3/4 inch dip in center of sub floor, what to do?

[bbcamp]

If the flooring is original to the '50s, it is likely Lineoleum. If that's the case, leave it and install backerboard.

Ditra can be installed over it, too, but the flooring must be secured with screws every 4 inches and there must not be any wax or bond breakers on the surface. Tough standard to meet.

[iagaoe]

put backer board ontop of the plywood? 1/2 inch backer plus mud and tile will probally leave me with a 1 inch transition from tile to wood floor? isn't that a big gap?

[jadnashua]

Linoleum can last a very long time, and being primarily green materials, is back in fashion. It can be a major bear to remove, though. Often, it is installed over 1/4" luan plywood, and if so, then it becomes somewhat problematic. Are you certain of the stackup of the materials in the flooring now? Also, being that old, they often used planks as the subfloor. Assuming you are adding the footer(s) and beam to reinforce the joists and help remove the dip, you still need to verify they will meet the requirements for tile at L/360 (use the Deflecto tool above). Note, jacking the floor will need to take some time or you'll crack things. And, because that group of joists have taken a set over all these years, they may not like to go back straight at all. You might find that supporting them where they are and sistering new ones along the sides to support the subfloor at the desired level may work better than trying to jack it up.

[iagaoe]

Ya i need to figure out my joists length to use the Deflecto tool.

If it fails the deflecto tool, is there anything i can do to still lay tile there?
Wife will die if i tell her i can't lay tile in the kitchen!

[jadnashua]

You indicated you were going to put in some piers and a new beam...that will shorten the span. If required, you might need a pair of beams, but you won't know until you determine the size, spacing, species, and length of the existing stuff. Sistering might do it as well, and if you did that, you could set the new ones level and attach new subflooring to those and that would get rid of the dip as well as strengthening the floor. That would likely mean tearing out the existing subfloor to work from above if the crawlspace isn't high enough.

[Bodie Powers]

If that composite flooring is 50-60 years old there's a good chance the old adhesive and/or flooring contains asbestos. Make sure you're wearing a good quality mask designed to filter all particulants, especially when you're sweeping/vacuuming up any dust.

[iagaoe]

Really don't want to have to pull up the sub floor, i so hope i can fix it with out doing that.

Ya we are putting in a new beam, hopefully!