Plywood subfloor bends

Can I install backerboard over the plywood subfloor if it bends between joists?
What do I do if not?

 

If it bends between the joists, you'll need to beef up the floor before installing ceramic tile. Adding more plywood would be one suggestion. What do you have for a subfloor now? Joist size? spacing? thickness of existing plywood. How long is the longest continuous span of the joist?

 

It's 15 years old house. Subfloor is a plywood (about 5/8, may be 1/2)
Joists are 2x10 in about 16" apart and have a perfect condition (do not bend). It bends when I step on the joint between joists.
I'm thinking would cement board add more strength to the floor?
What is beefing up?

Thanks,
Max

 

Hold the phone. You have joints in the plywood that aren't centered on the floor joists? That's bad. There's no way to make that stronger. You need a floor joist under that seam. Tongue and groove plywood (or even butting plywood pieces up against each other) need to be joined above a floor joist. Period.

Now if I've misunderstood I apologize, but consider how plywood is meant to work over floor joists. Plywood is thin sheets of wood sandwiched together using a resin to form a composite. It's strength is maximized when its forced to be perfectly flat when attached to a load bearing structure like floor joists in more than 2 points end to end. If an end is left in a cantilevered position it looses its strength, and over time will warp and break.

If the edges are supported by floor joists determine its thickness don't think of tile backer as a structural material. It's just effective in creating a substrate for tile to adhere to. It doesn't add strength per-se. You need another layer of plywood to add strength. Use a construction adhesive you can spread with a trowel and lay down another layer of plywood - at least another 1/4 or 3/8". Then think about creating a substrate for your floor with tile backer afterwards.

 

Well the joints that are paralel to the joists are over them, but there is perpendicular edges of plyood that cross the joists and that is the place of my concern.

1/4 looks like a good idea.
Thank you
Max

 

When you replace subfloors or lay new sheeting for subfloors you are supposed to block seams like that by framing in headers (cross-beams) between floor joists to provide both structural stability to the seam and to provide something to nail to.

 

Yes, I figured it wasn't done right.
Question is - what to do now?
I would change it, but there is a some walls steping on it.
The only way I see - cut the part of it to get acess from under (don't want to open seiling) block the seam.

Thank you,Max

 

If you can't reach it from underneath any more then I would recomend re-sheeting the floor from upstairs. If walls have been framed over it then use a skill saw and cut the floor out as close to the walls as possible. THEN, you'll have to block the seams between the old pieces of floor under the walls and the new one you put in, so you have your work cut out for you. Tear up the tongue and groove, block the seams, and recut new flooring material.

Who did this? If it was a paid contractor then order them back and have them fix it. You shouldn't pay for this. It was done wrong in the first place, and to fix it will require a whole hell of a lot more work than what would have originally been required.