Sub-floor and underlayment requirements

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Old 07-20-10, 01:52 PM
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Sub-floor and underlayment requirements

Hello,

I am going to be working on 2 ceramic tile projects and need a little help. The first is converting a 6'-6"x5'-0" (33SF) bathroom into a pantry and the other is tiling my 17'-0"x13'-0" (221SF) kitchen . The kitchen will be done at a later date (next year). The two areas abut each other so I will want consistency between the projects.

I have ripped up the vinyl flooring in the pantry and the subfloor is 3/4" T&G (OSB) Chip Board glued and screwed to 2x10 floor joists at 16" o.c. and I have calculated the deflection of my floor to L/505.

Can I put 1/2" cement board on top of my existing subfloor with thinset mortar between and screw it down as required and then install the tiles?

I have researched and found varying opinions on what is needed for a long lasting tile job and would like to know the options I have available. Some say I cant have OSB as subfloor some say I can. Some say I need 1 1/4" thick floor but do not say if that thickness is subfloor only or subfloor + underlayment. So this leads to my confusion and why I am posting here - I feel I get the best advice from this site. Thanks for the help.
 
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Old 07-20-10, 03:21 PM
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Your plans sound fine to me. You only need 1 1/4" thick subfloor when you are installing a natural stone, like granite, slate, marble, etc. Your subfloor exceeds the minimum requires for ceramic or porcelain.
 
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Old 07-20-10, 04:53 PM
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And you only need 1/4" cement board.
 
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Old 07-21-10, 09:18 AM
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Stephen

You can use 1/4" cement board over 3/4" t&g osb. Cement board isn't a structural component of the floor, so using thicker cement board doesn't make the floor any stronger. The cement board's purpose is to provide a good bonding surface for the thinset mortar. Make sure you follow the manufacturers installation instructions for the cement board, and don't skip any steps.

Add screws as necessary to make sure the osb is solidly secured to the joists.
 
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Old 07-21-10, 07:50 PM
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I agree with the answers given. You could also go with Ditra instead of cement board, Ditra adds only 1/8" installed. Or, you can first install min. 3/8" plywood underlayment to stiffen the area between the joists, then the Ditra. Total thickness under half inch........Best of both worlds.

Jaz
 
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Old 09-26-10, 06:02 PM
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Next step. I am finally ready to start installing the tile and had a few questions about the cement board. I purchased 1/4" HardiBacker and am going to install it next weekend. My room is about 4'-11"x6'-2". The 2" on the 6'-2" dimension is the nub at door opening. I found a 12x12 tile (11 3/4"x 11 3/4" actual)

Is it OK for me to cut a 2"x2'-8" piece of HardiBacker and set it in mortar and screw it to cover the area of the nub so I can have 2 full boards and 1 little piece at the door opening?

Can the tile joint line up with the HardiBacker joint? (My sub-floor joints and HardiBacker joints are offset)

What should I use to cut the HardiBacker? I have seen multiple suggestions. First I have to take an inch off of the 5'-0" dimension to get to 4'-11" and second I have to cut a few notches around the baseboard pipes. I have utility knives, dremel, table saw, chop saw, and circular saw at my disposal. Should I get a new/special blade for any of them?

As always thanks for the help, it is greatly appreciated.
 
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Old 09-27-10, 10:28 AM
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Is it OK for me to cut a 2"x2'-8" piece of HardiBacker and set it in mortar and screw it to cover the area of the nub so I can have 2 full boards and 1 little piece at the door opening?
Yeah, just be careful with your screw placement so you don't blow out the edges. Remember to use your mesh tape and thinset on the seams when you tile.

Can the tile joint line up with the HardiBacker joint? (My sub-floor joints and HardiBacker joints are offset
)

See my first response. The seams with be thinsettted. Remember also to thinset the board to the subfloo, so yes, you can.

What should I use to cut the HardiBacker? I have seen multiple suggestions. First I have to take an inch off of the 5'-0" dimension to get to 4'-11" and second I have to cut a few notches around the baseboard pipes. I have utility knives, dremel, table saw, chop saw, and circular saw at my disposal. Should I get a new/special blade for any of them?
I have used everything. Razor blades to score the material and snap it like drywall (not as easy), circular saws, jig saws, etc. If you use any power tool it will create a lot of dust so do this outside! This stuff will eat up your blades, but I think they make them for cutting the material.
 
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Old 09-27-10, 11:05 AM
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The blades made for cutting cement board kick out a lot less dust than a normal wood cutting blade
 
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Old 09-27-10, 01:05 PM
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Is it OK for me to cut a 2"x2'-8" piece of HardiBacker and set it in mortar and screw it to cover the area of the nub so I can have 2 full boards and 1 little piece at the door opening?
Sounds like you are gonna have to buy 3 sheets of hardi anyway, yes. A better solution along the 6 ft length would be to center a 3x5 sheet, and then 2 smaller pieces approx 1' 7" on each side. That will be a little more stable. I try to avoid little pieces like that as its not easy to screw them and get them flat without breaking the board. Remember to gap the boards approx 1/8" and mud and tape them. Leave a 1/4" gap around the perimeter of the room for movement.

Can the tile joint line up with the HardiBacker joint? (My sub-floor joints and HardiBacker joints are offset)
The tile joint could line up with the hardi. If you do the hardi the way I suggested, then possibly the joints will not line up.

What should I use to cut the HardiBacker? I have seen multiple suggestions. First I have to take an inch off of the 5'-0" dimension to get to 4'-11" and second I have to cut a few notches around the baseboard pipes. I have utility knives, dremel, table saw, chop saw, and circular saw at my disposal. Should I get a new/special blade for any of them?
1/4" hardi can pretty easily be scored and snapped. If you use a saw or grinder, cut outside and where a dust mask. Its messy stuff and you dont wanna be breathing that stuff in.
 
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Old 09-27-10, 01:49 PM
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I laid out the hardibacker and tiles on CAD and have it working nicely without any little pieces. Now my only concern is what to use to as a reducer/transition strip at the doorway. If I estimate the mortar/hardibacker/mortar/tile thicknesses correctly I will end up with at least a 7/8" height difference between the existing vinyl floor and the new tile floor. The thickest reducer I found is 1/2". If I can't find anything I will probably cut a nice piece of wood to create a reducer. What have you seen in the past as a solution?
 
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