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Old 02-19-2008, 02:55 PM   #1
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Subfloor / underlayment question...


Some questions about subfloor / underlayment with carpet and tile flooring...
My entire home currently only has a 3/4" plywood subfloor right now (no underlayment) with beat-up carpeting & parquet floors. The carpet will be replaced with new carpet while the parquet will be replaced with tile.

While the joists / structure appear to be in great shape (10' span 2x8's...16" OC), the subflooring is a disaster...soft spots everywhere & with every step the floor creaks. Whether defective or incorrect subfloor plywood was used or due to improper installation, I don't know. Although I could try to "patch" every soft spot, I believe the best approach to an eventual bullet-proof floor is to replace the subfloor and add the needed underlayment.

1) By cutting / replacing an entire subfloor away from the walls (toe kick saw), does this weaken the structure in any way? Note: I will confirm & pull a permit, if needed.

My goal is to have a nice transition from carpet to tile, no big "lip". Since I am basically rebuilding the floor, this is my plan of attack:

For the carpeting: new 3/4" plywood subfloor with another 1/2" plywood underlayment.

For the tile: Either 3/4" plywood subfloor and cementboard underlayment...or 1 1/8" plywood subfloor and fiber-cementboard underlayment (which I understand is thinner than cementboard?)

I would prefer to keep a 3/4" subfloor thru-out the home, and not use 1 1/8" plywood.

2) Your thoughts on this plan? Anything I am not considering?

So there it is...Bring it on & thanks in advance!

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Old 02-19-2008, 04:19 PM   #2
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Subfloor / underlayment question...


If it was my house or if I was hired to do the job. If it's as bad as your saying. I would cut the sub floor out and install new 3/4" along with 1/4" durock in-bedded in 1/8" thin-set on top of that, (Spec is 1 1/8" for tile, Then 1 1/4" for stone).
Remember if you do this don't glue and nail your 3/4" subfloor down this is a major no-no when it comes to tile. What is the spacing of the joisting underneath? is it's regular 2x6"?
Thanks
Adam Forsythe

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Originally Posted by scheenstra View Post
Some questions about subfloor / underlayment with carpet and tile flooring...
My entire home currently only has a 3/4" plywood subfloor right now (no underlayment) with beat-up carpeting & parquet floors. The carpet will be replaced with new carpet while the parquet will be replaced with tile.

While the joists / structure appear to be in great shape (10' span 2x8's...16" OC), the subflooring is a disaster...soft spots everywhere & with every step the floor creaks. Whether defective or incorrect subfloor plywood was used or due to improper installation, I don't know. Although I could try to "patch" every soft spot, I believe the best approach to an eventual bullet-proof floor is to replace the subfloor and add the needed underlayment.

1) By cutting / replacing an entire subfloor away from the walls (toe kick saw), does this weaken the structure in any way? Note: I will confirm & pull a permit, if needed.

My goal is to have a nice transition from carpet to tile, no big "lip". Since I am basically rebuilding the floor, this is my plan of attack:

For the carpeting: new 3/4" plywood subfloor with another 1/2" plywood underlayment.

For the tile: Either 3/4" plywood subfloor and cementboard underlayment...or 1 1/8" plywood subfloor and fiber-cementboard underlayment (which I understand is thinner than cementboard?)

I would prefer to keep a 3/4" subfloor thru-out the home, and not use 1 1/8" plywood.

2) Your thoughts on this plan? Anything I am not considering?

So there it is...Bring it on & thanks in advance!

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Old 02-19-2008, 11:05 PM   #3
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Subfloor / underlayment question...


I have no problem with 3/4" exposure 1 subfloor for the tiles areas. Be sure to block where the old meets the new since you no longer have a t&g joint at those spots. Do this everywhere including along walls. You may not have to remove all the old subfloor?

You can then install your favorite CBU or a product such as Ditra or a Noble membrane product. Basically, 1/4" CBU's or 5/16" Durock are preferred on floors over the 1/2" varieties. If you can install a second sheet of plywood over the 3/4", that would be a good thing. Remember, Ditra is better than cement boards, and is only 1/8" thick installed.

Adam,

If you're suggesting the new subfloor should only be fastened and not glued to the joists, I disagree with you. The subfloor should always be glued to the joists. Use 1/4x1/4 trowel to spread the thinset under the CBU. And the spec is NOT 1 1/8" for tile. One and 1/8" of what? Where did the 2x6" come from? I thought I read 2x8", 16" oc with a 10' span?

Jaz
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Old 02-19-2008, 11:21 PM   #4
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Subfloor / underlayment question...


Jaz, this is one that's gotten a little vague over the last few years. It used to be that the spec WAS as Adam stated-- 1 1/8" total subfloor and underlayment under ceramic and 1 1/4" under stone. However, TCNA DOES accept both 1/4" CBU over 3/4" subfloor, and even 1/8" Ditra over that same 3/4" subfloor. So more and more it seems that 1 1/8" spec is going out the window.
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Old 02-19-2008, 11:51 PM   #5
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Subfloor / underlayment question...


The 1 1/8 or 1 1/4" deal is/was always for tiles bonded direct to plywood, not in consideration when a CBU was installed. Although most floors with a 3/4" sub are stiff enough for tiles, a second sheet is needed as a new clean surface for tile. So 3/4" plywood plus 1/4" CBU is good enough for floors.

Heck, even 5/8" subfloor meets minimum specs, for those that strive to do things to the lowest possible standard. Yikes.. that is scary!

Jaz
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Old 02-19-2008, 11:55 PM   #6
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Subfloor / underlayment question...


You might wanna check on that!! 5/8" subfloor still requires that you use 1/2" CBU, just for that very reason-- to get back to the 1 1/8" requirement, and I think if you check Schluter's site, you'll find that you need to add more ply before going over that 5/8" with Ditra.

I know how much you like Ditra.
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Old 02-21-2008, 08:21 AM   #7
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Subfloor / underlayment question...


I have read that the CBUs do not contribute to the stiffness of the floor only provide a good bonding surface for adhesive. I don't really believe it. It seems more likely the CBU is a structural enhancer as well as a good bonding surface. The preceding discussion sort of alludes to this. Maybe one of you smarter floorers could confirm it for me.
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Old 02-21-2008, 08:39 AM   #8
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Subfloor / underlayment question...


There is very little that CBU contributes structurally speaking. Quite honestly, I don't understand how even using 1/2" CBU over 5/8" subfloor contributes enough to make it sturdy enough to tile over. I certainly wouldn't do it, but it is approved as a MINIMUM standard.

Thing is-- take a piece of CBU by itself, and lay it on the floor on top of a couple of 2x4's, and see what happens-- even with 1/2". Granted , 1/2" will hold more weight than 1/4" will, which is why I say there's some, but very little strength there. Neither one will hold very much, though.

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