Ok, now that I have your attention… as some of you know I earn my living as a frame carp…however we all like to dabble now and then. I just finished remodeling my kitchen and the bathroom is my next victim. Here’s the scoop. I tore out the old mortar bed which held a bed of tiny mosaic type tiles. The floor appeared to be in decent shape as far a cracking goes, but rather than tile over I tore it up because I didn’t want any more height (it was already 1 1/8 above the subfloor) The sub floor is 1×8 planks (not t&g) If I glue and screw 1/2 acx ply and lay 1/4 cement board with thinset over that, tape seams etc. Will my 6×6 tiles be on a suitable bed? The floor is only 25 sq feet on 2×8’s. Tough to say if my deflection is within the 1/132 or whatever it is that is recommended. How do you even measure 1/132 worth of defection???
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Replies
Diesel-
I used the 1/4" cement board in my bathroom over the 1x6 diagonal subflooring tat was there. I spread ttile mastic over the subfloor and nailed the cement board at about 6" o.c. both ways with galvanized roofing nails. The random-pattern 6x6, 6x12, 12x12 tiles were then set in thinset, and after 3 years, there's not even a hairline movement in the groutlines or the tile.
Bob
Personally, I would bump the ply up to 3/4" and screw the backer board down on top of a thin layer of thinset. Try to screw ply to joists. Gluing is a plus.
No, not really, that's not a suitable bed. Perhaps it's just a typo but you made no mention of a pan or membrane?
Also how do you plan to rake the tiles towards the drain show that the water doesn't pool up in places. I've seen some tradesmen who aren't tilesetters do tile jobs where they have used thinset like it was a thicker mortar bed to get that rake but that's not recommended or approved at all and could even possibly void a warranty. What exactly were you planning to do in that regard to get the pitch?
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"Function is based
on more than utilitarian factors. Ambiance invites use."- Sarah Susanka
i think he is talking about the floor not the shower jerrald.
james.
about the floor i would use 3/4 ply screwed ( possiably glued) to the joists with hardie backer bedded in thinset, then tile.
Hey on reading your post again I began to think that maybe there was no shower in this bath? I heard "mortar bed" and a shower imediatly came to mind so maybe I jumped the gun. Although we do install membranes with all our floors anyway for crack isolation and/or prevention and water proofing protection (nobleseal). Hey while it's not a shower floor a bathroom is still wet and also a place where leaks can happen and cause damage.
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"Function is based
on more than utilitarian factors. Ambiance invites use."- Sarah Susanka
Thanks to all for the speedy responses... Jerrald, it's just a floor, no shower pan. You still think I should use a waterproofing membrane? And back to my originial question, in your opinion will the 1/2 in ply over sub with cbu's over that be sufficient?
You don't have to use a membrane but it is a real good specification and selling point. It not that much more time and material, while it does make a signifgantly better "high-end" installation. I realize this is for you own home but I use that specification as a selling point when I am in a competitive bid situation to help sell our bid.
There is an article I think from JLC that presents a real good argument for using a membrane everywhere. I'll see if I can run it down for you. It may have also been from one of my stone or tile trade magazines too but for some reason I'm pretty sure the article I am thinking of was from JLC.
I'll be back...
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"Function is based
on more than utilitarian factors. Ambiance invites use."- Sarah Susanka
Thanks again for all the good info. If I can hassle you one more time... should I use an acrylic admix for the thinset between the ply and the cbu? Or just thinset?
That was quick huh? Yup it was Why You Need Membranes Under Ceramic Tile November 1987
By Steve Thompson. Are you a JLC Plus member or do you perhaps have a library
of old issues around? Let me know. I'll also give a look though my other trade
mags to see what I can dig up although that will be a lot slower.
With regard to "...in your opinion will the 1/2
in ply over sub with cbu's over that be sufficient?" With each
layer glued and screwed it sounds like it should be okay. Those 2x8 joists
are how long and where along them is the 25 sf of bathroom located? Obviously
in the center there is more "bounce" than right next to the bearing
wall that holds them up right? Also are they 16 in. O.C. I'm assuming too?
Hey
if you are in any doubt at all about cracking even more reason to use a membrane.
As for acrylic admix we use it everywhere. I have heard there are some reasons
not to use it but and there are some applications where it may not be necessary
but I haven't personally heard those arguments yet.
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"Function is based
on more than utilitarian factors. Ambiance invites use."- Sarah Susanka
should I use an acrylic admix for the thinset between the ply and the cbu? Or just thinset?
I would consult the mfg's instructions for whatever CBU you end up using. Some want latex modified thinset, some want "dry set" (aka un-modified thinset). It's usually pretty explicit in the instructions that come with the board, but if you have any doubts most mfg's have excellent websites and e-mail support.
Now, does it really matter? Probably not, but the mfg's design the products to perform to their maximum capability, so I try to do what I can to observe this. Also, if it's not installed to mfg spec, they may not honor any warranty. This can be significant; I know GP offers a lifetime guarantee on installations with DensShield...
As long as you're glueing all layers and screwing, it'll be fine. I'm doing the same with 5/8" ply right now. No give to notice at all with just the plywood glued down..
Excellence is its own reward!
>> How do you even measure 1/132 worth of defection???
With a dial indicator, clamped to something that doesn't rest on the floor. Here's a picture of one with a magnetic base.
http://www.gwrauto.com/phoenix/Dial_Indicator_with_base.gif
The 2x8 over 25 feet is not real kosher. The mud bed probably kept the cracks down to a minimum.
Soooo, I think there are a couple ways to deal with this problem. The best way and another way.
I think the best way would be to get the exact type of wood (Doug Fir, Pine etc) and post the span at JohnBridge.com. There is an engineer on staff there who has the span tables by species, load, size, and length who can help. I don't think your joists cut it. So you'll probably have to rip up the diagonal decking and bolster those joists. Sister some 2x10's to them, blocking, supports underneath, etc. I'd use inch and an eigth t&g plywood subfloor, called Sturdi-Floor in some necks of the woods, and then add heavy duty 3.5 diamond lathe and another mud bed floated on top. Then tile. With those skimpy joists, I wouldn't guaranty the job, but blocking, the HD t&g subfloor, and a mud bed would go a long way to giving you a good setting bed.
The s h i t t y way would be to forgo the joists. Heck, they lasted this long, anyway, right? Rip out the subfloor, use the inch and an eigth ply and the mud bed. You'd probably be OK.
By the way, another poster suggested that you screw the backerboard into the joists. Do not do this. This is against the TCA guidelines. You actually want the CBU's to be nailed to the floor, not the joists so it sorta floats, and will not be moved by the joists moving up and down and sideways etc. Unmodified thinset goes underneath to cushion and support the CBU. But as I said, on this wimpy floor, I wouldn't touch backerboard with a 10 foot pole. The mud bed with lathe is way, way stronger.
Hope this helps.
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927
Boris I don't think the span he's talking about is 25 feet. I think the 25 feet was the total square footage of the installation.
However I did just tell him to screw his floor sandwhich together however you are 100% correct that it SHOULD NOT BE SCREWED TO THE JOISTS. Do not screw the sandwich assembly through to the joists. I was not clear on that at all. Screw the ply and backer board to the subfloor. Good thing you brought that up.
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"Function is based
on more than utilitarian factors. Ambiance invites use."- Sarah Susanka
Tough to say if my deflection is within the 1/132 or whatever it is that is recommended. How do you even measure 1/132 worth of defection???
The other posters have all offered good ideas, so for lack of anything better I thought I'd point out that TCA spec for deflection in floors is 1/360, not 1/132..
And that's just about a RCH. I think, could be less...........or more.
Joe H
I advised screwing the ply to the joists. Not the CBU. I always screw and glue the ply to the joists, and believe this is the correct way of doing it. However whenever we do a complete bath remodel, the entire floor goes, including the sub floor. We then can see what we have and how bad the joists are and if they need to be replaced, sistered or whatever.
Regards"One measurement is worth a thousand expert opinions"
I should read these posts, huh? Yeah 25 SQUARE FEET is not much of a span. And yes, screw the plywood to the joists; do not not screw the CBU.
I would still remove the mortar bed and re-install it. Whether you want to beef up the joists and/or subfloor is up to you. Your old mortar bed obviously worked fine and did a splendid job. I would replicate it. If you wanted to use CBU, I would beef up the plywood considerably. I like the inch and an eighth stuff. I bet 3/4 t&g would be fine.
Regards,
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927