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Old 06-07-2009, 07:46 AM   #1
rrando
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Should I use 0.25 or 0.42 inch hardiboard

Hi folks,

I want to install backerboard on top of a repaired area of subfloor to match up to the level of pre-existing 7/16 inch (0.4375) hardwood that was installed directly on top of the original T&G subfloor. Should I use 0.25 inch hardiboard on a relatively thick layer (0.2) of thinset or 0.42 inch thick hardiboard with a almost no thinset, (in this case some high points would also have to be knocked down). Which size notched trowel is recommended? I guess I'm really asking what is the min and max practical thickness for thinset under backerboard?

The long story:
My bathroom redo was a success thanks to the forum, so I'm on to a new project. A section of rotted subfloor was cut back to the 4x4 joist, new floorboards nailed to joist with a sistered 2x4 for additional support (and to raise the new T&G floorboards ~1/8 inch to match the original full dimension material). If you must know, this is in preparation to install laminate rather than tile. We kept about half of the room's hardwood flooring intact and its sound enough to make a good substrate for new flooring. I know this will raise the floor about half an inch -- but that's OK, still plenty of height before anyone bumps their head on the doorway :~)

Robert

Last edited by rrando; 06-07-2009 at 08:07 AM.
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Old 06-07-2009, 08:16 AM   #2
Davestone
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If you are installing wood i would use plywood, you can get it any thickness,and Hd has small pieces to buy.
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Old 06-07-2009, 08:19 AM   #3
madronatile
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Just re-read. If your floor is t&g planks you can't put hardi backer right on them if they're getting tile on top. Plywood indeed as dave recommended-what's the finished floor?
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Last edited by madronatile; 06-07-2009 at 08:25 AM. Reason: hasty reading
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Old 06-07-2009, 08:33 AM   #4
rrando
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you guys are fast (that's a compliment).

I'm thinking of putting laminate, hardwood or even (ugh) carpet on top, considering hardiboard in case I wanted to tile in the future. I thought you could use hardi on top of T&G boards, is expansion/contraction the issue? -- I used it in my bathroom remodel, which has sheet vinyl on top of it and it made for a really sound floor -- its a relatively small area. --

edit: I just reread, and it now sounds like you're saying I cannot use hardi without thinset which rules out the thicker stuff. It would be ok to use the 0.25 inch with a good bed of thinset, right??

you can see the new exterior door where the water came in causing the floor to rot --
that's a full 3x5 sheet of 1/4 inch hardi. I'm ready to go as soon as you guys say its a good plan.
(here's a bigger version of the same picture)
http://members.dslextreme.com/users/...floorphoto.jpg
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Last edited by rrando; 06-07-2009 at 08:51 AM.
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Old 06-07-2009, 08:36 AM   #5
jgleason
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Attach a photo - yep. When composing a post, scroll down below the Submit Reply button and you will see the Additional Options menu. Select Manage attachments.
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Old 06-07-2009, 08:58 AM   #6
madronatile
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Use 1/2" ply. If you want to tile in the future, first make sure your joists are up for it then cover it with a thin membrane like Ditra or Noble CS.
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Old 06-07-2009, 09:00 AM   #7
rrando
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thanks, I was able upload a photo. This forum is awesome...

-- I'm going to go with laminate which is inexpensive, easy to do-it-yourself, lasts a long time and looks great. Reason for not tiling: my joists are 48 inch o.c. so i would need to
add additional support and, since I don't believe in cutting corners, I'd have to remove the rest of the hardwood and replace with the proper substrate.

The new subfloor is 1-1/2 inch T&G planks (pine) the old stuff is actually a full 1-5/8 really solid fir -- I think this type of floor is called "western" construction and is common for older houses here, ours was built in the '59-- the hardwood is solid 7/16 inch oak, not plywood with veneer -- what a difference 50 years makes.

So why is hardi NOT allowed here (as seems to be the case)? We were planning to remodel another bathroom, but maybe we'll hire a pro this time and have him do a real mud wall.
After all the last bathroom makeover only took me 15 years or so.

Edit: I did a bit of online research and although its not apparent from the hardibacker instructions, I think that hardi is NOT recommended with T&G planks due to different expansion rates -- My reasoning is that if you securely attach one structure (the tile and cbu) to another that is expanding and contracting at a different rate (the subfloor), this results in the entire floor bowing up and down as temperature and humidity change, this causes all sorts of problems with tiles. In one direction the subflooring can give because of the T&G, but in the lengthwise direction it cannot. I guess if you use plywood it has roughly the same expansion properties as the subfloor, so in this case, the tiles simply move apart or together a tiny amount, which is taken up mostly by the grout. I don't understand why cbu on top of thinset and plywood is OK, though. Why wouldn't this result in trying to bend the tiles. In any case this hasn't been a problem in my bathroom because I don't have a tile floor and its only a 6x6 ft area, -- so far no indication that the vinyl or flooring are failing in any way.

I guess I'll bite the bullet and get some plywood. I can use the hardiboard on the bathroom walls (I'll make sure the substrate is correct this time).

Another question: Is it a good idea to apply some sort of anti-rot preservative and/or water sealant to the new subfloor in the area around the external door where the water came in (was an old wood french door, and there were some roof issues) -- it could happen again but I'm going to do my best to not allow water in.

Last edited by rrando; 06-07-2009 at 10:14 AM.
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