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-   -   Subfloor question (https://www.johnbridge.com/vbulletin/showthread.php?t=77282)

UCbandit22 09-17-2009 08:39 PM

Subfloor question
Is 1/2" ply sub (Over the joist) and 3/4" in Exterior grade Ply, glued and nailed every 4 inches to the subfloor be adequate enough to install 3/8 ceramic tile on? I don't really want to add much more height and i was wondering if it's a problem installing Tile over plywood. I talked to a contractor and he does it all the time as long as the second layer is screwed and glued to the sub.

I just want to hear other opinions!

Dobri 09-17-2009 08:46 PM

1 Attachment(s)
You can install tile over plywood with modified thin set, BUT your problem is the the second layer is glued down. When did you glue it down and how by mean did you just randomly apply the glue? Wjat kind of adhesive did you apply?
The second layer needs to be nailed or screwed down with no penetration into the joist, do you have that?

UCbandit22 09-17-2009 08:48 PM

I haven't installed it yet..it is my next step. I was going to glue it down to the subfloor with liquid nails and then screw every 4" on the sheet to makes sure it doesn't go anywhere.....You say Glue is not necessary?

Deckert 09-17-2009 08:55 PM


Originally Posted by Bandit
I just want to hear other opinions!

Ok, heres mine.

Although there is an approved method for installing over double layers of ply I think its a terrible plan. Use Ditra and you can solve your height problem and end up with a superior floor.

If you want to install straight to plywood, read up on method F160-07 in the TCA handbook, and do some searching here on pros that use that method. I think Bill Vincent had a nice write up about it.

But to be clear, I can't think of one good reason in the world you'd do that.

Dobri 09-17-2009 08:57 PM

The article i posted will help you understand how the underlayment is installed. 2 layers of ply or minimum of 1 1/4 even without knowing the size of your joist and span should be more then adequate enough for ceramic installtion. If you had stone to install i would've ask you to deflection math

Deckert 09-17-2009 08:57 PM

Also, before you do that make sure your joists are suitable for a tile floor. Use the Deflecto tool at the top of the page. Be a lot easier to make corrections if needed without the plywood in the way.

Once it is time for ply, the 3/4" goes down first.

Dobri 09-17-2009 09:01 PM

How big is you floor. Depending on that and your budget you can save money from installing the tile directly to the subfloor (nothing that i will ever do )
If you decide to go with tile underlayment here are some options
Ditra runs about $1.5 per sq. ft plus the cost of modified thin set under.
Hardie runs about $0.8 per sq. ft plus the cost of unmodified thin set and screws or roofing nails

UCbandit22 09-17-2009 09:02 PM

the 3/4 could not go down first...The 1/2 is original to the house and do now ant to take that up due to hot water heating pipe through it...I do not want to disassemble those pipes for this...

bbcamp 09-18-2009 06:08 AM

Rob, the 1/2" subfloor isn't good enough for tile. The 3/4" you plan to add is fine, just screw (ring-shankled nails would be acceptable, but most prefer deck screws) it into the joists, not the subfloor. Do not glue the new plywood to the old. If you must drive screw into only plywood, such as near the walls, pre-drill and countersink the holes in the 3/4" plywood to reduce the possibility of stripping the holes in the 1/2" plywood. Once your new plywood is installed, install an underlayment, such as 1/4" backerboard, or a membrane, such a Ditra or one of Noble's membranes.

All the previous advice you received here is correct. Tiling on plywood is OK, but only if done correctly. Since you cannot remove the 1/2" subfloor, you will not be able to follow the approved method for tiling on plywood. Therefore, you will need an underlayment.

Dobri 09-18-2009 06:33 AM

Bob, according TCA the second layer of subfloor IT'S NOT srewed or nailed down to the joist. I posted article in post #2.

Quote:The 3/4" you plan to add is fine, just screw (ring-shankled nails would be acceptable, but most prefer deck screws) it into the joists, not the subfloor

bbcamp 09-18-2009 06:45 AM

Dobri, you are correct if I was advising a direct to plywood method. My advice basically treats the 1/2" subfloor as a spacer. I was trying to work around the 1/2" plywood as the bottom layer. A single layer of 3/4" plywood and an underlayment is acceptable without regards to the plywood underneath.

Dobri 09-18-2009 07:05 AM

I understand the spacer idea but adding a second layer ply is treated as a second layer from the TCA, correct? I remember calling Ditra and ask about how the second layer ply under Ditra should be installed and they said not to be nailed to the joist

UCbandit22 09-18-2009 07:27 AM

thanks for the great advice...So I will NOT glue down the 3/4 ply to the 1/2 sub...but now i don't know if i should screw to the joists or not....I decided that I will be using 1/8 Ditra, if that does make a difference in the final answer

Dobri 09-18-2009 08:28 AM

I just call Ditra and ask about your situtuion and they said first layer of 1/2 inch needs to come out. I was right, the second layer they don't recommend to be screw or nail down. That's if you are going with Ditra.

UCbandit22 09-18-2009 08:33 AM

well..the first layer cannot come out....could i still use Ditra

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