Cement backer board

[DaveE]

I know this is probably an old question for you guys. I am about to tile my bathroom floor, It's 5/8 plywood nailed to treated 2x6 joists that are sitting on a cement slab. I am planning to lay 1/2" Hardie Backer in thinset and screw it down. I opted for the 1/2" backer board for added rigidity. Is this the correct approach? I am going to use 12"x12" tiles.

[bbcamp]

The minimum CBU thickness for 5/8" plywood is 1/2", so that's OK for ceramic tiles. Stone tiles need a second layer of plywood.

Are the 2x6 joists fixed to the floor, or do they float? In either case, I recommend getting the dry treated wood, or kiln dried after treatment (KDAT) wood. Wet treated wood will shrink and warp real bad as it dries out. Not good for tile. Use screws to attach the plywood to the joists.

Do you plan on a vapor barrier under the sleepers (joists)?


Bob

[DaveE]

The 2x6's and plywood are already existing construction. The 2x6's have been in place for 10 years and are glued to the cement slab. Thanks for your input.

[cx]

The spacing between the floor joists is how much, Dave?

[DaveE]

The joists are 16" on center and glued to the slab with what looks like Liquid Nails or something similar.

[cx]

Like Bob said, sounds like your in good shape to add half inch CBU and tile, but not stone.

That's an interesting foundation structure, Dave. Are the 2x6 joists on edge, or laying flat? Not uncommon here to lay 2x4 on the flat over a slab to install hardwood flooring. We always put 4 or 6 mil poly over the sleepers in that sort of installation. What kind of flooring was over the plywood you have?

[John Bridge]

Hi Dave,

You've got my curiosity piqued, too. Are they laying flat against the slab?

[tileguytodd]

I Saw this once in an old Home in Duluth MN.The floor between joists however was filled with cement.The reason for this was it was a Below grade installation and they wanted to use Hardwood flooring.If you are putting down Ceramic tile, there is no need to have a buildup of 2x6" Joists and Plywood. Revove the bulk of the joists with a sledgehammer,Put down a layer of 15" Roofing felt and fill to the height required with cement.It will cost less and be the best possible substrate for the tile.
If you are concerned you cant get it level enough, Fill to within 3/8" and finish with a Self leveling cement for a Perfect floor.
Good Luck!!

[DaveE]

This was originally an outside patio slab. It's 6 inches thick and slopes away from the house. The previous owner & original builder enlcosed the patio into a large 16' x 35' family room. To get the floor level with the rest of the house, he cut treated 2x6's to match the slope, glued them on edge to the slab and sheated it with 5/8" plywood, no vapor barrier. The floor is actually very solid, no squeaks or noticeable deflection, and it's surprisinly level! The original floor covering was carpet. Moisture has never been a problem since this is a very dry area of Southern California. One wall of the room butted up to the master bedroom and I have remodeled a small portion (5'x9') of this room into the new master bath. Hopefully this gives you some more info. Let me know if I can provide anymore details.

[John Bridge]

Okay, I wouldn't worry about joist deflection, but how far apart are they?

[cx]

He already done said 16 inches, JB, try to pay a little more attention here, OK?


Your structure is good to go for tile, Dave, you just need to add a suitable substrate as discussed earlier.

[John Bridge]

Sorry.

[tileguytodd]

I sure nuf love new information!!
Like CX Said,go for it.Now get to work

[DaveE]

Thanks guys, now if we can just settle on a tile,I've got an itchy trowel finger. By the way my building inspector also thought it was an "intersting" floor structure. I think it would have been cheaper, faster and better if he had just poured a new slab even with the rest of the house.

Thanks again for the advice.

[JohnW]

I've used both CBU and hardibacker. I used hardibacker when I did my first tile job in the upstairs bath. When I did the kitchen counter tops I used regular CBU.

The only problem I've had with hardibacker is that the screws won't always drive flush. Most of the time I have to back the screw out and redrive. With the CBU they drive flush almost all the time.

Question, for us DIYs does one have any advantage over the other.