Sub flooring

My house is 30 years old. The sub floor is 1/2 plywood and 1/2 particalboard. I have removed the partical board and I am left with 1/2 plywood. The kitchen is 21 ft. X 13 ft. To get the tile to match up with the hard wood flooring I will need 3/4 inches more
Than what I have now, 1/2 inch plywood. What would be best adding a nother layer of 1/2 inch plywood and 1/4 inch backer board or 1/2 backer board and 1/4 inch
Backer board. The floor joyces are 16 inch on center.
Also I plan on using thinset between the two layers
Of backer board befor installing the tile. All thoughts and experences with this problem welcome.
Thanks Doug 0

Hi Doug,

First of all your new tile does not care about the height of the adjoining room. We do not sacrifice quality just so the rooms match.

Chances are your 1/2" subfloor is not in perfect condition. Homes haven't been built that way in a long time when it was learned two 1/2" sheets is not so good.

You need to add as much plywood as possible to strengthen the floor. You might get by with 1/2" B/C underlayment grade, but I'd like to have you install 5/8" instead. Then install 1/4" cement backer or a membrane such as Ditra. (adds only 1/8").

Remember cement backers do NOT add strength to the floor.

Tell me more about the framing. Type and size of joists, we know spacing is 16" oc, do you know the species and grade, and need the unsupported span of the joists. Measure to the inch from face to face of supports. What kind or tiles and size.

Jaz

Thanks Jaz
Just measured, joists are 16" on center, the size of the joists is 9 1/4" x 1 3/4 ", today it's called a 2 x 10, not. The joists are pine and look to be a #1 garde, no knots. Face to face measures 15 ", the joists are not perfectly spaced some are 14 1/2 " face to face. The unsupported span is 12 " 4 "
The sub floor and the joists are in very good shape. There is one peace with water damage from the sink, I will replace it befor installing the next layer.
I can go with 5/8 or 3/4 and then 1/4 backer board. When I install the new sub floor should I screw it or nail it. Shoud I hit the joists or try to miss.
I'm using a 13" x 13" porcelin glase tile. Doug 0

OK, although saying the joists are pine, and grade #1 doesn't tell me much, I'm confident your joists meet L360 max deflection code for ceramic tiles.

Install the 3/4" subfloor grade ply if you can. Install it in the normal manner as if the 1/2" wasn't there. Normally we would say to avoid hitting the joists. But in your case because of the wimpy 1/2" ply, I recommend fastening to the joists, then placing some 1 1/4" coarse thread screws between the joists.

Then install your favorite 1/4" backer into fresh thin set and fasten/tape etc. Or, go premium with Ditra.

Jaz

thanks Jaz.
I will follow your sub floor recamindations.

Doug 0