Underlayment for ceramic tile

I have 3/4 inch plywood subfloor. I want to put ceramic tile down. I plan to put down 1/2 inch 3x5 foot concrete board underlayment over the plywood.

How should I secure the concrete board to the plywood? I have special 1 1/2 screws for the concrete board. Should I use construction adhesive or thinset between the plywood and concrete board?

What can be used to level the plywood subfloor before the concrete board is put down?

smile7777

 

For starters, you can use 1/4" cement board (cbu) for floors. cbu doesnt add any structural strength to the floor. Its purpose is to isolate the tile from the movement of the structure below and to provide a good bonding surface for the thinset. You can use 1/2" but its not needed. If you are using the 1/2" cbu because you want the floor height to be higher then adding an additional layer of plywood and then 1/4" cbu is a better option. If you go that route 3/8" plywood is the minimum you can use.

Use the special square drive screws for the cbu. They are easier to set flust with the top of the cbu. You can find these screws at the big box stores in the general area that you'll find the cbu. All manufacturers of cbu require that there board be bedded in a fresh bed of thinset and then mechanically fastened with screws or roofing nails. The screws are better in my opinion. Construction cannot and should not be used in place of thinset. All the cbu manufacturers have installation instructions on the website. Its always a good idea to print out these instructions and follow them to the last detail.

If you need to do some minor leveling/flattening of the floor, it should be done on top of the cbu after its installed. You can use cement based patching compounds or self leveling compounds to do this. If theres some major dips and humps in the floor let us know before you proceed. Tile doesnt need a level floor, it just needs a flat floor. If the floor is way out of level, you might want to find out why that is before you proceed.

Have you checked to make sure the joist system is adequate to support a ceramic tile installation?

 

I'm not a tile guy, but I would be wary about laying ceramic tile over only 3/4" of floor. The backer board isn't going to add a whole lot of rigidity and ceramic is prone to cracking from floor movement.

If the finish floor height allows, think about adding a minimum of 1/2" plywood over the existing subfloor. 3/4" is better if you have the room. Nail it with ring nails and stagger the joints between the subfloor and the new plywood. Use 1/4" backer board with thinset. If your budget is tight you can use roofing nails to secure it. The square drive screws sold by HardiBacker and others can get pricy if you have a large area. Once the thinset cures the fasteners probably don't do much. I tried using drywall screws once, but they are a PITA. Too hard to drive flush.

IMO your finished floor should be the 3/4" subfloor, 1/2" to 3/4" underlayment on top, 1/4" of cement backer board and 1/4" of ceramic tile plus 1/4" or so of thinset. If your house was built to code, your existing joists are adequate to support ceramic tile.

 

cw

5/8" of plywood is minimum industry standard for ceramic tile and 3/4" is even better. Yes more plywood is always better but not required in this situation. 2 layers of plywood are required for natural stone products but not for ceramic.

As to thinset under cbu, its not there to bond the cbu to the plywood, the screws or roofing nails do that. The thinset is there to fully support the cbu and fill any voids between the ply and cbu. Thinset is not used to bond the cbu to the plywood. Never use drywall screws with cbu. The heads will snap off them. The screws are what holds the cbu down, not the thinset. I dont know where you got your info from but its wrong. Yes the square drives are pricey, but they are a small price to pay for a good quality install that will last. Drywall screws can and will fail.

As to the floor joists being built to code, we dont know anything about the joists from the op's info. Probably they meet deflection for ceramic tile, but maybe not. It never hurts to make sure.

 

I just visited a couple of DIY sites for tiling information. Both recommended at least 1 1/4" of flooring for ceramic tile. Here's a quote from the diY site
Floor Underlayment
Proper backing for the tiling surface is very important. Backing for the tile floor may consist of exterior grade heel-proof plywood, lauan underlayment panel, mortar based backer board, or exterior plywood underlayment. It should be at least 11/4" thick over a minimum of 16" on center floor joists. Otherwise a "flex" can cause tile to pop out of place. The other site said essentially the same.

A couple of years ago I demo'd a ceramic tile kitchen floor that I had installed 15 years earlier. The tiles were placed over a 1/2" subfloor and a 3/4" plywood underlayment as recommended by the tile store and required by my permit. That floor lasted 15 years without a crack and was a serious bi*** to demo.

Unless we're talking two different things here, I don't see how a ceramic tile floor is going to hold up over nothing more than 5/8" plywood and 1/4" backer board.

HeresJohnny - If you think I recommended DW screws in my post you need to reread what I wrote.

 

[QUOTE=cwbuff;1259670]
Floor Underlayment
Proper backing for the tiling surface is very important. Backing for the tile floor may consist of exterior grade heel-proof plywood, lauan underlayment panel, mortar based backer board, or exterior plywood underlayment. It should be at least 11/4" thick over a minimum of 16" on center floor joists. Otherwise a "flex" can cause tile to pop out of place.

LMAO reading that and can tell you its garbage. For starters, luan is never ever used in any tile installation. Read this.

http://www.custombuildingproducts.co...er=diy&lang=en

I hope you didn't find that on this site, but wouldnt be surprised if you did.

Go to the wonderboard website and the hardibacker website and read. They all say there cbu can be installed over 5/8" exterior plywood. What the heck is heel-proof plywood anyway.

The Tile Council of North America (TCNA) says minimum subfloor for cbu is "19/32" exterior-glue tongue and groove plywood with 1/8" gap between sheets on joists at 16" o.c. max variation in plywood surface shall not exceed 1/4" in 10'.0" from the required plane. Adjacent edges of plywood sheets-max. 1/32" above or below each other."

Custom Building Products for 1/4" wonderboard says 5/8" exterior grade plywood or OSB panels. Go to the link and read it for yourself.

http://www.custombuildingproducts.co...er=diy&lang=en

James Hardie for 1/4" hardibacker says 5/8" exterior grade plywood or 23/32" OSB with Exposure classification or better.
Go to the link and read it for yourself.

http://www.jameshardie.com/homeowner...install-us.pdf

Go to other board manufacturer sites and check them out as well. Im not aware of any that don't claim that you can install there 1/4" board over 5/8" t&g plywood. I will tell you that I havent used them all though.

I commend you for adding extra plywood to your installation. My comments didnt say that you cant add extra plywood, only that its not necessary in this case.

TCNA (the bible for us tile setters) and the backerboard manufacturers say its ok and the manufactureres will warranty the installation.

You may be thinking of setting tile directly to plywood. If thats the case (no cbu, no membrane) then yes you need 2 layers of plywood totalling 1 1/4". If you are using cbu or an isolation membrane then 5/8" subfloor is the minimum.

I don't make the rules here, we have the TCNA, cement board and membrane manufacturers and others who make them. I dont make them but I do know them, follow them and recommend them.

 

cw

I saw that you got your quote from this site. This information has been criticized for being wrong many many times. We have had some old threads from pros here criticizing that info as a matter of fact. Unfortunately, the powers to be here choose to do nothing to correct it. If you or anyone else are contemplating a tile project, the info on the "Home Repair Projects" section of this website regarding the installation of ceramic tile are terrible and in many cases dead wrong. I doubt that whoever wrote those instructions even knows who or what the TCNA is. Enough said here.

If you are looking for some good "how to" tutorials on ceramic tile, check out the John Bridge website, The Ontario Tile website and the Floors Transformed websites. Theres also a newer website, The Floor Pro that has some good articles. They are articles written by real pros who know the right and the wrong way to do things.

 

I would like to clear up a few misconceptions in this thread...
1) Ring nails are acceptable, but not the best option
2) Flooring screws are the best option
3) Drywall screws are NEVER recommended, they are too brittle for flooring use
4) Roofing nails are not acceptable regardless of your budget. They have no holding power for your underlayment
5) Lauan Plywood is not a recommended substrate for ANY flooring - the resins break down, the surface is too porous, and delamination is an expected occurrence
6)All of the discussions with regard to subfloor thickness are based on joists with 16" centres. that all goes out the window if your joist spacing is greater than 16", in which case you have to go to the L/360 spec for porcelain tile or L/480 for stone, which measures the deflection of your floor.