tile over plywood

redpearl99 wrote: ↑ Im confused about the purpose of the plywood attached to the OSB subfloor.

Its to make your job easier right now! If you're stronk just rip it all out now, but likely you're just going to have to smashy smashy the tile, and after you've taken the bulk out, just rip up the plywood and toss it.

Seeing as ditra + tile is usually close to 3/4" thickness, I assume you will likely want to install your ditra ontop of the OSB to make for better transitions. Will you be doing the tiling yourself?

Yes I'll be doing the tiling myself.

I'd like to keep the plywood if I can but I get what you're saying about the added thickness

So ditra is absolutely necessary and the layer of plywood doesn't do the same job?

redpearl99 wrote: ↑ So ditra is absolutely necessary and the layer of plywood doesn't do the same job?

Is it absolutely necessary? No. Is it superior in every regard at a marginal cost, yes.

I'd go with the ditra. Saving the plywood is a pain most likely, and buying new plywood, and installing it might be close to same cost and effort.

Also, as you are DIYing it, clearly you should use ditra and put in some heated floors while you're at it

The plywood is over the OSB because thinset will have a more challenging time bonding to OSB. Here is a more technical article: https://www.custombuildingproducts.com/ ... to_osb.pdf

Ditra states it can work with OSB, however the problem is more with your thinset. You just need to make sure the thinset you use states it is fine with OSB.


One consideration is whether you are willing to move to a luxury vinyl tile floor. It will be warmer, easier to install, and with the more modern patterns can look quite nice. You can then lay it right over top of your OSB without issue, and never have to maintain the grout again. There are some higher end options ($7/sq ft) which can be very nice and are either floating or glue down.

Ditra and Ply do different jobs. The plywood is likely there to strengthen the floor and reduce flex (and thus cracking). The uncoupling membrane (Ditra) is there to reduce the chance of any floor movement cracking the tiles.

Ideally you want 3/4”+ sheathing (OSB or Ply) and then install ditra on top, then tiles on top of that assuming normal sized ceramic/porcelain tiles (I.e up to 12x24” - larger or using stone tiles thing change a bit). How thick is the OSB? If it’s less than 3/4” you may want to reinstall plywood again.

Schluter have a simple to follow guide you can use here: https://resources.schluter.com/media/ps ... ndbook.pdf

When I redid my master bath I had same setup of wire and thinset. When you stomp feet on floor it would shake the floor and vanity.

When I redid the tile I glued and screwed an additional layer of 3/4" ply then ditra then tile. Now when you stomp the floor it it rock solid. No flex or vibration. So I'd recommend doing the same.

I also did kerdi band on the ditra floor to wall seems and then tiled the walls 2/3 way up all around to keep it waterproof. Always feared a water spill from a toilet overflow or other mishap would seep in under the baseboards. No longer a concern.

Couple things to keep in mind.

I would recommend using the Schluter All Set (available at Home Depot and some Lowes) for the thinset. It works well both under the Ditra and over it for the tiles. It's a little bit more expensive than other thinsets but honestly you're talking about a bag maybe two for a bathroom floor. Plus it's made specifically to work with Ditra and Kerdi so you don't have to worry about it being good or not for it.

When putting down the Ditra make sure to clean the OSB then wet it well (no standing water) with a sponge. This helps the thinset not dry out prematurely (the wood sucks water out of it) so you get a better bond.

As mentioned you can seal the joints between Ditra sheets and the floor to wall transitions with Kerdi band to make the floor waterproof. You can use Kerdi fix around the toilet flange to help waterproof that as well (just not on top of it where the wax ring would go). You can also use the Kerdi fix to fill the joints between the tub and floor (and on the walls). This helps keep that area waterproof as that is usually the worst from what I've seen.

Thanks everyone. I am going with 12x24 porcelain tiles small bathroom of 60sqft total. With the advice, I'm going to keep the plywood on top of the osb. Will go with ditra onto of plywood and used kurdi waterproofing for seams