I just discovered this site and am eager to get some concrete advice on how to approach this issue. My husband and I have just built a home and I'm getting ready to tile. We have "I" joists on 24" centers. The plans called for only 7/8" OSB, but we used 1 1/8". We have already purchased and cut 1/4" Wonderboard. I've read a little about the deflection issue, will this subfloor be strong enough? Also, don't laugh, because I know this sounds strange, but where there were some low spots we built it up under the Wonderboard using layers of 15 lb. roofing felt. We're tight on money and looking to avoid the expense of a self-leveling agent. Can this technique work when using thinset and screws?

2nd ???
We goofed and installed greenboard in the shower. We were told we should have put 15 lb. roofing felt on studs with 1/2" backerboard. Can we now sandwich felt in between the greenboard and use 1/4" backerboard? Will the 1/4" hold up as a vertical surface? Thanks!

First things first. With reference to the floor, your plans for using 1/4" cement board over the subfloor would be fine if this were over 16" on center joisting. With 24" on center, the first thing you need is two layers of plywood under the cement board. Secondly, they need to add up to a MINIMUM of 1 1/4".

As for using the roofing felt to build up low spots, that is a GUARANTEED FAILURE for that floor!! That needs to come out of there, and once you have the second layer of plywood (screwed and NOT glued to the first), lay the cement board directly over your subfloor (thinsetted snd screwed down-- NO construction adhesive!). Once the cement board is down, THEN you fix any dips in the floor-- not under the cement board.

As for the shower, yes, 1/4" will work ONLY if it's backed up by the sheetrock. It would be way too flimsy by itself. But yes-- what you've proposed is possible, although I'd rather see the sheetrock come down and 1/2" cement board go up.

Green board is not even approve for any "wet surface" anymore it should have been paperless sheetrock if any was used in that area. (which there should not have been) Also it should have been 6 mil. plastic not roofing felt so there would be no seams and totaly water proof.
If the rock is not removed and tile board is installed over it there may be trouble when you try to do the plumbing with the lengths of the stems for the handles. (the walls to thick)

quote:

Originally posted by joecaption:
Green board is not even approve for any "wet surface" anymore it should have been paperless sheetrock if any was used in that area. (which there should not have been) Also it should have been 6 mil. plastic not roofing felt so there would be no seams and totaly water proof.
If the rock is not removed and tile board is installed over it there may be trouble when you try to do the plumbing with the lengths of the stems for the handles. (the walls to thick)

Much as I disagree with it, you CAN go over sheetrock with a vapor barrier and backerboard. AS for the vapor barrier, either one is acceptable-- tar paper or poly.

Next time your in the Big box store make a side trip down the shingle isle and look at the wraping on a roll of roofing felt. It says to store standing on end and to "keep dry".

Joe, if you check ANSI ot TCNA, 15 pound asphalt roofing felt (tar paper) is just as acceptable as 6 mil poly. Now why belabor the point?

Exactly. TCA approves 15lb roof felt.
If they give it the ok, it's good to go.

We use DensShield for our backerboard systems.
No need for a vapor barrier, the stuff is water-resistant.


Matt
Cupan Custom Tile
Charlotte Tile

Must disagree with youy CupanTile. You must use a vapor barrier regardless of what you use on the walls. Moisture vapor will go right through the DensShield and condense within the walls. The only thing however you will not see it until its to late as it will not break down like regular rock will.
You can however use Kerdi over the DensShield which is what we do. It provides the vapor barrier, Much better then behind the wall as no nails or screws puncture it causing moisture leakage.

HCC-- AS much as I agree with your thought process, Denshield actually does require that you don't use a vapor barrier because of its "plastic" skin. It would cause two vapor barriers to be face to face which is a mold situation waiting to happen.

I think we are or at least I am mixing things up. My prior post may not have been clear. But here goes once again.
The Dens-shield is a fiberglass faced gypsym rock board. It offers no resistance to moisure or vapor pressure. The Kerdi will provide the necessary moisture barrier required to prevent the vapor or any leaks from getting into the wall. I do agree you do not want two vapor barriers on the same side.

Thank you all so much for your input. Bill, I wanted to explore the first issue a bit more. This tile floor is transitioning in two different places to 1/2" engineered wood flooring (about 700 sq. ft.). With over an inch of plywood buildup plus the backboard and tile, it's hard to believe that's feasible without building up all of the rest of the floor. Could you put some bracing between the joists or might there be another consideration? So is the extra thickness of the OSB we used, even though not required, already factored in?

I don't quite know how you'd anchor crossblocking into I-joists, but if you can, then yes. You could conceiveably add cross blocking and then go right over what you have with the 1/4" cement board.