Just now getting to replace a plywood subfloor for a bathroom that will receive ceramic tile (after 1.4″ CBU is put down). I was planning on putting down two layers of 5/8″ CDX (first layer running perpendiculaar to joists and the second layer parallel.) Is CDX the correct grade of plywood that I should be using?
Also the bathroom dimensions are 5’6″ x 9′. I’ve been made to understand that there are some minimum widths (i.e. 24″) of plywood that should be installed to ensure a structurally sound floor. Any other tips along these lines before I get started?
Thanks,
Tark
Replies
CDX will work fine.
I would run you second layer perpendicular to the long axis of the room. This will give you two off-cuts of approximately 2'6". Use these to finish off the area under the tub/shower.
It's good to avoid short rips of ply but since your tile will likely only cover 6'6" of the 9' depth don't have anything to worry about.
Glue down with either construction adhesive (good) or PL Premium (better). You will have a solid floor.
Jon Blakemore
Thanks Jon
Glue between layers too
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Typically, one wants about an inch of plywood below the typical tile floor. That depends of course on your structure below that plywood. Give me your joist size, and unsupported span length, and I'll tell you if the floor is structurally sound for tile.Once you have your plywood size put to bed, you need EGP plwood. The rest may swell. I happen to like inch and an eigth t&g ply--it is just awesome to install and solid as a rock. And you only need one layer.One you have your ply picked out, nail the ply to the joists running against the direction of the joists for the 1st run. Glue if you want, but its not necessary. Blocking between sheets adds structure and strength.If you have a second layer, nail it in the opposition direction. If you use my inch and an eigth, you won't need any 2nd layer.You said an inch and half CBU? Was that a typo? CBU is in nominal half inch thicknesses (Hardibacker is actually about 7/16ths). You should only need one layer of CBU, not three. The CBU will not add any strength to the construction. If you have to make up that kind of space, I think I would float a mud bed. Its about a quarter of the price of CBU, probably 10% if you are doing 3 layers, and you'll end up with a dead flat and level floor, something I can't guaranty with ply and CBU. Do not nail the CBU over the joists. Nail them to the ply between the joists. It needs to be installed over a thin bed of unmodified thinset to cushion the CBU. The CBU should be spaced about a 16th apart. Tape and mud the joints as you lay tile, not before--avoid speed bumps. Some manufacturers don't want or need tape on floors, but I do it anyway.Good Luck Buddy!Regards,
Boris"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
Hi Boris,The joists are 2x8 (1.5 x 7.5, 16" o.c., with an 11 foot span. I did check johnbridges deflectometerand it seems that the span should be fine, but if you think different, I'd be happy to take in your opinionI will check into the availability of EGP plywood at the local yard. If I can't find EGP, would any other inch and an eighth plywood be fine?After writting my original post, I realized that I have a copy of the TCA's 2003-2004 Handbook. Method F144-03 is pretty much the method I plan on using, although it seems to indicate only a need for one layer of 5/8" plywood. I still plan on either two layers of 5/8 or the EGP plywood you have suggested. . .As for the CBU, I meant a quarter inch to minimize any height at the door transition. Having had the benefit of reading your advice for the last several months, everything else you've written is already well entrenched in my notes.Thank you for the advice and Happy New Year!Tark
Edited 12/31/2004 2:01 am ET by tark
What's EGP?
Jon Blakemore
Tark I would say your on track with the info your getting. The only thing I would do different is use some const. adhesive over the joist before nailing, if you used nails which would be ring shank. However I would use galvanized deck screws and not nails. Then the CBU would be half inch not quarter inch. I only use quarter for walls and install the CBU over a thinset with additive, usually comes pre-mixed, spread with a quarter inch square notch trowel then screw the CBU down with CBU screw. Spread some thinset over your joints and some mesh tape one more layer of thinset to smooth out the joint and you're ready to go.
John:I wish to correct something that Santi said, and that is that half inch is for floors and quarter inch is for walls. With all respect, Santi has it backwards. Quarter should only be used on floors, and half may be used on either walls or floors. Actually Hardi says quarter can be used for walls or floors, see
http://www.jameshardie.com/backerboard/homeowner/hardibacker.phpBut in my opinion because it is unsupported between studs, it has too much flex. Quarter inch on floors is fully supported by plywood (no voids between studs, OK?) and perfectly fine on floors.If you want to minimize elevation gains, and lose another eighth inch, consider using a membrane like Nobelseal, one of the PVC clones, or Schulter's Ditra. These are applied directly over the plywood over unmodified thinset. You tile right over these membranes.Good Luck and Happy New Year.Regards,
Boris"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
Hey Boris, no disrespect, but if you're going to correct me get your facts straight. I never said half is only for floors or quarter only for walls. I was simply explaining to Terk how I would do the install, and I don't have it backwards. The ultimate decision as to wether to use 1/4 or 1/2 really depends on the thickness of the actual subfloor. For a ceramic tile install you really want the most solid subfloor as possible and the install method I described I feel is solid. I myself have actually used both 1/2 and 1/4 CBU's for floors but have only used 1/4 when then subfloor was greater than 5/8 or 3/4 and properly fastened to joist. Again don't take this personal I was just giving my opinion of how I would do this subfloor and underlayment prep for ceramic install.
Happy New Years Boris
I don't take nuthin personal. I don't use quarter inch for walls because it flexes. Hardi says its OK, but ya gotta do a quarter inch install and push against it to understand. It actually flexes, so quarter inch is a no-go for me on walls.You are perfectly OK and within Hardi's warranty to do so, so I guess that legally your advise was OK, but I won't put quarter inch on walls.We agree that either quarter or half is fine for floors. Regards,
Boris"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
I agree with you 1/4 does flex on walls. If you I were going to use 1/4 on walls it would be over green board as a base. Otherwise I would also use 1/2 on walls if was going directly over the studs with a layer of 3 mil for added moister protection. Opinion well taken. I'm always looking to learn from someone else.
The man who thinks he knows it all really has a lot to learn.
We are on the same page now.Regards,
Boris"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
Boris: Do you ever thinset floor tile directly to plywood, say AC? It's done this way often in my area.
Al Mollitor, Sharon MA
santi ... my spec sheets say that durock & hardibacker have no deflection strength.. so if you are trying to reduce flexure.. increase the thickness of your subfloor or decrease the spacing on the framing..
the thickness issue becomes which one will bring your finish closest to the desired level
but, hey, whadda i no ?Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike, I didnt know we were getting so into this whole tile thing. Your are right about the deflection I never called into judgement the tolence or deflection on durock or hardibacker. If you check some specs for min thickness of a setting bed to recieve ceramic tile you will find that there is a min requirement. The setting bed should be a min thickness of 1 1/8 in. thick. This can be done with the 1/2 in. plywood to the joist and a second layer of 5/8 to that layer or use the hardibacker or durock for the second layer.
But what do I know ?
yes... that 1 1/4 seems to be the magic number... and they don't give any value to the hardibacker or duroc......
since we usually have a 3/4 subfloor, we follow with 1/2 uunderlayment and no hardiMike Smith Rhode Island : Design / Build / Repair / Restore
No way do I thinset directly to ply. Custom's site says its OK, but in my some 27 years experience, the application cracks. If you decide to do this, use a super modified thinset, like Ultraflex by Custom.I have thinsetting directly to ply but only using a thin membrane, like Nobelseal or Ditra.Regards,
Boris"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1934
Thanks, Boris. I have a bathroom remodel coming up, and I'll be sure to discuss this in detail with my tile guy.
Al
Boris-- Actually, tile over plywood is a VERY safe installation, so long as you know what you're doing. 99% of my residential installations have been over plywood, and have lasted for years and years. However, the thinset you cited wouldn't be good enough. It has to be unmodified mixed with an additive. my favorite is Mapei's keralastic/ kerabond. SO long as your subfloor is prepped with a plywood to tile floor in mind, it'll work.
Bill Vincent
http://www.creativeceramicandmarble.com
Edited 1/1/2005 11:54 pm ET by Bill
Not for nothin, but I have to agree with Boris. You're better off building up the floor with plywood for the elevation, and using 1/4" CBU, being that CBU has no structural strength, whereas plywood does. The only reason for using CBU at all is as a more stable bonding surface.
Bill Vincent
http://www.creativeceramicandmarble.com
Edited 1/1/2005 11:55 pm ET by Bill
Edited 1/1/2005 11:56 pm ET by Bill
Here you go. Try this. Put down your plywood build frame around the outside within the studs, put a rebar grid in, open the window and have a concrete truck pour in some cement with a psi of 4500 or better to your desired height and you'll be ok. How is that for a solid base. Can we all agree this would be a satisfactory base to install some ceramic tile.
Holy Cow what a tough crowd.
Santi-- I'm not trying to give you a hard time. I'm just trying to say that although yes, manufacturers say go ahead and use either one, you're better off with the 1/4" and building the rest of your height with plywood for a stronger floor.
(besides, then you'd have to use lightweight concrete unless it's slab on grade, which is problematic as a tile subfloor. :) )
Blame it all on that guy "Billy". I was going through my stat counter, and wondered who'd quoted my website on a forum where I hadn't been yet. :)
Bill Vincent
http://www.creativeceramicandmarble.com
Edited 1/2/2005 12:16 am ET by Bill
Hey Bill,
You've gotta hang out on more than two forums, right? Good to have you here.
But now you have started up the thinset to plywood debate. ;-) Boris ain't gonna like it, and too many people will be tempted to try it with the wrong thinset or with m-m-m-mastic. It seems to me that it is safer to put down 1/4" Durock first, but apparently your installs have worked fine. Do you bond directly to plywood in bathrooms and wet areas as well?
Happy New Year!
Billy
Hey Billy--
Bathrooms, yes. Countertops, yes. Wet areas, such as tub decks, no, and never on walls (there's no need for the strength that plywood provides). Once again, I'll say it here, as I eluded to in my first post, and I've said in every other thread in every other forum that it's come up-- It's a risky installation method if you're not absolutely sure of what you're doing, which is why so many pros have even had problems with it. On my website page that you referred to, there's a link to an excellent article by Dave Gobis outlining alot of the things to watch out for when tiling over plywood. But just as a fer instance, the thinset recommended previously in this thread would have definitley put the installation at risk. Yes the bag says you can use it over plywood. It might even last a year or two, but no more. That's just one of the major issues with tiling over plywood that can cause problems. There are several smaller issues, as well which must be addressed when tiling directly over plywood that aren't as important, or aren't even issues when tiling over CBU, and any one of them could sink the installation. One was brought up by you in the beginning of the thread-- the use of CDX plywood. AS you said, there are way too many voids in it, which could cause failure. Even the SPECIES of the plywood is important. Fir or spruce are best, and if you were to use yellow pine, the tile would lift as if you'd first sprayed the whole floor down with wd-40. These are just a FEW of the reasons why alot of people have problems when setting over plywood.
Again, I'll reiterate-- if you're not absolutely sure of what you're doing, do NOT tile directly over plywood. Add the CBU, but use the 1/4" CBU, and use the plywood for the rest of the elevation to beef the floor up.
Oh, and just for the record, this makes FIVE forums I go into now. If I keep going, my wife is going to forget what I look like!! LOL
Edited 1/2/2005 10:51 am ET by Bill
"Oh, and just for the record, this makes FIVE forums I go into now. If I keep going, my wife is going to forget what I look like!! LOL"
I know what you mean! These things can chew up lot of time. Like many places, there's a good group of people here. Tile & stone questions pop up every day or two.
Billy
Edited 12/30/2004 11:19 pm ET by Billy