Question on proper subfloor prep

[cruzmisl]

Hi All,
I'm planning on laying down some porcelain tiles in my living room but concerned about having the appropriate subfloor.

Currently I have 5/8' plywood glued and screwed to 2x10's which are 16" on center and span 10'. I have another tile floor that the new install is going to butt up against so I need to plan this properly so I have enough space to level the floor as I go and ensure I don't end up too high

The tile I have measures 7/16". I'm also installing in floor heating which will be secured to the plywood and then completely submerged in self leveling concrete.

Does the self leveling concrete contribute to the floor strength? Meaning, do I still need to add 1/4 or 3/8" cement board (or similar) and then add the concrete or is just the concrete adequate? My gut says the concrete will add the stability required for a porcelain install but I want to be sure.

Any other tips?

Thanks!
J.

[cx]

Welcome, Joe.

I would personally not even consider installing ceramic tile over a single layer of nominal 5/8ths" plywood even over 16" oc joist spacing.

Your SLC manufacturer or/and your heating system manufacturer on the other hand, may tell you it's fine. And if you wanna do that, that's fine.

The SLC adds nothing at all in the way of structural help for your subfloor. A CBU adds nothing at all in the way of structural help for your subfloor. If you want a better subfloor for your tile installation, and I would, you must add more subflooring.

My opinion; worth price charged.

[jadnashua]

if the 5/8" ply isn't T&G or the edge seams aren't reinforced by blocking, I wouldn't tile it without adding a second layer of ply, and I'd want one, regardless. Now, if it was originally 3/4", maybe go with one. Most slc manufacturers want lath on the floor and some thickness above the highest point (often 1/2" or so). They might take a heating mat as suitable substitute for lath, but probably not. You can tile directly on the slc, but you may want to consider something like Ditra to give you a little more flexibility with the heating expansion/contraction from the added heat. It's not required, but useful.

[cruzmisl]

Meh! In my heart I knew the answer but didn't "want" to know the answer

I was planning on using the Suntouch warm wire system and their literature suggests a minimum 1/4" to 3/8" rise. I was hoping the 5/8' ply (which is T&G), the SLC , ditra then tile but by the sounds of it I may be pressing my luck with stability. The deflection calculator gives me a thumbs up but calculators and real world experience are surely different. If I added even 1/4' ply it would raise my new floor beyond what exists and I don't want that. Even with what I was planning on using it was going to be close.

Unless anyone else has any suggestions it seems the heated floor is out.

Thanks a ton for the advice!

Joe

[cx]

Joe, our Deflectometer is a pretty conservative tool. If you've entered accurate information into it and gotten a thumbs up, chances are very good that you have no problem there.

Don't know what kind of floor covering you have adjacent to your project, but making reducers/transitions from higher tile to other floor coverings is frequently a pretty easy task.

My opinion; worth price charged.

[cruzmisl]

Hi All,

Starting installation of Florida Tile Berkshire porcelain. I currently have glued and screwed 5/8" T&G plywood now and will be adding 1/4" fir and ditra. That's the thickest I can go without going above the tile already installed.

What's the best way to secure the 1/4" ply? I was thinking of troweling a latex adhesive with screws every 6 inches. Speaking of screws how long should they be? 1"? Is the adhesive necessary? I just want it as solid as possible since it's the bare minimum as it is.

Thanks for any tips.

Joe

[cx]

Joe, it'll help if you'll keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered. We can give it a more generic title any time you'd like to suggest one.

The 1/4" plywood is not a suitable addition to your existing subflooring. You can do it if you like, but don't expect to get it installed sufficiently flat and solid for your intended purpose.

The minimum thickness in the common exterior glue plywood that is suitable for a structural application is nominal 3/8ths" and I would recommend you use nothing less than nominal 1/2" material.

My opinion; worth price charged.

[cruzmisl]

Ok. I didn't know if I should start a new thread.

So are you saying 1/2" on top of the existing 5/8" or 1/2" total?

The neighbouring tile is 3/4" high so I'd like to keep it flush with that.

[Brad Denny]

Joe,
They are saying 1/2" over the 5/8". The decision to weigh out is...

1. Roll the dice with what you have
2. Place 1/2" over what you have and be higher (substantially) than the adjacent flooring
3. Remove the old subflooring and go with 3/4" T&G (still higher than adjacent, but saving 3/8" over the double layer)

[cruzmisl]

I think I'll take my chances. I'm not pulling up the existing sub and the wife won't tolerate a half inch disparity between the two floors. Should I not even bother with the 1/4" ply? The new tile is so thick and with the ditra just not enough room. Will the ditra help with any potential cracking?

[Yooper Jim]

Hi Joe,

I'm new here but here is my $.02. If going up in height is not an option what is the access like below? There are many ways to stiffen up a floor from below. Depending on space available you could.

1. Add more joists cutting the span of subfloor in half
2. Sister up existing joists to stiffen everything
3. Add blocking between joists at regular intervals
4. Add beam under center of span of joists (depending on head room)

Any of these would help stiffen things up and help give you a better piece of mind.

And yes, definitely use ditra!

[cx]

Up to you, Joe. The subflooring you have meets the minimum requirements of the manufacturer's of any number of substrate products, including Ditra.

I think you're more likely to introduce problems with the added quarter-inch plywood than to solve any, but that, too, is entirely up to you.

My opinion; worth price charged.

[Brad Denny]

Jim, the big issue here is stiffening between the joists. You can have monster strong joists, but if your ply is weak, tile will fail. You are right though, joist strength is important.

[Yooper Jim]

Thanks Brad but here is my reasoning.

1. Add more joists cutting the span of subfloor in half
If you doubled the amount of floor joists your span for the 1/2 in ply would go from 14 1/2" down to 6 1/2" greatly decreasing the chance for deflection.


2. Sister up existing joists to stiffen everything
This would help stiffen up the joists and reduce the span of the subfloor to 13" which is over 7% reduction of the span.

3. Add blocking between joists at regular intervals
This would solely stiffen up the 1/2 ply

4. Add beam under center of span of joists (depending on head room)
Stiffening up the whole floor system would help with reducing deflection in the whole system. I added this one because I don't know what is underneath the floor and if it is finished this would help some.

[cruzmisl]

Up to you, Joe. The subflooring you have meets the minimum requirements of the manufacturer's of any number of substrate products, including Ditra.

I think you're more likely to introduce problems with the added quarter-inch plywood than to solve any, but that, too, is entirely up to you.

I don't need any more problems, I have enough of those

I should also point out I'm not doing the in floor heat.

Basement is finished drywall ceiling so going in from the bottom is out too.

The Florida Tile is 11mm thick which is about 7/16" The ditra adds another 1/8" which, not including thinset leaves 3/16" residual to meet my max 3/4". I'm thinking even if I wanted to I couldn't add the plywood

I guess I'll see if the ditra works as well as they claim. I'd hate to waste $3k in tile....anyone care to wager on the potential for issues?