Will adding floor joist supports help my slate install?

[fenderskirt66]

Hey all, I started a thread yesterday entitled "white thinset over grey fiber joint" and the direction has moved to floor support for a slate tile installation, so I figured it would be better to start a new thread. I need to know what needs to be done "without installing additional plywood" so that my 12x12 gauged slate tile install will not fail. Here are my flooring specs-
9x2" floor joists spaced 16" apart
Floor joists are approx 12' long with cross support pieces installed between the joists at 6' all the way across
5/8" plywood subfloor
1/4" hardibacker thinset'd and screwed every 6-8"

Kitchen to be tiled is approx 9x10'
Entryway to be tile is 4x8'

According to the deflecto, im gonna be short. How can I successfully beef up the floor from the underside so that my slate will last?
Thanks in Advance!
Kelly
fenderskirt66

[bbcamp]

Kelly, you have 2 options: 1) add a support beam somewhere close to the middle of the span. 2) add more 2x10s between the ones you have now, so that they will be 8 inches apart. Looking at the pictures, I'm not sure which will be easier for you.

Either way, you need more plywood on the subfloor. Stone is fragile, so it needs twice the support of ceramic tile. It is also susceptible to structural movement, so the second layer of plywood, when installed properly, acts as an isolation layer. 5/8" is not much subfloor for a ceramic tile floor.


So: if you add the beam: add 5/8" (minimum) plywood and your backerboard. The beam should be 2 2x10s with supports every 6 or so feet.

If you add the joists, add 3/8" (minimum) and your backerboard.

[fenderskirt66]

Thanks for schooling me BBCamp...could there be an option 3, "Take the natural slate back to HD and settle for a ceramic tile and not do anything to the subfloor support system?" Is this the more "reasonable" option? Since my backer board is already laid, the thought of taking it up again "and I know it's not gonna come out in 1 piece" doesn't sound very appealing.

Thank You!

[bbcamp]

Yes, Option 3 looks good from here.