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

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'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.

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.

"

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

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!

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.

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

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.

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.

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...

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?

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).

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?

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.

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.

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.

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

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!

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.

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" bc 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 bc 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 Lowes 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 Lowes will be fine.

Thanks for butting in, if that's what you want to call it. I'll take any and all advice possible -- I'm perfectly capable of making a final decision after collecting the information. :p

As of this moment, we're going to put down 3/4" AC plywood on top of the subfloor and then the 1/4" cement board on top of that. There is some debate as to whether to use 1/2" plywood, but we're probably not going to do that because: (1) my wife actually likes the higher floor because she thinks it'll make the cutting of the transition tile easier (FYI: she does the tile work; I do everything underneath), (2) we like the idea of a stronger, better floor, and (3) we got the 3/4" AC plywood at a ridiculously good price (i.e. better than 1/2" bc plywood).

We actually have a decent amount of experience laying tile -- enough to know that it's absolutely critical to have strong, non-flexing floor underneath. In our last house, though, the subfloor was conceptually simpler for us because it had 1" x 6" boards installed diagonally across floor joists 16" on center(OC) (and we could obviously see it).

Also, that's a very good point about natural stone having weakness/fissures in it. I don't think that we're going to be using natural stone tile, but thanks for the heads up (my wife might want to try natural stone... so that's good to know).

Bljack.. if I understand correctly you are implying that a stone floor would require a
Minimum substrate of 2 1/2" from the joist up?

bljack..if i understand correctly you are implying that a stone floor would require a
minimum substrate of 2 1/2" from the joist up?

No, a layer of 3/4 t-g Properly installed and a layer of 1/2" or 5/8" properly fastened would be sufficient for the subfloor and underlayment layers. It's the joist structure that would need the strengthening, either by supporting it from below with posts and a beam to effectively reduce the span of the joists or by sistering them to effectively double the strength.

For ceramic, the minimum deflection criteria is L/360, or for every inch of unsupported span, when a load (300lbs for a ceramic assembly) is placed at mid span, the joists cannot deflect more than one 360th of an inch for every inch of span. This would mean that for a 10 foot span, the joist cannot deflect in the middle more than 1/3". That might seem like a lot, but you wouldn't feel it. More often, the killer is deflection between the joists. That too, must be at L/360, or in numeric terms, the subfloor panels cannot sag between the joists more than 4-one-hundredths-of-an-inch. (.04"). Where on earth the "You must have 1.25" of subfloor" comes from I don't know. If you know what you are doing, how to select the right grade and species of plywood and the proper setting materials and exactly how to dot your I's and cross your T's, that would be the case for a direct to plywood installation.

More plywood is always better since cement boards are not a structural part of the floor, but the minimum is 3/4" tongue and groove plywood on joists 16" oc, attached to the joists with a single bead of subfloor panel adhesive where panels cross the joist and a double bead where two panels join on a joist. The panels must be fastened a minimum of every 6" where they meet on a joist and every 12" where they cross a joist. Providing the joist system is designed to achieve L/360 or better using 40/10 loading, your subfloor between the joists will be between .04 and .025, well within the limits. Follow?

Note: If plywood is to be added to the subfloor, the minimum thickness of plywood allowed in a ceramic floor assembly is 3/8" plywood and the minimum quality is cc plugged and sanded , rated as "exterior" or "exposure 1." CDX, Sheathing rated plywood, pressure treated and all 1/4" plywood products are specifically excluded from ceramic and stone floor assemblies. I'll cover the fastening below.


Frazwood

INSTALLATION OF PLYWOOD UNDERLAYMENT PANELS

Install the panels so the long edge crosses the joist. Do not use glue. Using beads of construction adhesive creates voids between the panels as it does not compress into a full spread layer between the two (as does the thinset used under cement boards) and in time the underlayment panel fatigues (creep) and results in failure. The exception would be full lamination of the panels with a trowelable adhesive but even that is not desirable as you lose the effect of the underlayment helping to isolate the floor above from some of the seasonal movement of the dimensional lumber joists.

The underlayment panels should be placed so that the short edge of the panel ends 2-4 inches past the joist, not on the joist. Offset the long edge from the subfloor panels by a foot or more. Stagger the panels so that 4 corners do not come together. 1/8" gap between panels and 1/4" gap around the room. Fasten the panels with coated screws every 6" around the perimeter and every 8" within the field, fastening in straight rows so any bowing in the plywood is removed. Do not fasten the underlayment into the joists, fasten it only to the subfloor panels. Excessive bowing in the underlayment panels can be removed by setting a circular saw at the depth of the bottom veneer and making a series of cuts along the length of the ply, in dashes, to release some of the pressure of the odd number of veneers. Since it is along the strength axis, it does not reduce the strength of the panel.

Happy tiling :D

You are correct in the maximun deflection. The 1.25" comes from ansi spec A108.
It is good to see so much info from a pro like yourself. People like you make us all better..
I never stop learning myself.. thanks again

ANSI 108.01.3.4 19/32" sub with 1.25" mud bed, 19/32" sub with 7/16 cement board, 19/32" sub w/ 15/32" underlay with organic adhesive bond coat.

ANSI 108.11.4.4.1 (covering interior cementitious backer units) 19/32" or thicker subfloor (with deferring to manufacturer minimum subfloor requirements, often 3/4" t-g ply for 1/4" cbu)

:)