Tile Floor: Hardie or Plywood

[tminihan]

I am re-tiling my bathroom floor. The original floor was 5/8 subfloor, 1/2 plywood followed by tile. In researching this forum it seems like I need CBU or Hardiebacker in the new floor but I don't want to raise the height of the floor. Can I go back with the 1/2 plywood and then tile as before (no cracking problem noticed) or should I install the Hardie over the subfloor? I feel like the lack of structural stiffness I should go with the plywood. Thanks in advance. Tom

[jadnashua]

Most pros would not tile on 5/8" ply. The specs say it is okay (assuming the joist spacing is 16" OC) when new and properly installed...yours isn't. There is an industry procedure for tiling on two layers of plywood, but again, most pros don't like to do that.

Should you want to install your tile directly to two layers of plywood, the procedure calls for using a highly modified thinset.

The size of the room has nothing to do with whether the tile installation will be successful. You should check your joists for suitability by plugging in info in the 'Deflecto' tool in the blue bar at the top.

Plywood (subflooring) provides strength between the joists, the joists provide strength along them. You need both to have a successful installation.

[cx]

Welcome, Tom.

Originally Posted by Tom

The original floor was 5/8 subfloor...

Five-eighths-inch what?

[tminihan]

Sorry, appears to be 5/8" thick plywood, 16" OC. The home was built in the mid 70's.
Thanks, Tom

[tminihan]

So it sounds like the best I can do is 1/2" ply on top of the existing subfloor, 1/4" hardie on top of that.

[jadnashua]

Something like Ditra can go over the plywood second layer and it's only about 1/8" thick, if that's big enough to make a difference to you. It is easier to carry home, cut and install than cbu. The tile size needs to be at least 2x2", but that's not usually a problem.

You can tile over two layers of ply, but it is less forgiving to any mistakes.

[cx]

If that subfloor is T&G style, you could install a nominal half-inch exterior glue plywood second layer and tile directly to that. Presuming both layers were correctly, and differently, installed. As pointed out above, we (TYW) generally do not recommend that method, but it is a published method in the TCNA Handbook (F150).

But that makes a good subfloor and you could also install any tile substrate product you want over that. There are many such, ranging in thickness from less than 1/16th-inch to 1/2-inch.

My opinion; worth price charged.

[tminihan]

Thanks CX, the subfloor is T&G.

[Tiger Mountain Tile Inc]

I live in the PNW and I would estimate that 85% of the floors that I install tile over are wood-framed structures over a crawl space. We go over 5/8 inch subfloors with tile all of the time.

The floor framing has to meet industry standards and the subfloor needs to be in adequate condition- this is a judgment call.

But, assuming everything is OK, I would typically proceed with an uncoupling membrane and tile over that.

There's certainly nothing wrong with adding another layer of plywood underlayment. In fact, there may be a reason that yours had it in the first place.

Oftentimes though, people want the heights of their floors to match up and that's usually tough to do with an extra layer of plywood.

[jadnashua]

New construction with 5/8" is one thing...after tearing out layers above it, not necessarily as good.