Cement Backer Board Thickness

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Old 12-31-08, 12:58 PM
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Cement Backer Board Thickness

How thick should my backer board be if I am installing an 18 inch porcelain tile on the diagonal in my kitchen and foyer?

I have been told that 1/4" is two thin for the size of the tile and I should actually install 1/2" backer board.

Any advise is greatly approciated!
 
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Old 12-31-08, 02:40 PM
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You heard wrong.

Cement backboard is not used for strength on the floor.

You can use 1/2 if you need the height, or else just use 1/4".

The main concern is to make sure your joist and subfloor can handle a tile installation.

Can you provide us with your joist sizes (2x?), their spacing, and the unsupported span (the length the joist runs for before it is supported by another wall or post).

How thick is your current subfloor, and is it plywood?

Alot of questions there, but that needs to be answered to make sure you have a long lasting installation.
 
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Old 01-03-09, 11:50 PM
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As HotinOKC mentioned, a CBU like Durock, Wonder or Hardi add no structural rigidity to a subfloor so using a 1/2" CBU will not give you any added benefit over 1/4", again unless if you need the extra finished floor height. Also as he mentioned, adding CBU, thinset and tile can add quite a bit of weight to a subfloor (assuming you have a wood subfloor due to the mention of cement board) so you really should be sure that the subfloor is suitable to carry the load, without causing deflection or flex in the floor that can cause the tile job to fail.
 
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Old 01-04-09, 06:44 PM
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Cement Backer Board

Thank you HotinOKC and Shooter for repyling to my message.

HotinOKC, I checked the measurements you asked for and this is what I measured:

Joist size - 1 1/2" x 9"
Joist spacing - 14" apart
Unsupported span - for the foyer 14 feet and for the kitchen - 12 feet 4 inches.

The subfloor is 5/8" thick. I'm not sure if it is plywood or not.

The tile flooring will be ajoining other room(s) that will have 3/4" solid hardwood flooring.

Thanks again for all your help.

Newflooring
 
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Old 01-04-09, 07:14 PM
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Based on your info, you are on the border of having an acceptable floor for ceramic tile, but you fall way short for stone tile. You would help things alot if you could put in a mid span support for those joists. The 5/8" sheet is at the bare minimum. It really should have another layer of 1/2" underlayment rated ply added to it. If you do that you could install Ditra instead of Hardibacker, then set the tile.
 
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Old 01-05-09, 01:35 PM
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Cement Backer Board Thickness

Thanks TileGuyBob,

The tile I am using is porcelain tile (18"x18") not stone.

If I use Ditra do I still need to have a mid span support?

Thanks,
Newflooring
 
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Old 01-05-09, 02:31 PM
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Ditra does not make up for a substandard floor.

I'm curious though, when you give your unsupported span, if you understand what that means. The unsupported span is the distance between the 2 bearing walls or beams supporting the floor joists from below.

If the spans you gave are correct, I'd not install 18" porcelain on that floor. TGB is right on with his assessment.
 
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Old 01-05-09, 02:58 PM
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Tile Installation

ok.

When I measured the foyer, I measured from the front of the house to the steel beam running lengthwise from the right of the house to the where the garage begins (left side of the house).

For the kitchen, which is in the rear of the house, I measured from the steel beam to the back of the house.

Is that correct?

So what can I do to ensure that the sub floor can support the new tile installation?

Thanks much!!
 
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Old 01-05-09, 03:17 PM
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So, the joists are resting on the steel beam and the other side of the joist is resting on the sill plate of the foundation and nothing supporting the joist in between?
 
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Old 01-05-09, 03:34 PM
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Yes. As far as I can tell.
 
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Old 01-05-09, 04:04 PM
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I'm showing you are good with what you have in regards to joist deflection.

Foyer: Deflection of L405

Kitchen: L539
 
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Old 01-05-09, 09:17 PM
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I suspect that when Newflooring said his joists are 14" apart, he really meant to say they are 16" on center. If so, those joists in the foyer are right at the edge if they are SYP #2. The kitchen is fine.

It would be helpful if we also knew the species and grade of those joists. It can make a big difference.

The plywood is a different matter. You need more.

Jaz
 
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Old 01-06-09, 03:24 PM
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Jaz,

That very well could be the case here. I'm betting the OP measured between the joists instead of center to center. Good observation.
 
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