Remodel/18x18 Porcelain-subfloor prep help
We are putting 18 x 18 porcelain tile in a small screened porch area. The house is 30 y/o, and the joists are 16 OC, with either 1/2 or possibly 5/8 plywood subfloor. My husband got a little over-eager and put 1/2 Durock on top of the plywood. He used Liquid Nails and screws to attach the Durock... The area gets very little traffic, but I'd like to have this right from the beginning if possible. My husband is willing to tear out the Durock and add more plywood at this point. I've read several posts about this general topic but want to confirm opinions... We should put another 1/2 of exterior grade plywood on top of what's already there, correct? That 2nd ply should be screwed, but not glued, to the first layer of ply. We then put *thinset* on the 2nd layer of plywood, under the backerboard. We will go back with only 1/4 backer. Should the thinset under the backer be modified or unmodified? We'd then look at level or flatness of surface and determine whether to use some type of SLC before the tile. Assuming none is needed, we'd tape the joints on the backer (with 1/8 spaces at backer seams) as we apply modified thinset and lay the tile. Does this sound like a plan, or am I missing something? THANKS! Karen Hi Karen, That's pretty close I think. Most manufacturers of backer board claim you can install ceramic or porcelain over their CBU if the subfloor is 5/8 plywood as long as the deflection of the system is above 1/360. That's the catch 1/360 deflection. However, that is bare bones minimum and considering the size of tiles and where located, I certainly would never recommend it. So yes, removed the Durock and start over as you described. Use whatever thinset the manufacturer recommends to install the CBU. Most want modified both under and above. What brand will you be using? There are many other factors I should know, like where are you located? Since it's only screened, it's gonna get wet. Is the floor slopped? If slopped, can the water get out? More later. Before I go much further we need to be sure the subfloor structure is strong enough for tiles in the first place. I need to know the size, spacing and if possible the species and grade of the joists. Also the unsupported span of the joists. That is very important. Jaz Jaz --
A: The joists are 2 x 10, No. 2 (pine...dunno?), 16 O/C. The unsupported span is maybe 8.5 ft., and the screened area itself is about 6.5' x 7'. We are in Arkansas. As for the area getting wet, it is possible but should not be a huge concern. If we had hard, blowing rain, it could certainly get wet/damp. We've had lots of rain recently with no probs, though. The screened porch is open/exposed only on two sides, with two sides protected against interior walls. Also, the exposed sides of the porch are covered by approx. 4-ft. overhangs from the roof. We have deep eaves.
A: Not sure but am thinking of HardieBacker this time v. Durock CBU. Any thoughts? Let me know if you need more info or have other ?'s. Thx, Karen By your numbers those joists are plenty strong enough. You just need to add more plywood. Hardi is NOT for exterior use, so use Durock, Wonderboard or my favorite PermaBase. It is going to get wet, so if you wanna do a real good job I recommend installing Schluter's Ditra over the CBU and also apply Kerdi Band on the seams and the floor/wall intersection to waterproof those areas. ( schluter.com ) Also visit the sites of whatever CBU you end up using. Jaz Jaz -- Thanks for the help. I've read the info on the USG site about Durock and on JamesHardie.com on the HardieBacker. I saw that Hardie was interior-rated, but for some reason was thinking our application wasn't true exterior (i.e., it's all covered and protected). BUT, if we're going to do it right, we should start right. We'll likely buy the thinner, 5/16 Durock board...or whatever cement board is readily available through our tile supplier or HD/Lowe's stores. We need the thinner backer board b/c of adding another 1/2 plywood subfloor and working with transition/threshold at a door. Another question about Ditra -- can it be installed over the plywood, vs. applying it over the CBU? I went to the Schluter site and read some about Ditra last night. I was intrigued, but also had some concerns about ease of application -- or maybe about our understanding the application -- for first-time tile DIY'ers. Thx, Karen I categorize your project as an outdoor installation. Therefore Ditra is installed over cement backer. If it was your kitchen floors then of course, it takes the place of the CBU. There are other ways to surface waterproof and provide an isolation membrane, but Ditra is the best way to go. Some other products are roller or trowel applied liquid membranes such as Redguard, which is available at HD. Laticrete also makes similar products. Jaz OK, we've installed the Wonderboard and have some high/low spots. Since we're installing 18x18 tiles, we want to ensure this is as level as possible. We are priming the CBU w/SLC primer and will be pouring SLC. However, we need direction on treat the CBU seams prior to pouring the SLC. I thought I saw a thread on this but can't find it now. I've ready some John Bridge tile forum instructions, as well, but none are specific to this particular issue. Do we go ahead and tape the seams/joints on the CBU and pour the SLC over that? If we weren't using SLC, I know we should tape the CBU joints and apply thinset as we tile. However, it seems backwards to tape and thinset the CBU joints, then use SLC. OR...can we caulk the CBU joints prior to pouring the SLC? Just want to do this right and am not that familiar w/SLC... THANKS! Karen Actually with Wonderboard brand only, taping the seams on floor installations is not required. We usually recommend doing it anyway. So go ahead and tape the joints prior to pouring the SLC. Be careful not to build up over the tape too much. Probably could just use self stick tape and just pour? Be sure to follow instructions for the SLC to a T. Many people create more humps then they correct. SLC need a little help finding its level and they set up fast. Make sure you plug any holes and cracks or you'll have SLC under the porch too. Jaz Thanks, Jaz. I read that tape is not req'd for the Wonderboard joints on floor installations, but it seemed (is it that seamed? ) to be another why not deal... ALSO, this may be a silly question, but... We will attach self-stick foam insulating tape (1 wide by 1/4 thick) around the perimeter of the room/floor to eliminate gaps and keep the SLC from seeping where it shouldn't. We attach that foam tape to the floor, right? It occurred to me that it *could* be stuck to the bottom edge of the wall vs. on the floor surface. I'm becoming completely anal and paranoid about working with the SLC -- can you tell????? I just don't want us to be in the middle of pouring the SLC and have that oh, no! moment! THANKS for all, and we'll keep you posted. Karen I guess Custom Building Products has determined that floor seams just don't need attention because they'll get filled with thinset and since they are fully supported will be fine? They don't require horizontal seams on walls to be taped either if it's an uncut factory edge. Foam tape or rods are fine for that purpose. Remember, SLC's are made to make floors level, however floor do not have to be level to install tiles, they need to be flat. Of course flat and level is ideal. Problem is that some floors are to much out of level and making them level would create problems at the doorways that are now low. Do you follow? If you decide you can't do level, a patching material would be in order to make it flat. Jaz
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