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Old 04-27-2009, 01:29 PM   #1
Darren Deese
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Location: Columbia, South Carolina
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New floors, old house

Need advice on two bathroom floors. First has two layers of 3/4 sturdi floor, which is a tongue and groove OSB. Area tiling is only 4ft. by 9ft. on 2x8's 16oc. Steps planned are red guard, thinset with hardiboard screwed down. Question on this one is can I skip the red guard and thinset and just screw down the hardiboard since I have no bounce at all in this floor and the floor is a full sheet of sturdi floor and a 1ft. piece.

Other bath is on a converted back porch with a smooth concrete slab which is dry and no cracks after 60 years. Plan to clean paint overspray off and tile directly on top of slab. Total area of slab may be close to 12ft.x6ft.
Going to be using 16x16 ceramic on both.

Any advice or recommendations are greatly appreciated.
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Old 04-27-2009, 01:54 PM   #2
bbcamp
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Darren, no need for Redgard on your OSB floor, but you do need to use thinset under the backerboard. One of the biggest cause for failure in a Hardibacker installation is to skip the thinset. The next is to skip taping the joints. What's the unsupported span of the joists?

Do not use chemical strippers to remove the paint from the concrete. Grinding or water-blasting is recommended since strippers may seal the concrete and prevent thinset bonding. On that subject, check the concrete for sealers by sprinkling water on it and noting if it soaks in (good) or beads up (bad). Run a straight edge over the surface to see how flat it is. The standard is no more than 1/4" out of plane in any 10 foot direction. The slab doesn't not need to be level, unless you want it to be.


Consider using a medium set mortar (granite and marble mortar) for those big tiles. It will allow you to build up more thickness to account for some out-of-plane conditions, but it won't let the heavy tile sink as thinset might.
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Old 04-27-2009, 01:54 PM   #3
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Welcome, Daren.

You need to know the unsupported span of your 2x8 joists. Not the size of the room to be tiled, the span of the joists below.

No reason for RedGard at all over the plywood, no matter what else you do. But you must follow the CBU manufacturer's installation instructions, which call for the board to be bedded in wet thinset.

You don't indicate any geographic location, but I'd strongly recommend you use some sort of waterproofing membrane over your exterior slab. You could use your RedGard there, but Schluter's Ditra would give you waterproofing as well as isolating your tile installation from seasonal movement of your slab. You need to seal the seams in the Ditra with KerdiBand, also if you go that route.

My opinion; worth price charged.

Ol' Injineer Bob's fast since he came back from vacation, eh?
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Old 04-28-2009, 10:00 PM   #4
Darren Deese
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Thanks for all the advice, greatly appreciated. I left the house in a hurry this morning and didn't check the post. When I got back to the job I went ahead and did the redguard, then the hardiboard with thinset. The joist are only 4ft. They're sitting on a sill and running to a doubled 2x8 beam that is supported by brick pillar and another sill. Length of this beam may be close to 10ft. Thinking I may put a jack post about middle ways just to be safe, even though I feel no movement.

On the slab side I picked up a cup shaped concrete grinding wheel for my side grinder and went over the entire surface with it while my redguard was drying. This did great, a little dusty but well worth the peace of mind. Slab has a good bonding surface to it now. Put a straight edge on it and near perfect. Going to go with the redguard on the slab before tiling.

Didn't know about the medium set mortar on the larger tiles. I'll give it a shot.

Thanks again, Darren
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