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How to Prep a Plywood Subfloor for Tile - Fine Homebuilding

How to Prep a Plywood Subfloor for Tile - Fine Homebuilding

By Official Fine Homebuilding Post Oct 22, 2015
For residential use, one form of underlayment the Tile Council of North America’s handbook allows is plywood over joists that are 16 in. O.C. The go-to product is 1/2-in. void-free underlayment grade panels, which both stiffen the existing floor and provide uniform support to prevent cracks in the tile above. Installation is a little counterintuitive though. Carpenters are used to installing all sorts of panels, from subfloor to wall sheathing to drywall so that the panel edges fall on joists, rafters, or studs. That’s exactly what you don’t want to do when installing underlayment. The panel edges should fall between the floor joists, and avoid fastening into them. The idea is to isolate the underlayment from movement in the framing below.
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eason December 31st
I thought!!! here is the one hour video i have always been waiting for.... How to prep your self for a disappointment.
AndyEngel November 19th
All good comments. In residential use, the Tile Council of North America specifically allows ceramic tile to be installed directly over 1/2 in. plywood underlayment in dry locations. Wet locations such as bathrooms require the addition of a waterproofing layer. There's a caveat though. The TCNA refs ANSI A108, and neither of those documents provide details for installation over joists on 24 in. centers. Alternatively, the APA publishes details for subfloor installation it says meets the same design spec (Max L/360 deflection) for a variety of joist spacings and subfloors. And yes, the subfloor is best glued and nailed.
None of this is to say that cement board or a plastic underlayment such as Schluter makes aren't also acceptable, but either of them would benefit from underlayment below, if you have the height.
ANSI A108 calls for 1/8 in. spacing between the panels, and 1/4 in. between the panels and plumbing fixtures or walls. As to gluing the underlayment down, I'm on the fence. A108 calls for it, but doesn't spec what kind of glue or how much to use. The APA document doesn't require it. I like the idea of making construction reversible when possible. Glue the underlayment down, and you've hideously complicated any future remodeling of that floor. I compensate for not using glue by increasing the fastening schedule, and by using screws instead of the ring-shanked nails allowed by all the standards.
Finally, I wouldn't hesitate to install a hardwood floor over underlayment installed in this way. But the Wood Flooring Manufacturer's Association does allow hardwood to be installed directly over OSB subfloor.
enduringcharm November 15th
Two things in this video make me cringe. First, using 1/2 inch plywood underlayment is inadequate by my standards. Typically I'm replacing a previous "mud job" in a bathroom and sometimes in a kitchen, which means I'm making up for much more thickness. More critically, I want to add strength and stiffness to what is often an inadequate subfloor. I aim for 3/4 inch underlayment when possible and I'll settle for 5/8 inch if the transition to another room will be trouble.
Secondly, I would never tile directly over the plywood as this video seems to imply is acceptable. I'll either use 1/4 inch cement backerboard or a proprietary product like Schluter Ditra. In my experience tile applied directly to plywood will be a short lived installation and more likely to encourage rot in areas near a tub or shower. In fact, when doing demolition I cheer if the tile was applied directly to plywood because I know it will come up easily in a few minutes. Cement backerboard or a proprietary decoupling mat provides a better bond and a means to prevent future rot.
AlMack56 November 11th
Does this hold true for hardwood floor I have a small dining room with OSB sub floor nailed 16" o/c I want to provide a better nailing surface for 3/4 7" wide engineered hardwood. I had planned on laying down 5/8 ply underlayment and I don't think the OSB would hold it well and wanted to screw it down hitting the joists. Existing sub floor Floor is over unheated floor space and appears in good condition. any suggestions or comments??
copywriter November 9th
First, you don't explain why the underlayment should not be screwed to the joists. It's often helpful to explain why instead of just in effect make it "because I said so." Second, your very helpful video is overwhelmed by someone's stupid idea of entertaining music. Is louder better? Maybe have your video stand on its merits instead of on some music tech's idea of "cool." I do appreciate the useful video, otherwise.
Mike_Guertin November 9th
Does the plywood and the installation practice shown comply with ANSI A108/A118/A136.1 American National Specifications for the Installation of Ceramic Tile that is referenced in the 2012 and 2015 IRC for installation of tile?
http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI+A108%2fA118%2fA136.1%3a2014
I didn't realize until recently that the building code had a section that covering tile installation. Rather than having prescriptive measures spelled out the code just references the ANSI standard. I haven't wanted to spend the $40 to buy a copy of the standard so I'm not sure what's in it.
user-900674 November 9th
My experience is it is recommended to leave a 1/8" gap around perimeter of room, and 1/16" at joints between underlayment panels.
Also, I agree with above, that new recommendations are to use a cement board over plywood underlayment. And, last; always screw and glue subfloor to joists.