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frazwood · Nov 15, 2009, 06:57 PM

My wife and I have decided to replace the vinyl floor in our bathroom and we'd like to replace it with tile (ceramic, porcelain, or natural stone... we haven't picked it out yet).

My question is: what should be underneath a tile floor?

I am in the process of pulling out the vinyl and its underlayment. This leaves the plywood-subfloor that's nailed to the joists.

My specific question:
(a) Do I simply put down 1/4" Hardibacker cement board onto the existing plywood subfloor (it doesn't look like it's the highest grade of plywood) and then place the tile on that?

or

(b) Do I tear out the existing plywood subfloor and replace it with 3/4" AC-grade plywood and then place down the Hardibacker and then the tile?

I've heard both but I've heard that (b) was better so I was planning on doing that. But, now, I'm not so sure, so I thought that I'd ask for opinions.

Thanks for your help,
Tim

ballengerb1 · Nov 15, 2009, 08:08 PM

I'd go with B since you do not know the thickness or quality of the old sub. New 3/4" T&G ply glued and screwed then Hardiebacker also glued and screwed. I use the same modofied thinset under the backer as I use for the ceramic. How big is the room?

frazwood · Nov 16, 2009, 09:15 AM

Originally Posted by ballengerb1

I'd go with B since you do not know the thickness or quality of the old sub. New 3/4" T&G ply glued and screwed then Hardiebacker also glued and screwed. I use the same modofied thinset under the backer as I use for the ceramic. How big is the room?

I think that I am getting more confused than before. I apologize for the lack of the details in the original post -- I'm learning as I go.

I think that the existing floor is 3/4" plywood T&G like you describe -- the grade is OSB. I can verify the thickness of the floor by drilling a small hole through it in an area that will not have tile (for example, under a location where a cabinet will sit).

I am pretty sure that the existing subfloor is in very good condition -- the house is only 15 years old. So, my biggest worry is that I'll remove a perfectly good subfloor that would actually be better than the one that I re-install.

My next question is: does the grade of plywood matter? I thought that I had heard that AC-grade was necessary, but now I'm not sure of that (I can't figure out where I read that). I know that particle board is a bad idea for tile; I'm sure that it's not particle board.

The bathroom is perhaps 12' x 12', not including the areas occupied by shower/tub/cabinets (that is, the area that will have tile will be 12x12).

Thank you for you help and your patience.

ballengerb1 · Nov 16, 2009, 09:32 AM

OSB is not a grade of plywood, it isn't even plywood at all but oriented strand board. We really must know exactly what you have to give you good advice. I copied this from Hardwoodinstaller.com for your information.

"Approved wood subfloors are listed in order; best to least preferable. This is based purely on the materials nail holding capability.

• Best: 1" x 6" solid #2 or better Douglas fir or frame grade pine boards installed diagonally across floor joists 16" on center(OC)
• Next: 3/4" T&G Sturdi Floor plywood subflooring installed at 90 degrees to joists 16" OC.
• Next: 3/4" OSB (Orientated Strand Board) T&G subflooring installed 90 degrees to joists 16" OC.
• Next: 5/8" T&G plywood subfloor installed at 90 degrees to joists 16" OC.

"

mtconc · Nov 16, 2009, 11:59 AM

How old is the house?. if it is newer is was inspected to be 3/4" plywood..
Keep in mind you will need 1 1/4 inches from the floor up from the joist

frazwood · Nov 16, 2009, 12:37 PM

Originally Posted by ballengerb1

OSB is not a grade of plywood, it isn't even plywood at all but oriented strand board. We really must know exactly what you have to give you good advice. I copied this from Hardwoodinstaller.com for your information.

"Approved wood subfloors are listed in order; best to least preferable. This is based purely on the materials nail holding capability.

• Best: 1" x 6" solid #2 or better Douglas fir or frame grade pine boards installed diagonally across floor joists 16" on center(OC)
• Next: 3/4" T&G Sturdi Floor plywood subflooring installed at 90 degrees to joists 16" OC.
• Next: 3/4" OSB (Orientated Strand Board) T&G subflooring installed 90 degrees to joists 16" OC.
• Next: 5/8" T&G plywood subfloor installed at 90 degrees to joists 16" OC.

"

My guess (I'm not at home) is that I have the third one on the list.

One thing that I am 100% sure about... the plywood that I purchased in not T&G, so I need to return it.

So, if my subfloor is #3 on the list, do you recommend removing it?

Thanks!

frazwood · Nov 16, 2009, 12:37 PM

Originally Posted by mtconc

how old is the house?..if it is newer is was inspected to be 3/4" plywood..
keep in mind you will need 1 1/4 inches from the floor up from the joist

The home was built in 1995, so I am pretty sure that it was 3/4" plywood.

mtconc · Nov 16, 2009, 01:32 PM

Why can't you use 1/2" cement board adhered to the substrate that is already there? If it in a bathroom cement board is always a good choice

frazwood · Nov 16, 2009, 05:48 PM

Originally Posted by frazwood

My guess (I'm not at home) is that I have the third one on the list.

One thing that I am 100% sure about... the plywood that I purchased in not T&G, so I need to return it.

So, if my subfloor is #3 on the list, do you recommend removing it?

Thanks!

Now that I am at home... I just looked in the basement where I can see underneath the subfloor (I'm assuming that the subfloor is the same throughout the house).

It says that it's 23/32 inch rated "sturd-i-floor" 24 OC. It's also labeled "Nordbord" sold by the Northwood Panel Board Company in Solway, Minnesota

So, on your list above... I'm option#2 for the subfloor. Thanks for all of the input. I think that I am starting to figure this out.

frazwood · Nov 16, 2009, 05:57 PM

Originally Posted by mtconc

why can't you use 1/2" cement board adhered to the substrate that is already there? if it in a bathroom cement board is always a good choice

I don't know where, but I had heard to use not 1/2" cement board on floors -- basically, it was invented for situations where you apply tile to walls (like tub/shower surrounds).

Admittedly, I don't know... if I did... I wouldn't be asking here.

frazwood · Nov 16, 2009, 06:20 PM

A final comment... Here is what I am thinking about doing...

mtconc mentioned that I need at least 1 1/4" below the tile. So, with that in mind and given the fact that I already have 3/4" plywood... I am going to glue and screw the 3/4" plywood onto the existing subfloor and then place 1/4" cementboard on top of that (1 3/4" total). It'll be a little high, but we can deal with the height difference.

Please let me know if you think this is a bad idea...

ballengerb1 · Nov 16, 2009, 08:35 PM

The thickness of cement board varies, not for strength, but mainly so walls and floors can be made to match up with other surfaces better. Is that sub floor mounted 24 OC or 16 OC?

frazwood · Nov 17, 2009, 05:53 AM

Originally Posted by ballengerb1

The thickness of cement board varies, not for strength, but mainly so walls and floors can be made to match up with other surfaces better. Is that sub floor mounted 24 OC or 16 OC?

It's mounted 16 OC (assuming that the second level is the same as the first).

mtconc · Nov 17, 2009, 08:33 AM

In your reply you stated that it says"sturd-i-floor 24" oc. I reason that the floor joists are, in fact, 16 oc and that is just what is stamped on the panel,correct?

ballengerb1 · Nov 17, 2009, 09:03 AM

Odd that this sub is stamped with names from 2 different companies who both make OSB. Sturd-i-floor is made by Georgia Pacific. In any case they both are suitable substrates for your app. If in good condition you can now glue and screw down your Hardieboard per James Hardie's instructions. Use the same modified thinset to glue the Hardie down that you use for your tile.

frazwood · Nov 17, 2009, 09:41 AM

Originally Posted by mtconc

in your reply you stated that it says"sturd-i-floor 24" oc. i reason that the floor joists are, in fact, 16 oc and that is just what is stamped on the panel,correct?

Yes, that's correct... that is what is stamped on the plywood. The joists are 16" OC.

frazwood · Nov 17, 2009, 09:43 AM

Originally Posted by ballengerb1

Odd that this sub is stamped with names from 2 different companies who both make OSB. sturd-i-floor is made by Georgia Pacific. In any case they both are suitable substrates for your app. If in good condition you can now glue and screw down your Hardieboard per James Hardie's instructions. Use the same modified thinset to glue the Hardie down that you use for your tile.

Thank you for your help! I would rate your answer, but it tells that I've rated too many of your answers recently to rate you again.

ballengerb1 · Nov 17, 2009, 09:44 AM

OSB, not plywood. You should be good to go if its in good shape. You mustrate at least 5 other folks before you can come back to rate me again. Thanks

mtconc · Nov 17, 2009, 10:30 AM

Hardibacker is good.. glued and screwed.. I agree with ballengerb1.. you should be good to go if the osb is in good shape.. good luck!

Bljack · Nov 17, 2009, 11:11 AM

Hello, I just want to butt in for a quick second to not only provide you with some additional info, but also because I firmly believe that for each person who posts a question, there is probably 7 to 10 who read to research but never post a question of their own.

Your floor, as it is described by you, meets the minimum standard for a ceramic installation but not stone. Stone needs a floor that is twice as rigid as that which is required for ceramic and unless the floor is designed at the time of construction for a stone surface, it is not structurally rigid enough for a stone installation. It is not because of the weight of the stone but because of the natural fissures and fault lines within the stone that create weak spots along which it will easily crack. You would need to add an aditional layer of at least 1/2" bc ply prior to installing your cement board. This second layer is never to be glued, only screwed. It is also to run 90 degrees to the joists, even if the subfloor panels run 90 degrees to the joists. The strength axis of the ply is along the face grain.

Your joist structure would also need to be twice as rigid for stone. That's not as easy to accomodate on an upper level of the home

Whatever was listed on a hardwood floro site for nail holding power may apply for hardwood, but you cannot install a cement board over a dimensional lumber subfloor. To clarify for those who may read this thread doing research, you would need to install a layer of 1/2" because plywood over the t-g plank subfloor first then you could install your cement board or membrane.

It is good that you did not follow the advice to take out the original subfloor and replace with AC plywood. If you had, you would have needed to overlay the new subfloor with an additional layer of because or better 3/8" plywood as the minimum of properly installed glued and nailed 3/4" subfloor requires that it be t-g plywood. Replacing it would have significantly reduced the rigidity of your floor.

Good luck with your project and come back with any other questions. If you have not used Hardi before, it is a very thirsty cement board. Make sure you wet it with a sponge before combing any thinset onto it. Tape your joints as you set the tile so you don't create any ridges to try to ride over. Use a good quality unmodified underneath. If you have a Lowe's near you, Megabond with just water, or from Home Depot, Multiset with just water are great choices for the bedding layer. Over top, any modifed thinset, versabond from HD or "Multipurpose" from Lowe's will be fine.