Hardibacker subfloor requirements

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Old 01-21-08, 10:37 PM
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Hardibacker subfloor requirements

My older home has 2 x 8 redwood tongue and groove decking as the sublfoor spanning 4' over 4 x 8 girders. The subfloor is in good shape, and I would like to install 1/4" hardibacker and tile in the kitchen. This worked fine in the bathroom several years ago, but everything I read recommends installing plywood or OSB under the backer board, which is not practical for my application. Any comments?
 
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Old 01-22-08, 10:00 AM
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Why isn't it practical? Is cracked grout and or broken tiles practical? You need minimum 3/8" plywood, thicker is better of course.

Your type of construction is usually only found on the left coast and can be problematic. There are no specs that approve tile over that wide a span. However many of these installations have worked in the past.

Jaz
 
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Old 01-22-08, 02:03 PM
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Adding plywood+thinset+hardiboard+thinset+tile will put me considerably higher than the other flooring throughout the house.

One bathroom has 1/4" hardi over the subfloor with no problems (with 18" tile). The other bathroom has a 1/2" mortar bed floated over the subfloor with 12" tile, again, no problems.
 
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Old 01-22-08, 02:35 PM
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If you don't want to do the job right why did you bother coming here? What did you expect to happen to the other floor with Hardie over planks? I mean since it's still there does that mean it's fine and it'll last a long time? Maybe it'll be ok under average conditions 60, 70, or even 85% of the time? A professional cannot recommend methods that can fail 20-30% of the time. I would bet you a coffee that you now have hairline cracks in the grout, but don't know it.

Do this just for a test. Tap the floor with the plastic handle of a screwdriver. Listen to the sound. It may still be all solid, but chances are that within 4-6 years you'll find some hollow sounds. Not always but often. As I said, 85% success rate is not good enough.

Your situation has been tested my me, all of the manufacturers of CBU's and every testing organization in the US and Canada, and has never meet the standards.

Adding 3/8" thick underlayment plywood will add just under 3/8" in height. Anything close to or under 1" in variation is no big deal, and is easily transitioned with a bevel threshold or similar manner. In any case you don't choose methods to accommodate heights of adjoining floors. Or you could do what most of the rest of the world does and install tile in every room?

Jaz
 
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Old 01-22-08, 06:21 PM
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If I build up the height of the subfloor, then I won't be able to smoothly transition the same tile floor into the entry way (which currently has slate on top of about a 1" mortar bet on top of a recessed 2 x 8 subfloor) without pulling out all of the floor in the entire house.

Also, it seems that adding 3/8" plywood could cause problems in itself unless it is attached with mortar or some other compound to fill in any irregularities, no?
 
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Old 01-22-08, 08:54 PM
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OK then, do the kitchen the way the the slate was installed. That's a lot more work and you probably won't be able to do it yourself?

Then is the last sentence above you now, all of a sudden think the plank subfloor is too uneven to installed plywood to, yet you want to put Hardie on it. ME confused.

I don't understand why you think all floors should be the same height when they are all different types and installed at different times. How is that possible?

Jaz
 
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Old 01-22-08, 10:27 PM
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Dropping the subfloor down and adding a mortar bed on top is not a cost-effective option.

Regarding my concern about he plywood--I would be applying thinset under the hardi to account for any imperfections in the subfloor, but I am not sure if that is the case for the plywood. So potentially 1/2" plywood would be spanning any imperfections in the subfloor, but the hardi would not.

Adding plywood to the subfloor also means my appliances won't fit unless the plywood is installed under the cabinets.

Does anyone know of a good deflection calculator?
 
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Old 01-22-08, 10:58 PM
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There's no deflection calculator that I've ever seen for your type of construction. Although there must be? You should check in your area because they don't build houses like that anywhere else.

BUT, deflection is not the issue here anyway. The movement of the individual boards is what makes your planned installation fail. Maybe you should consider some other type of flooring?

As for appliances not fitting, I guess you mean the dishwasher? That is a possibility, if the base cabinets weren't shimmed up at all. Easy to figure that out, look at the adjustable feet of the machine. You usually have plenty to work with. When I did my kitchen I had to pull all the base cabinets out, but that is rarely necessary.

Jaz
 
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Old 01-23-08, 08:28 AM
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Yes, the dishwasher and the refrigerator. An extra 1/2" would mess me up. Perhaps hardwood is a better option, but I hate to have wood in the kitchen.

Thanks for your input.
 
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Old 01-23-08, 09:10 AM
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The clearance for the ref. is a common problem. But again usually can be easily corrected. Just remove that upper cabinet and have it trimmed. It's done all the time, and is no reason to switched to some inferior flooring.

Jaz
 
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Old 01-24-08, 11:57 AM
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Theres probably a reason why the slate floor in the adjoining room is framed differently and has a mud bed. 48" span between joists aint gonna do it. Add some 2 X between the 4 x 8 beams to cut the oc spacing down to 16". Then you gotta do something about the dimensional deck boards. Either go over them with 1/2" plywood as Jazman has suggested or remove them and go with 2 layers of 3/4 plywood. Then ditra then your tile.

You got lucky if you used cement board over those 2x8's and no problems so far. Sometimes you get lucky and sometimes you dont. Its not worth taking the chance. You came here asking for advice and you got good advice. I makes sense to use it.
 
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