20 inch porcelain tile

I have 4" x 8" joists solid with a span of 4 ft and the subfloor is 1 1/4" tongue and grooved plywood. I want to install 20 inch porcelain tile that is 1/2" thick. Do you think my subfloor can withstand it? The area is is 10'x20'.

That 4 ft. Span, is it between those joist or are the joist unsupported for only 4 ft?

The Joists are 4ft. Apart either 8ft long or 16ft long and are supported every 4ft.

Still trying to picture this, is this a beam framed house?

It was built in 1970 and the joist beams run parallel to each other with a 4 ft distance between them and are supporte every 4 ft. I assume it was a stick built house. I guess I don't know what beam framed means?

Rd thats PIER AND BEAM

Post and Beam

Or Timber Frame.

Thanks, now back to my original question. Do you think this will support hardyback and 1/2"porcelain tile. I realize we will have to take any sway out of the plywood if there is any. Would cross beams be necessary?

I believe they would be. Even putting another layer of plywood down (which brings up another question-- that 1 1/4" plywood-- is that one layer?) I don't believe would be enough to make sure there's no deflection (or minimal) between the beams. I'd be putting two cross beams between each main beam, and bring that down to 16" OC, or atleast one bringing it down to 24"OC, and another layer of 3/4" plywood. Especially with larger format tile, you want that floor just as stiff as you can get it.

If I use and additonal sheet of 3/4 inch plywood it would appear to raise the floor too high unless I did not use the hardyback if that is what you call the heavy material used under tile. The 1 1/4 inch plywood is one sheet. Would any additional plywood like maybe 1/4 inch help? Thanks for the information on how to cross brace the beams.

I would recommend schluter-ditra over your 1 1/4 t & g plywood floor and than set the tile with a latex modified thinset mixed straight with a flexible additive like laticrete has. Use a 1/2 " x 1/2 " saltillo trowel and enjoy that floor for the rest of your life.

Here in smallville texas almost all the houses are PEIR AND BEAM and the peirs are every 4' each direction and I have done the above in every one of them I have done. Good luck

I won't use Ditra with pocelain tile, due to the fact that it says to use dry set for porcelain, I'll only use modified like William said, but it will take 2 weeks to cure before you can grout, if Ditra is used, this has been an on going debate for months now.

William, I agree, but have you had any problems grouting the next day with porcelain over ditra with a modified thinset? Richie.

Sorry William about the post and beam thing. Is the pier and beam just a foundation thing? Have not run into it before.

It's known by different names in different regions of the country. Up here it's known as post and beam, as well.

Also, the floor still needs to meet the L/360 requirement BEFORE the Ditra goes down, and from what I'm hearing, it doesn't-- not between the beams.

I really appreciate all the replys. Could somebody tell me what the L/360 requirement is? I assume it is something to do with whether the floor is level in between the beams and is checked by having weight in the middle and checking for levelness. I live in the high desert with less than 8" of moisture per year with 50-60% humidity if that would have any effect on the installation?

I am planning to build a peer & beam house and would like to have about 75% porcelain tile flooring. I have planned the 1 1/4" T&G sub-floor and a 1/2" concrete backer board, but my question is, how do I know if the floor joist are stiff enough to prevent tile movement? I have considered two: A conventional 2" x 10 or 12" floor joist 12" apart that spans 17ft to make a 34ft wide house. The other could be a 16" deep floor truss 17ft span and 16" to 24" apart. Would someone tell me "schluter-ditra" is? And could someone direct me in the right direction for advice. The soil has tested very hard and secure and the footings have been enlarged over the recommended engineered sizes. I just don't know about the tile floors.

Schluter DITRA:
http://schlutersystems.com

"Very hard and secure" are not terms used in the findings of soil tests.

Your builder should know about deflection minimums. L/360 is an absolute minimum and should be increased.