I am getting ready to install 18″ ceramic tile in my kitchen and have run into a small problem. After removing the 1/2″ fiberboard and vinyl, I found out the subfloor is only 1/2″ plywood and it has a few soft spots. I had figured on it being 3/4″ and planned to use 1/4″ Hardibacker to meet the height of the floors in the next room. Assuming I can fix all the soft areas, what is the best way to proceed? I have thought about these options so far: using 5/8″ plywood instead and no cement board, using a thinner layer (1/4″ or 1/2″) of plywood and keeping the 1/4″ cement board, or use only 1/2″ cement board. I realize that keeping the same floor height is probably not an option.
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Replies
Cal
I'm assuming your kitchen is over a basement. If so then go down below and toe nail and PL some 2x12's on the flat under the soft spots.
Personally I wouldnt be as concerned with the floor height as much as the solidness of your floor. Especially with tile that big. I'd screw down another layer of 1/2" ply "at least" and then 1/2" Wonderboard thinseted and screwed on top of that. Really would suckk to see your tile cracked after your done..and it will.....Wouldnt it be a drag to have skimpped on material? Build up your door saddles if need be.
BE well
Namaste
Andy
It's not who's right, it's who's left ~ http://CLIFFORDRENOVATIONS.COM
Being a relative notice at laying tile, I need to ask a question. Everything I've read talks about using a 3/4" plywood subfloor and cement board, but I've not been able to find anything that really explains the science behind this. Can you refer me to a good book, or provide an explanation as to what the cement board really does other than provide a moisture barrier? Assuming I can fix the soft spots, how bad will the 1/2" subfloor really be?
1. Cement board is NOT a moisture barrier. It provides a bonding surface for the tile and an isolation feature so that most movement between the wood framing system and the masonry flooring occurs between the plywood subfloor and the underside of the cement board leaving the tile unaffected.
2. Rip up the 1/2" ply and install 3/4" ply and 1/2" cement board. Do the job right the first time otherwise it is a waste of time. A circular saw makes quick work of it. You do not have to go all the way to the room edges. Cut along the face of the first and last joists, sister a 2x4 to these joists and laydown the 3/4" ply. Lay down 1/4" ply at the perimeter you were unable to cut out. Then lay 1/2" cement board throughout.
3. You want documentation and tables for allowable spans of different thicknesses of ply!? I am sure it exists but there are certain standards in good building. This is one of them. Sounds like you want to be told everything is ok at 1/2". This is a dangerous way of approaching any construction issue.
4. Are the soft spots random or due to water damage from above? I suspect they are random and thus you have concern. That is the nature of 1/2" ply on joists 16"OC. The 1/2"fiberboard you removed "hid" the soft spots, but beleive me, the flex was always there. The previous builder may have been cutting costs and may have thought that any flexing in the flooring would not be visually noticed in the vinyl surface applied. It WILL be enough to crack your 18" tile.
The soft spots are not water related, but are due to the thin nature of the 1/2" board. I have noticed the flex for some time, but guessed it was water damage. What is normally used for subfloor? I had always thought 3/4" T&G.
I have thought about laying another 1/4" or 1/2" of plywood over the current 1/2" and turning them 90 degrees to the current subfloor. Would this total 3/4" to 1" layer be enough to constitute an adequate subfloor? If so, is there any real difference between 1/4"and 1/2" cement board? I have used 1/2" Wonderboard and 1/4" Hardibacker but prefer the Hardibacker.
My thoughts about cement board are basically what you said, but it brings to mind some additional questions. What about securing the cement board to the plywood with mastic and nails/screws? This is what I've read, but now I wonder how much flex it allows.
I definately want to be sure this is done correctly since I'll only get one chance.
Thanks
Adding another 1/2" layer is acceptable. Layout matters only so that seams do not align between layers. Glue and screw.
I have never used 1/4" cement board. min. is 1/2". Glue and screw. I use PL Premium with 1 5/8" screws.
Don't understand your question of flex relating to the cement board installation.