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billinak 09-28-2006 02:16 PM

Tile Underlayment
I know this subject has been addressed in previous posts, but I would like some feedback on my specific situation. Kitchen floor is linoleum then 3/8" plywood on top of old 3/8" plywood. I'm planning on using 1/2 hardiboard as a tile underlay over the entire floor, but should i remove ALL of the subfloor down to the original 1952 T&G flooring, OR should I leave the first layer of plywood for stiffness and place the hardiboard over that?

dougrus 09-28-2006 02:45 PM

3/8" is very thin for a underlayment for CBU and tile...not necessarily in terms of strength but in terms of stability (stopping movement). What is the thickness of the original T&G plywood subfloor? If it is 3/4 inch then, if it were me, I would add at least 1/2 inch over that then CBU then tile...remember to thinset AND screw the cement board to the plywood and remember to use thinset and alkaline resistent fiberglass tape on the joints of cementboard as you tile.... Note: The 1/2 cementboard does NOT add "strength" to the floor. My first couple floors I did lay CBU over the original plywood subfloor, but I have been educated since that it is better to add additional support in the form of an extra layer of at least 1/2" ply...also be sure that your joists/floor structure is suitable for a tile installation in the first place.
PS...if height is an issue, you can use 1/4 CBU like haridbacker...I also like this stuff because it has a screw fastening grid to follow...
Hope that helps :)

billinak 09-28-2006 04:21 PM

So it sounds like I want at least 1/2" of plywood over the subfloor, which is at least 3/4" thick, probably more. Since the hardiboard does not add strength, I could go with the 1/4" hardiboard on top of the 1/2" plywood and be OK. Why do you suggest thinset between the plywood and hardiboard?

So I looked a bit more online and it looks like using thinset between the CBU and plywood is standard procedure, so it looks like my 'sandwich' will be 3/4"+ subfloor, under 1/2" plywood, with thinset and 1/4" CBU on top of that. Sound good?

dougrus 09-28-2006 05:48 PM

I was a little confused but what you said there at the end but it should be:

Subfloor (probably 3/4)
1/2 ply glued and screwed into the plywood NOT the other words, try to miss the joists.
CBU thinsetted and screwed into the 1/2 ply

Note: if your subfloor is sound and more than 3/4" you may be able to get away with no extra ply, but I always feel uneasy when I do the minimum.


billinak 09-28-2006 09:31 PM

Looks like the subfloor is 2x6 t&g, but I think I'll still throw down the 1/2" plywood, then the CBU as suggested. Thanks! :thumbsup:

R&D Tile 09-29-2006 05:21 AM

Install the plywood, CBUs can not be installed over demensional lumber, only plywood or OSB.:)

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