Floating Wonderboard??

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Old 12-25-06, 08:30 AM
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Question Floating Wonderboard??

Remodeling new laundry/utility room. Room is 9 X 12, and I ultimately plan to lay 6 X 6 ceramic tile. It originally had 2 X 6" joists (16" on center) that ran the 12' span. I installed a midway beam such that the span is only 6' now, and I also sistered all the joists will LVLs across their whole span. Perhaps a little overkill but the floor is very solid now.

The underlayment has been removed, and the subfloor is constructed of dimensional lumber (planks) and a couple patches of 3/4" interior grade plywood. The problem is that the range of thickness of these planks varies between 3/4" to 1-3/16". I plan to install 1/2" wonderboard over these planks but the varying thickness won't allow me to use the classic construction adhesive installation of the wonder board.

I was hoping to first level the floorboards a bit with a cement-based floor leveler. ANy suggestions on product and do I need to seal the floorboards with some latex-based bonding agent?

After the above preliminary level, i was hoping to install the wonderboard with a thinset. This would allow me to float the wonderboard a bit and create a level enough surface for tiling. I would go back after the thinset dried and screw the wonderboard down.

Will this process work? Is there a better way? what thinset product should I use?

Shouldn't be any problem with the flooring height--there will be an easy transition into the adjacent hallway.
 
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Old 12-25-06, 08:53 AM
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Your best bet is to replace the subfloor with plywood so that your starting with a flat surface. All self leveling compounds are supposed to be used as the top layer, not under cbu. You could probably do a mud floor with what you have. It may wind up a little higher than you wanted the finished floor height to be but it will work.
 
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Old 12-25-06, 09:01 AM
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For such a small area, replace the subfloor. You want a solid, flat subfloor with no movement or other problems.
 
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Old 12-25-06, 09:10 AM
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You CAN NOT install CBU over demensional lumber or SLC, it has to be installed over plywood only,unless the plywood can be installed over flat plank flooring, remove it and start over.
 
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Old 12-25-06, 09:04 PM
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Unhappy Can't replace subfloor

Unfortunately, I can't replace the subfloor because it will undermine a non weight bearing wall. I can't get a new joist in under that wall because it sits directly above my furnace in a crawl space.
I guess I could cut back the floorboars to the last joist before this wall but this would leave 10" area of floorboard exposed on one wall. This would be very similiar to the situation I am in now-- a mix of differently thick dimensional lumber and plywood.
Would a "mud floor" be the answer?? I would imagine this is just floating a thin layer of cement over my floorboards?? There are large gaps between the floorboards so I would have to come up with some means to seal this before proceeding.
 
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Old 12-25-06, 10:07 PM
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How about going back to that last joist, screw the dickens out of the existing floor to the joist so it won't move, and then cut along the side of the joist toward where the decking will be replaced. Scab on more framing to this joist to fasten the new decking to and then redeck up to it with good plywood as close to or slightly thicker than the thickness of the existing floor left as you can get. Then add a second layer of plywood that covers both surfaces and ties the new first layer to the remaining old layer by spanning them. This will give you a subfloor you can install the Wonderboard to fairly safely. You really don't want to put anything tileish over those planks.
 
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Old 12-26-06, 06:31 AM
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Question new proposal

Actually there are unsupported walls on both sides of the room. Cutting the floorboard back to the joist before each of the walls doesn't seem to make sense to me--There will still be alot of this plank flooring exposed. As it is now I already have a 4 X 8 sheet of 3/4 plywood used as a sub floor in the middle of the room; cutting the planking back really wouldn't allow me to replace that much more of the planking with plywood.

Remember the underpinnings of this floor are very solid with a mid-span beam and all joist sistered with LVLs.

I am thinking about taking the belt sander to the existing floor and leveling it a bit. then rescrewing is necessary (due to sanded screw heads,...) After that I would put down a 3/8 or 1/2 layer of underlayment (exterior grade??) then put 1/4 hardibacker over that before tiling. I think I could use a floor leveler on the underlayment and the hardibacker to take care of some of my disparities. (latex or cement-based??)

THanks for all your time.
 
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Old 12-26-06, 10:04 AM
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A "mud floor" would require a minimum of 1 1/4" of deck mud. This would go over roofing felt and lathe. This may work for you but as I indicated before you may end up a little higher than you like. The end result however would be a solid, flat and level floor.
 
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Old 12-26-06, 10:39 AM
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You can not use self leveler under the hardie, you must have a flat subfloor before starting, NO 2 different thicknesses in the subflooring side by side, cut out what you can and install the same thickness wood side by side, then install 1/2" or 3/4" plywood over that, then a cbu or membrane.

Or as stated above, do a full mud job, that's it.
 
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