Bathroom Subfloor Prep for Tile

[1990C4S]

I have gutted my bathroom, the subfloor is 5/8" construction plywood on 16" centers. Subfloor is in good shape, but I can fell a bit of flex in the floor between the joists even after screwing it down.

The plan is to add Schluter uncoupling membrane, then 8"x8" tile.

Technically, the Schluter membrane says 5/8" plywood on 16" centers, so I do meet the spec. It also looks like OSB is approved, so my plywood has to be better than than the minimum spec (5/8" OSB 16" OC).

Should I just go ahead and add the Schluter (this is my inclination)? Maybe add some 5 mm cedar plywood underlay (glued and screwed)? Pull the floor and glue/screw 3/4" plywood?

[908/930]

What material are the tiles made out of? Natural stone won't take much to crack but Porcelain tiles are usually able to take some flex. That membrane works pretty well just make sure you use the correct thin set to bond it down, as they spec. Try placing a tile on the floor and standing on it to see if it will break? I guess you can not access the area under the floor?

[1990C4S]

I'm putting down porcelain tile.

I'm being OCD about the mortar. I will be done right.

If I go far enough to access the floor and stiffen things up, I think I might as well just replace the plywood (or add more).

I honestly think I will be fine with what I have. The flex will probably disappear completely when the membrane and tile are done.

[908/930]

Hopefully others will arrive soon. I'm pretty good with overkill so would likely make it stiffer. How hard would it be to lift the sheet and build up between the joist with some 5/8" ply glue & screw , at least in high traffic areas?

[look 171]

Block off the joist and that will add in a fair amount of support and it may even eliminate bounce. If you already screwed down the plywood, just set tile on top. It should be OK. If not, go down there pour a 14x14" 18" deep concrete pad ( or just a pier from home depot) and a girder. A 4x4 is plenty if it is a small bath. It will completely remove any bounce

[Black968]

I have done a fair amount of tiling in my day. The standard use to be 1 1/4" of plywood without the membrane. For a bathroom (small space) I would lay extra 5/8" plywood down and you are good to go. I am leary about the membrane, I have seen popped tiles on OSB. Not sure why OSB is approved for flooring or roofs, it sucks.

[porsche930dude]

Iv never dealt with the rolled stuff so i dont know how much stifness it would add. I would lay hardibacker it would stiffen it up for sure.

[look 171]

We spec out Hydroban with Laticrete 255 MultiMax thinset. Its match made in heaven ( from same co.). No leaks and we usually use a few layers over and under a waterproof fabric just for added insurance. Their tech seems to think the fabric isn't necessary at all, but I like to see it in there for some added insurance for leaks and strength. That or we float our showers the old fashion way still

[VINMAN]

The bathroom I'm doing for a customer now, porcelain tile on the floor I'm using 1/2 Hardie over 5/8 ply floor. Ditra between Hardie and tile.

.The Ditra will not eliminate any flex in the floor. You still risk cracking the tile or the grout joints

.

[1990C4S]

Originally Posted by look 171

Block off the joist and that will add in a fair amount of support and it may even eliminate bounce.

I think this is the next step. There's a 4x4 sheet in the high traffic area I can pull up, block, then glue and screw the subfloor.

I think the blocking will solve the problem with minimal cost and effort.

[gregpark]

You're already in spec with the ⅝, just go for it. Make sure you use plenty of thin set on both layers. Every layer of the process stiffens the floor and disperses weight, including the actual tile. If it still seems like it needs it after installation, you can hang some blocking in from below. But I'll bet you won't need to

[1990C4S]

^^^ Thank you Greg. You have never steered me wrong before; I appreciate your input.

[jyl]

Curiosity - is there such a thing as flexible grout?

[gregpark]

Yes, it's called epoxy grout. Doesn't even require sealing but doesn't hurt to smear sealer if you have a can laying around

[Zeke]

Originally Posted by look 171

We spec out Hydroban with Laticrete 255 MultiMax thinset. Its match made in heaven ( from same co.). No leaks and we usually use a few layers over and under a waterproof fabric just for added insurance. Their tech seems to think the fabric isn't necessary at all, but I like to see it in there for some added insurance for leaks and strength. That or we float our showers the old fashion way still

He didn't mention a tiled shower so a leak proof floor is overkill.

Why do people ignore 1/4" backer?

[KFC911]

^^^^ I've always used 1/4" Hardie in 3'x5' (easy to handle & work with) secured with good thinset (Mapei) and screwed down ... lots of screws per Hardie ... about eleventy dozen... at least... it ain't moving .

I always enjoy reading Greg's (and other pros here) posts .... if I ever have to redo a tile or hardwood floor, I'll use what I learn here ....

Thankfully I won't need to...

Hardie

[FA-18C]

They used this system in our upstairs hall batch remodel a couple of months ago - also the entire barrier system in the shower. Really impressed. Stable, quiet - we are really happy.

[look 171]

Originally Posted by Zeke

He didn't mention a tiled shower so a leak proof floor is overkill.

Why do people ignore 1/4" backer?

I think most people associate quality with heft, something thicker or price? We normally use 1" TnG plywood screw and glue to the joist to eliminate bounce (always blocking for bath) and apply 1/4 Hardi-board on top for the tile to stick to. It normally works out perfect with the adjacent floor height. Due to various height of the adjacent floors, we sometimes glue Ditra directly to 1" TnG plywood and tile on top of that for bath floor to try and match the height of other flooring.

When we get into the baths, the 1x6 sub floor always goes and get replaced with plywood. I don't like sitting cast iron tub on top of the 1x6.

[look 171]

Originally Posted by VINMAN

The bathroom I'm doing for a customer now, porcelain tile on the floor I'm using 1/2 Hardie over 5/8 ply floor. Ditra between Hardie and tile.

.The Ditra will not eliminate any flex in the floor. You still risk cracking the tile or the grout joints

.

Ditra or similar material is a great crack isolater. We no longer set tile directly on top of slab floors but always use a crack isolation membrane or roll Hydroban over it. If not, cheap Red Guard will achieve the same result. Its cheap insurance from movement coming thought the slab causing crack tiles.

[VINMAN]

Originally Posted by look 171

Ditra or similar material is a great crack isolater. We no longer set tile directly on top of slab floors but always use a crack isolation membrane or roll Hydroban over it. If not, cheap Red Guard will achieve the same result. Its cheap insurance from movement coming thought the slab causing crack tiles.

I've been using Ditra on almost all my floor tile jobs, since it came out. Excellent product.

I use 1/4 board on most jobs when I use it, unless I need something thicker to make up some height, like on the current job I'm doing.

.