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Putting down 1/4″ Hardibacker over 3/4″ T&G decking (2×10 joists on 16″ centers). Instructions/books say to tape the joints with F/G tape and mortar which I have done. My question: how thick does the mortar really need to be over the joints? Haven’t read anything about that anywhere. What is the purpose of the tape exactly? I have several ideas, but thought some of you might know for certain. Any “links” to further info on this?
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Crusty,
I had the same question when I started using the backer board. It only needs to be a very thin layer. The theory is that it stabilizes the backer board and prevents the seams from showing through in the form of cracks. We did a very time sensitive job and we wanted to keep working after setting the board without having to step around wet seams so I called James Hardie and they said you could install the tape when you lay the tile so that's what we do now. Hope this helps.
Eric
*CrustyYour backer board should have a depressed edge like the edges on a piece of drywall........just keep those tape joints flush with the face of the sheet, and keep the butt joints as thin as possible while still covering up the tape.......hope this was some help... ........SW
*Purpose of the tape and thinset is to laminate the CBU and make it stronger, by attaching to it's neighbor. The thickness should be very thin. You do not want a buldge at the edges. Make sure the two CBU's have some spacing in between them, like about 1/16", to allow for expansion.
*Sandy,I must admit that I've never seen a cement backer board that had beveled edges like drywall.What brand are you refering to?James DuHamel
*Wonderboard I think has rounded edges, maybe even tapered. But the Hardibacker I'm using appears to be flat. Sounds like I'm doing it right. I fill the 1/8" or so joints with mortar, put down a very thin layer, bed the tape, and cover it with another thin layer, tapered away from the joint. Corners get a little more cause they don't lay down flat since you can't put a screw too close to the corner. But again I taper it quite a ways out.But wait, if I fill the joints with mortar, how is it going to expand? Maybe it'll just contract because I'm keeping the house fairly warm and humidity is very high right now (lots of rain lately).Still can't see that this adds much strength. Must be a reason for it though. (To sell tape?)
*Which ever cement board product you use, the purpose of embeding tape and anchoring it with the thinset is to minimize or eliminate movement between the adjacent sheets. After all, the cement board is being used over a wood subfloor that will move somewhat over its life and it will move the cement board with it. At least when the cement board is solidly joined it will (or should) act as a single unit thereby preventing grout or tile cracking because the individual sheets were joined to preclude independent movement at the whim of the subfloor.So there.
*Think of drywall. Same principle.A drywall joint that has only mud and no tape will eventually crack.A drywall joint that has mud and tape will remain stable and unblemished.If you don't mud and tape the joints in the floor's CBU, you'll likely develop a crack in the tile's grout line that falls over the un-mudded and un-taped joint. For a non-reinforced CBU joint, a crack in the grout line is more likely with small tiles with many grout lines. Cracking is less likely with larger tiles with fewer grout lines.
*James....I think Durock is the brand name...looks like a piece of drywall thats made from cement
*Makes sense... guess I'll keep on doin' what I'm doing.One more tile question. What size notch do you use for 12" tiles? 14" tiles? I used a 1/4 x 1/4 for some 8" tile I was "learning" with and the mortar really didn't seem to be thick enough to allow much adjustment. I'm thinking that 1/4 x 3/8 would be better for the bigger tile.
*You want whatever size of knotch gives you 80% coverage.Here's how: Start with 1/4". Comb out the thinset. Slap down the tile and firmly set it. Then lift it off, and look at the back. Is there thinset on at least 80% of the tile? If so, you are fine. If not, switch to a bigger trowel or back butter the tiles.
*I'm getting a good bond on the 8" tiles with the 1/4 x 1/4 trowel, but I think I'll need more wiggle room to keep the 14" tiles level. That's why I thought the 1/4 x 3/8 might be better. That's also what it calls for on the thinset bag for 12" and larger tiles. Well, on one brand anyway -- the Versabond just says 1/4 x 1/4 period.
*Also, I read something about "expansion joints" on larger rooms. Is this really necessary?
*Crusty,Pick up a copy of the TCA Handbook for Ceramic Tile Installation (www.tileusa.com). It provides a summary of the ANSI spec, which explains the necessary requirements for a successful installation.To answer your question, expansion joints are critical to a proper installation. Joints are required whenever tile meets a dissimilar material, i.e. a perimter wall, and also where changes occur in backing material. Expansion joints are also needed in floors every 24-36 feet or every 8-12 feet if the tiles are exposed to direct sunlight or moisture. Expansion joints must then be sealed with an appropriate sealer, typically silicone (NOT grout.)Failure to include expansion joints per spec can result in cracking of tiles and/or grout.-Andy
*I put expansion joints along the perimeter of every room. No big deal, just a 1/4" gap, filled with foam rubber rope and Nobel 151 Sealant or Silicone Calk.
*I've learned to tape as I lay tile so no humps in surface and no tendency to cheat on thinset in the joint area. If tile is to follow in your application aren't you concerned about 1" thickness?
*Randy, everything I've read calls for a minimum 5/8" underlayment, 1/4" tile board, and then tile. Is that not correct? Floor is framed with 2x10 SYP on 16" centers, with 3/4" sturdifloor (T&G) underlayment. Is this not sufficient? I am screwing the heck out of it with 3"-4" spacing at the edges and 6" in the field, using cement board screws with hi/low threads. Seems plenty stiff to me, but I could of course be wrong. Your idea about taping as you go has some merit. Any others out there do the same thing?
*CrustyI usually tape as I go also.On thickness of the floor, I think the TCA calls for minimum of 1-1/8". Anyway that's the number I use.I've never used Hardibacker, but did you bond it to the underlayment with thinset like we do with Durock? Or doesn't Hardibacker call for that?Barry
*Crusty,Your subfloor sounds just fine.. ANSI spec calls for minimum 5/8" exterior-grade ply, nailed every 6" o.c. at edges and every 12" o.c. at intermediate supports.Barry,For Hardibacker, the subfloor requirements are the same as for other CBUs, and you bond it to the subfloor with modified thinset.I also tape as I go... particularly to avoid setting tile over any areas that didn't get flattened down before they dried.Merry Christmas!!-Andy
*Real tile men float everything.
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Putting down 1/4" Hardibacker over 3/4" T&G decking (2x10 joists on 16" centers). Instructions/books say to tape the joints with F/G tape and mortar which I have done. My question: how thick does the mortar really need to be over the joints? Haven't read anything about that anywhere. What is the purpose of the tape exactly? I have several ideas, but thought some of you might know for certain. Any "links" to further info on this?