||11-08-2011 10:04 AM
I don't know who you asked, but there can be no conflicting accurate info on the subject. You should NOT install any backerboard direct over planks whether it's the subfloor or finished flooring. You need to install underlayment grade, (not cheapo CDX sheathing), plywood first. I recommend 1/2" if the planks are as in good condition as you say, thicker if not. More on that later when you ask how to install it.
JMdesign, OMG, where did you get this?
Do not use hardy board on the floor, they are made for walls and certainly don't use 1/4". Use concrete board 1/2" only on the floors. If you still want to just stay with the tongue and groove and do not want to add or replace with a subfloor you might want to use a membrane. Check with the tile companies to see which membranes they carry. Avoid having the joints on the concrete board line up with the tongue and groove. Screw down well. Don't skimp. Ripping tile out because of cracks is not fun..... Hope that helps...
The only accurate statement is when you said, "Screw down well. Don't skimp. Ripping tile out because of cracks is not fun....."
To clarify; Hardie is a very acceptable, (#1 seller), CBU. 1/4" CBU's are made for floors and counters, 1/2" CBU's can be used anywhere 1/4" can, but mainly for walls cuz it's compatible with the thickness of wallboard. CBU's add no structural strength, just a "tile-friendly" surface. There is no advantage in using 1/2" on floors unless you want the xtra height.
In addition, membranes such as Ditra also require plywood to bond to. No membrane over planks ever.