur probably has
done so already, I was wondering if there is any way to fill the valley
created by the sagging with a material that will level the floor
enough to install new tile over the existing tile. My guess is that the
floor's lowest point is 3/8" to 1/2" lower than its highest point.

Any suggestions would be greatly appreciated.

John

Sure, they make floor leveling products that are easy to use.
You better be absolutely sure the substrate is secure though because
those 12" tiles are going to break much quicker than the little 1"
jobs. Any bounce at all on that field and they crack. It's a small
area though and you could do this fast and take the chance.

PS. It probably sagged because water got through and rotted the
subfloor. The rot is still there.

Adding more weight to a sagging floor system doesn't seem like a great
Idea.

I own a 1930 house, both bathrooms are tiled with 4" hex over 2x8 joist
w/ ~1' bed.

If / when I repair/ replace, I'll either remove all the tile & bed &
sister to the joists OR I'll just jreplace the cracked tiles.

In your case if the all the sag is in the mddile of the floor, then
probably it's due to deflection of the joists /subfloor because
they're carrying too flexible.

How big is the bathroom? Can you do without for a while?

Since you're going with a new floor, I'd remove all the fixtures & have
at it with an electric demo hammer (Hilti, Bosch, etc).

Check out the joists; remove, replace, sister as needed.

For tile, you want the maximum deflection under a point load (~250lbs)
to be less than the "span" / 360. (This is a minimum criteria usually
applied to the total uniform live & dead loads and usually estimated at
~ 50lbs per sq ft) Tile & backer board can add another 6 psf.

Both the joists AND the subfloor material that spans between joists
have to be checked; the stiffer the better. If it's too flexible, the
tile will crack just like before. TIle placed over "older" wood floor
systems typically crack.


cheers
Bob