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Old 05-10-2009, 07:38 AM   #1
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Cement hardy board


Hello, I'm remodeling a house that was rented out for 13 years and have taken up the 1/2" sub-floor in the bathroom and kitchen where the vinyl used to be, in order to install tile.

My question is whether I should use 1/2" or 1/4" cement hardy board under the new tile floor? My thinking is the 1/2" is for floors and the 1/4" is for walls, with the 1/2" giving more support and less chance of the grout cracking. Though for ease of handling, I'd like to know if the 1/4" board is sufficient and won't cause future cracks in the grout? The actual flooring was nailed down and I plan to use screws to better secure the floor to the joists and lessen the squeaks prior to installing the new hardy board.

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Old 05-10-2009, 08:46 AM   #2
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Cement hardy board


Before proceeding any further, you first need to check your floor for deflection ( how much flex the floor has) You will need to find out your joist size, material, spacing and unsupported span. Go to the John Bridge Forum and enter your data in the 'deflecto' to find out if you need any additional support. Minimum of 3/4" plywood on top of joists, and backerboard of your choice on top of that. Make sure you thinset and attach backerboard to plywood following the backerboard Mfgs instructions. Tape and thinset the seams of the backerboard, and then you will be ready to tile.

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Old 05-10-2009, 09:43 AM   #3
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Cement hardy board


Quote:
Originally Posted by Rehabber View Post
Before proceeding any further, you first need to check your floor for deflection ( how much flex the floor has) You will need to find out your joist size, material, spacing and unsupported span. Go to the John Bridge Forum and enter your data in the 'deflecto' to find out if you need any additional support. Minimum of 3/4" plywood on top of joists, and backerboard of your choice on top of that. Make sure you thinset and attach backerboard to plywood following the backerboard Mfgs instructions. Tape and thinset the seams of the backerboard, and then you will be ready to tile.
My kitchen =
Thank you for using the John Bridge Forums Deflect-O-Lator :-)
For joists that are Unknown wood, but in good condition, 7.25 inches tall, 1.5 inches wide, 12 inches on center, and 12 feet long between supports, the deflection calculated is 0.468 inches.
This translates to a deflection of L / 307.
Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.


My bath =

For joists that are Unknown wood, but in good condition, 7.25 inches tall, 1.5 inches wide, 16 inches on center, and 11 feet long between supports, the deflection calculated is 0.457 inches.
This translates to a deflection of L / 289.
Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.


My current flooring is 3/4" thick. what would you recommend to beef up the floor? Will the hardy board work to reinforce or beef up the floor if I went with 1/2"?
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Old 05-10-2009, 12:00 PM   #4
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Cement hardy board


Plywood or backerboard will not reduce deflection. You can sister your floor joists to stiffen the floor. Do a search on the JBF and you will find a number of discussions on the subject.
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Old 05-10-2009, 05:49 PM   #5
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Cement hardy board


Let's re think this. We know that minimum building code in every (?) state in the union is L360 at 40/10 live load/ dead load. So unless this is a very old house, or was built with not plans and inspections, we have to believe they meet those standards at minimum.

We also know that the most commonly used species of joists are, Southern Pine & Douglas Fir, in #2 grade. Both of those species would meet code up to a span of 14' 2" for the pine at 12" o.c. & 12' 9" for the Fir at 16" o.c. OK, so that is at 40/10 loads. This is bare bones minimum.

I like to specify a load of 50/20 for ceramic tile installations. So let's see what my span chart says now. Now the Pine at 12" o.c. meets my higher standards at up to 13' 1", while the Fir at 16" o.c. if good up to 12' 6". So what' the problem? The problem is we are not sure of the species of the joists. Why don't you take a look and find what it says of your joists. Everyone should have been marked at the mill.

In addition to all the above, it is always good if you can easily sister the joists to make them stiffer, or build a supporting at approx. mid-span.

As to your subfloor, there is no way your subfloor could have been only a single 1/2" plywood. Please let us know what, how thick and how many layers your subfloor system is made of. You say it's 3/4", is that plywood or planks or?

!/4" backer boards are made for floors, half inch must be used on walls but can also be used on floors. Neither add structural strength, so use which ever you want. Thinset under it of course, screws, tape etc... I assume you're going with glazed quarry or porcelain tiles.

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Old 05-10-2009, 07:37 PM   #6
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Cement hardy board


Thanks Jaz, I'm with you on thinking the floor would support the tile. I took off 1/2" (or 5/8", not sure as I'm into the end of the week drinking phase of the project) of flooring to accommodate the vinyl flooring, which remains the actual 3/4 inch flooring. I'm planning on porcelain tiles and the house was built in 1996.
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Old 05-10-2009, 08:18 PM   #7
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Cement hardy board


OK, I am sure your house meets L360 max deflection, just don't know by how much. How does the floor feel? Can you sister or add a supporting wall under the room? Probably no reason to do so but? Try to figure out what species and grade those joists are.

Worse come to worse. If your floor had more deflection then the recommended max, and no way to fix it, there is a special mortar that claims will work even when the floor is as flimsy as L240. You should be fine, but that info is for others.

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Old 05-11-2009, 12:42 PM   #8
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Cement hardy board


Quote:
Plywood or ........... will not reduce deflection.
What about deflection between the joists? Will additional plywood reduce deflection in that case?
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Old 05-11-2009, 03:08 PM   #9
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Cement hardy board


You bet it will.
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Old 05-11-2009, 04:10 PM   #10
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Cement hardy board


I could not find any markings on the wood, so I took a picture or two. There are metal cross brackets going down the middle of all the joist spans. Pictures of the bath and kitchen, both not very big areas. The second picture is the area under the bathroom.

The floor is solid, no bouncing. Just squeaking from what I believe to be the lack of glue and nails either not present or loose and popping up. I heard that if I screw the floor down, moving in one direction it would take care of the squeaking.
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