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kbuchholtz · Feb 28, 2009, 02:39 PM

I'm putting in a new porcelain floor in a 1/2 bath. Tiles will either be 13x13 or 6.5x6.5 (waiting for my wife to decide).

What kind of subfloor should I put underneath the tile? Can I use a plywood floor, or do I need something stronger? Just a toilet and sink in this small space. There is plywood down now. It's pretty thin, maybe 1/4". I'm afraid of going thicker than that and having the floor be higher than the toilet.

Any suggestions?

JazMan · Feb 28, 2009, 02:59 PM

Hi,

You already have the subfloor, the subfloor is the thick sheet of ply that the walls are built onto. That 1/4" ply is an underlayment which is part of a vinyl installation, but needs to be removed for ceramic. After some prep of the subfloor, you should then install cement backer board and then the tiles.

Before you do any of that though, you should evaluate the joists system to make sure they are stiff enough for tile. I need the type,size,spacing and unsupported span of the joists. Would be nice to know the species and grade too. What type, thickness and number of plywood or OSB sheets? How old, what condition, what is/was there, how it feels etc.

Jaz

ballengerb1 · Mar 1, 2009, 12:30 PM

I pretty much agree with Jazman. Sounds like you aren't really sure of what you have and you can't just guess about flooring. I doubt that you have only a 1/4' of ply right now but you need to actually see what is there. I install 1/4" Hardibacker board , screwed and glued, over at least 3/4" ply. I suspect Jazman has another product in mind which also works quite well.

kbuchholtz · Mar 1, 2009, 07:23 PM

Here are the layers of floor in the bathroom right now, from top to bottom (what I can see):

Tile, then 1/4" plywood, then boards that are nailed down to the joists 5 inches wide, look like they could be tongue/groove.

I think the plywood is the subfloor and underlayment, and these are sitting on top of the flooring nailed to the joists.

Any suggestions how to proceed. I'm planning on ripping up the tile and plywood. What do I need to install? My concern is the floor coming up way higher than the toilet flange

ballengerb1 · Mar 1, 2009, 08:16 PM

That 5" wide board, how thick is it? I think you are calling a floor board a joist. A normal floor joist is made from 2x10 standing on its edge.

JazMan · Mar 1, 2009, 08:17 PM

You should be worrying about how to install the tiles correctly, not the toilet so much.

The planks make your subfloor. The 1/4" is probably luaun and is an underlayment.

Once the tile and plywood is gone, you prep the planks, and replace any bad ones.

Then install minimum 3/8", but I recommend 1/2" underlayment grade plywood, then either 1/4" backer or a product like Ditra or Noble CIS. Then the tiles.

You didn't report about the joists I asked about before.

Jaz

kbuchholtz · Mar 2, 2009, 06:18 PM

Not sure on the species of wood, nor how thick the 5" wide board is. The 5" board is nailed to the joists, which are the 2x10s turned on their side.

The bathroom is 4'x4', so the space is very small. The joists are 16" on center, and there are 3 in the room. There is about 8" of space between the last joist and the wall.

So I should pull up tile and luan, repair the subfloor, lay down plywood underlayment, backer board, and then tile? Is that correct?

Since the room is so small can I get away without one of the layers of flooring (underlayment or backer board)? Just curious. The tiles are 6.5"x6.5".

Thanks for all your help and suggestions!

JazMan · Mar 2, 2009, 07:41 PM

The size of the room has nothing to do with what I was asking. I asked for the unsupported span of the joists. And... you say the 2x10 joists are on their side? What?:eek:

You cannot put backer board on planks, you need plywood first. You could install tile on plywood if it was a dry area and you didn't care to do a good job. You could substitute Ditra or Noble CIS for the backer board and save about an 1/8".

Otherwise... So I should pull up tile and luan, repair the subfloor, lay down plywood underlayment, backer board, and then tile? Is that correct?

Jaz

kbuchholtz · Mar 3, 2009, 02:42 PM

I meant to say the joists are on their end, not on the wide side. My mistake.

I'm not understanding what you mean about unsupported span of the joists, Jazman?

ballengerb1 · Mar 3, 2009, 02:56 PM

I hink you answered that in post #7, 16" on center. It is siuch a small area this is what I would do. remove all the tongue and groove flooring and lay down a single piece of 3/4" plywood screwed and glued to the 2x10. Now glue and screw down 1/4" Hardibacker board which can be purchased in a 3x5 sheet. If your room is truly 4x4 you'll be a tad short and need a second sheet. Now lay your tile using the same modified thinset used to glue down the Hardiebacker board.

JazMan · Mar 3, 2009, 03:17 PM

NO, the 16" o.c. is the spacing, not the joists' span. Measure the distance from the two supports that the joists are resting on, that is the span. Chances are you'll be OK for ceramic or porcelain tile, but that is the first thing to look at to determine if the floor meets standards for stiffness.

You could remove the planks as ballenger suggested, but I do not recommend it. Lots of extra work. Remember, the walls are resting on the planks, and you'd also have to install blocking and maybe add a joist too. I say add 1/2" ply and then the 1/4" CBU.

Jaz