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KarenSB 08-01-2013 11:45 PM

How thick subfloor needed under tile?

I am renovating my bathroom in a century old home ~ nothing is plumb, level, etc. After putting in all new plumbing I am ready to put the floor down. Presently, there is 3/4" plywood screwed to the joists. Despite the fact that it is a small room (6' x 6'), the plywood is down in five pieces (long story). Additional 2' x 6' and 2' x 4' boards were added to the joists for extra support.

I want to put 3/4" lime stone tiles down as flooring and someone suggested that I need to put a full sheet of 1/4 or 1/2 inch plywood down first on top of the 3/4" plywood.

What is the total thickness that I need before putting down the tile? Or is the issue not thickness but rather uniformity of movement (therefore covering over the five pieces with one full piece). Of course, it still wouldn't fully cover the floor as sheets come 4' x 8'. Do I screw the top layer of plywood just to the joists or all over? Or is it better to use an electric staple gun?

Also, where I'm using limestone as opposed to regular tile, do I need any special mortar or grout and do they cut the same using a wet saw as regular tile do?

Thanks kindly for your input!

poppameth 08-02-2013 05:26 AM

I'd put 3/4 Advantech over it. Then Ditra Membrane (never tile directly to wood). Screw between joists. The top layer should attach to the subfloor, not the joists. Also put a layer of tar paper down before the underlayment to avoid squeaking. Mortar depends on how large of a tile but use a white thinset so it doesn't bleed through the stone. Limestone is soft and cuts easy but any stone can be easy to break compared to tile.

oh'mike 08-02-2013 05:34 AM

You need to understand the basics----natural stone requires a strong floor---

Floor strength starts with the floor joists. How big are yours? What is the unsupported length? What kind of wood?

As to the subfloor? If the joists are 16" apart---you will want to add 1/2" or thicker to the existing subfloor---

Pictures showing the joists would help----

Google 'deflection chart' --tiles don't bend---so the deflection must be within accepted range----

joecaption 08-02-2013 05:46 AM

Being a soft, porous tile Lime stone would not have been on my list of tiles to be used in a bathroom.

poppameth 08-02-2013 05:49 AM

Yes agreed. You need to measure your span between points the joists are setting on, how big the joists are, and spacing between them. I'd consider possibly pulling up all the existing pieces of subloor and putting a more solid layer down of 1/2 plywood and then another layer of 3/4" advantech. This will give you a very rigid floor, up to the TCNA spec for natural stone, as long and the deflection is also acceptable. The biggest concern you will have is a pretty good rise in the floor due to all these layers.

joecaption 08-02-2013 06:20 AM

Sounds backwards to me.
I'd be using the 3/4" T & G Advantech first then the subfloor rated 1/2".
A 1/2" subfloor would do nothing but act like shims on top of the joist. And offer almost no support plus it barely hold the needed fasteners.

KarenSB 08-02-2013 06:35 AM

Thank you for all of your input.... I think I've got the gist of the "collective" voice. It all makes sense to me ~ I need to know the strength and support is there first!

With regards to the comment on the porousness (is that a word?) of limestone.... it was also suggested to me that I seal each tile BEFORE I lay it as the mortar/grout would stain it. I would assume the sealing process would also help the issue of the fact that it is in a bathroom??

I googled "deflection chart" and I gotta be honest, it just kinda scared me to look at it. I'm just not that smart when it comes to geometry and charts to understand what I'm looking at. I'll have to take it slowly on that one but I LOVED it that you all went into such detail of EXPLANING your answers instead of just giving me the bare minimum.

Many thanks!


oh'mike 08-02-2013 03:29 PM

We have a secret one here----just give us the length---from support to support---the size (2x10--2x12 etc. ) the spacing and one of us will run the numbers for you---

Limestone and marble are easily damaged by acid and chemicals found in bathrooms---so be aware that your chosen product will need to be protected and might look like a mess in short order if you have any 'accidents' in the bathroom.

firehawkmph 08-02-2013 09:00 PM

there are plenty of porcelain tiles that look like limestone. Stronger, no real maintainence, more forgiving to install, and a lot less expensive. Visit a real tile shop, not the big box stores.
Mike Hawkins:)

KarenSB 08-02-2013 09:19 PM

Thanks Mike,

I went to pick up the limestone today and when I got there it was soooo different than my recollection. I ended up getting a really nice marble instead. According to the salesperson it is a lot easier to deal with and I dickered down to good price.

Do I need a special blade to cut with my wet saw? What about making the hole for the toilet. The last time I tiled a floor, luckily the hole for the toilet wasn't in the middle of one tile, it was a half moon in two tiles. So I cut it out by making thousands (I exaggerate) of cuts across the "moon" and then just broke them off. What do I use if the hole is in the middle of a tile (same question for the tub drain as well.)


oh'mike 08-03-2013 06:03 AM

No special blade needed for a small job like that----marble is soft and easy to cut---

Being soft, the diamond blade might need to be dressed once or twice---

This is simple--if the blade stops cutting you will need to expose more diamonds---so cut a brick or slice a hard tile to wear away more bronze---soft marble may not abrade the bronze edge fast enough ---

To make a toilet cut into the center of a tile---use a 4" angle grinder with a diamond blade-----In some cases I will set the tile into a shallow pan of water before cutting---

Make sure the tool is plugged into a ground fault outlet.

KarenSB 08-03-2013 06:38 AM

Awesome, that sounds very do-able! Thanks!

Okay, so I ran into a little hiccup.....

When the plumber did the demolition, he left the first floor board under the threshold and ripped out everything else. When I put down the 3/4" plywood and butted it up against the pre-existing floor board I noticed that they were not flush. The original floor board is 1/4" higher than the 3/4" plywood. I had a carpenter say I should take it out and start over. But when I took a closer look at it, the original floor board goes right underneath the studs and original plank flooring outside of the bathroom.

So I left it. Instead I installed a sheet of 1/4" plywood ontop of the 3/4" plywood ~ butting it up against the pre-existing floorboard. It's nice and tight, good fit, except the original floor board is still an eeny weeny smidge higher that the rest of the floor.

My thought is to carefully hand plane the floor board to feather it down to be flush with the 1/4" plywood. Then add another sheet of 1/4" plywood overtop of the whole thing ~ going in the opposite direction.

Am I headed for a disaster or does this sound okay.

Once I've put down the top 1/4" sheet of plywood, do I still need to lay membrane. Also, would you use deck screw in lew of ring nails. I tried to rent an electric staple gun today because my friend told me to staple down the plywood but the rental place didn't have the right staples and suggested the ring nails. Screws would make me much happier.... I'm much better with a drill than a hammer. =)

Cheers, Karen

oh'mike 08-03-2013 06:46 AM

1/4" ply does not belong in a tiled room---to thin and it will move---change that to 3/8" or 1/2" ---BC grade-- exterior glue---(exposure 1 )

Thin out the back of the ply where it laps over the existing board----

KarenSB 08-03-2013 07:01 AM

Are you saying then that I CAN use the 3/8" plywood as the final material before putting on the mortar ~ I don't have to use the really really expensive orange roll of plastic stuff?

oh'mike 08-03-2013 07:09 AM

No-----you still need to add a tile backer----The 'orange stuff' or 1/4" Durrock or wonder board----

I was just telling you that 1/4" ply is not a safe product to use under tile or stone---

If you use 1/4" Durrock---set that into a fresh bed of thinset and nail or screw it down----I use roofing nails---but screws work well also---

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