Help with Tub Surround Prep [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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03-04-2007, 08:58 PM
Hi all. I am in the midst of a big home addition which includes a kids bathroom which I am going to prep myself. This bath has a cast iron tub recessed into a little 5x3 alcove which we will be tiling. When the Cast Iron tub was already in place I installed poly to the studs then I had the boarding crew (that hung the blue board for the plasterer) install perm-base CBU around shower enclosure. If I had to do it again I would have had done it myself because the guys that hung it where not very accurate. Regardless, it is in place and the plasterer feathered into the (first couple inches of the) CBU nicely. Now I have the following questions that I am hoping the experts can help me with.

1. I did not caulk the poly to the flange of the tub and I wish I did. There is still about a 1/2 gap between where the CBU stops and the tub starts so I was planning to load it up with silicone caulk. Is that the best thing to do? Or should I tape and RedGuard? Or should I tape and apply thinset?

2. For all the of the seams (corners and where two boards meet) I was planning to tape, then thinset (Flex Bond), then RedGuard just the seams. Should I tape the seams and just use the RedGuard. Some of my buddies who are in the business swear that I should just use RedGuard and tape to take care of the seams since they claim the stuff will be more flexible and prevent cracking in the future. They all agree but I have not heard of anyone else doing this.

3. I am aware of the warnings that you should not RedGuard if you use poly behind the CBU. Is it OK to just RedGuard the seams?

4. In the same room I had to cut in 3' x 2' patch in the plywood subfloor in order repair a real bad patch left by the plumbers. Mine is a little better. But I still feel a little movement where the plywood extends between the I-Joists (spacing varies between 12" - 16" OC). I can rip it out and add some blocking but I glued it and fastened very well. I was planning to throw 1/4" Hardi Backer over plywood and I suspect that should eliminate the little bit of movement if I step right on seam in the subfloor. However I am obsessive compulsive and was wondering if you guys think I should use 1/2" Hardi Backer or even rip up the piece of subfloor again.

Thanks for your time,

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03-05-2007, 07:53 AM
Hi Joe,

I'm not a pro but know enough to be dangerous :monster: So here's my take FWIW:

1. The CBU is going to get moist, so you want to direct that moisture into the tub, not the walls. Taping and thinset/reguard will probably do the latter to some extent. I'm not understanding your poly situation. Was the poly cut flush with the CBU or is it hanging low enough so that you could still caulk it to the tub flange? If so then great, do that. If not then you should try to add a waterproof layer to channel moisture from the poly into the tub. Not sure how "permanent" your CBU is in place due to plastering but maybe you could loosen it near the tub enough to install a ~3-6" high layer of poly behind the existing poly to bridge the gap (think roofing shingles) or maybe you can do something similar with 100% silicone caulk, BUT it would have to be behind the poly because otherwise you will direct the moisture into the wall by "plugging" the path for moisture to follow into the tub (a picture would help - I hate to mention this option without a visual.)

2. Your method is what I would use - thinset for the tape (mesh tape NOT drywall tape!) then redguard. I have never heard of applying tape with redguard, maybe you can, but I wouldn't unless a pro here chimes in with approval.

3. Yes just redguarding the seams is OK when a poly barrier is behind - there will be no "moisture sandwich" because the moisture will be able to migrate out of the seams into the rest of the panel.

4. I would like to know more about this before commenting. Do you have a picture or diagram? I would be comfortable if the perimeter of the patch was supported by blocking spanning the joist width and secured to said joists. The hardi isn't going to serve as a sufficient structural layer. I've also mentioned here before that I think 1/2" and 1/4" Hardi have similar properties (I think 1/2" is simply an expanded [less dense] version of the 1/4" stuff) but noone here has commented on this. Either way, don't count on Hardi for structural support, OK?

03-05-2007, 08:06 AM
One more thing, with a 1/2" gap to tile over make sure your tiles are large and durable enough to span that joint. I can't tell you definitively what size is too small but I would try to stay away from anything less than 4"x4" in size.

Please share more with us before getting worried, it may all be OK and there may be other options not mentioned yet. If you want a water tight shower enclosure you could always Kerdi over the Hardi but it would have to be done very well to prevent moisture from getting into the Hardi and becoming trapped between the poly and Kerdi. I would discuss this one here before moving forward, to get some other opinions...

03-05-2007, 08:13 PM
Thanks Mike,

For the Poly situation, when I applied the poly I tucked the remainder behind the perimeter of the tub (I do not know what I was thinking) so I think can just cut out a foot of the CBU around the perimeter, fix up the poly and caulk, then path in anther piece of CBU (this would also let me get the CBU tight to the tub).

Regarding the floor I will try to post a diagram but essentially I cut a patch in to span 2 bays (width of the patch is about 18"). The first bay was a 16" space between two IJoists and I blocked the perimeter and a couple in the middle. The second bay is about 12" and I relied on my buddy to block in that bay and glue and nail the plywood. He decided to omit the blocking but did a great job with the glue and nails. Of course the bay I blocked is fine but the plywood around the perimeter of the patch in the unblocked bay flexes. I can up with the following options.

1. Cut out the only the patch that is over the unblocked bay and replace it (18" x 12"). This would not be a lot of effort since the other side is the one with all the blocking and all the glue and nails. However is it OK to have so many patches?

2. Just re-patch the whole area. It was fastened with the intention it would not be removed so this will be a pain but not the end of the world.

3. Put 1/4" plywood then 1/4" hardi. This may benefit other areas since the cause of the original issue was the plumbers beat up the subfloor pretty bad patching and another 1/4" may add some more rigidity.

Also I have one other fear. The boarders hung the board and a gave them a box of CBU screws. They used them but I suspect they ran out then I noticed some other screws being used in some areas. I feared that these were drywall screws and asked the plasterer that oversaw the job (who is a prey honest guy) and he said that they were absolutely anti corrosive screws. I still think they may be drywall screws. What should I do? Remove and re-screw (in different spot) or don't worry about it and just thinset and RedGuard all the screws? Is there anyway to identify a screw as anti-corrosive and does that really mean anything. Again I should have just done this myself from the beginning.

Let me know what you think.


03-06-2007, 10:50 AM
Hi Joe,

I'm not going to say that your CBU/poly construction is doomed to fail. It's just not the proper way to do it. I just demoed a 40 year old tub surround with mud walls and no waterproofing, shower was used regularly and no signs of water problems. But I say, why not do it right now vs. worrying about what might go wrong in the future, right?

Your idea to remove CBU near the bottom of the tub would work. If you choose to do that leave a 1/8" gap between the CBU and the top of the tub lip. Try to stuff your new layer of poly behind the old one so that there is at least a 2" overlap (shingle approach), this time let the poly drape down INTO the tub :D then install the new CBU piece. Then I would caulk the poly to the tub and after the caulk dries, trim it a little long past the caulk joint (that's the way I do it anyway.) Thinset, tiles and tile/tub caulk joint will hide the poly. Tape all your CBU joints and you're good to go. By the way, redguard is not necessary at all if you go with this approach assuming your poly overlaps in the corners and unless you need to waterproof something like a niche. Using a dab of redguard over screws will not protect them from moisture.

Regarding the screws, if they are black they're probably drywall screws (you probably knew that already.) There are colored screws made specifically for CBU (Hardibackers usually are green, don't remember the others) and those would be OK. Galvanized screws are usually a dull silver/grey. Maybe this helps? If you're convinced you have drywall screws you could remove and replace them or drive new, appropriate screws inbetween the existing ones per CBU screwing schedule. Putting thinset or redguard over drywall screws won't help prevent corrosion.

Having said all this, I'm wondering how easy it will be to remove sections of the CBU as you propose? Another option that might remedy both issues discussed so far (poly installed wrong and drywall screws holding up CBU) is to Kerdi or Redguard the entire enclosure. This is especially appealing if you have access to the poly from another wall (you mention it is an addition) and you could slash and/or remove poly between studs. If not then this idea comes with the warning that you would have two vapor barriers that could potentially trap moisture in the CBU, and this not a recommended approach. However if no outside walls are involved and your waterproofing is done properly that shouldn't be a moisture prone area anyway, I would think, assuming you don't have wild temperature fluctuations between walls. If you're interested in this idea and can't cut the poly from the opposing side of the wall, I would bounce it off some of the pros here before moving considering it.

Finally, the floor. It sounds like at least the area your friend :shake: patched needs to be cut out. I would redo the whole thing if it's not too much trouble. Hardibacker won't contribute much support regardless of thickness. I hope the 1/4" ply was a typo? 5/8" would be the minimum to consider for a subfloor, depends on flooring material and joist spacing/span (use deflecto on this site).

03-10-2007, 10:48 PM
Thanks again,

I ended up pulling off the cbu, sealing the poly to the tub, and re-fastnening with the right screws. I also fixed the floor appropriately. I am now going to tape and thinset the joints (then RedGard). Also I did not tape the deams in the poly (I did overlap) do you think that makes a differenece? I am still thinking that I should RedGard all the interior walls as a precaution.

I have one finally concern and that is that the crew that boarded the tub in that bath also did a shower stall in another bath.

- I am sure that they did not take the poly into the pan (which I will fix).

- They did not take the cbu to the bottom of the copper pan (they left about 2 inches of space I am assuming for the tile guy to form his bed. But I am not so sure anymore. What do you think? I am also going to get my tile guy in to take a look.

- These guys used drywall screws to screw the cbu to the threshold of the shower stall (screwing through the copper pan with drywall screws). I am planning to removed and use some galvanized exterior screws.

- This shower is mostly glass but the two walls that are going to be tiled are interior walls. Again I am thinking that I may kerdi or RedGard this entire shower, even though there is poly (seams not taped). Especially since there are a bunch of jets in this shower.


03-10-2007, 10:52 PM
Hi Joe, putting the Redgard on will create a moisture sandwich in where mold can form. Either remove the poly or fergit the Redgard. As for the 2" gap, prolly will be fine once the final mud bed is done. There will be about a 1/2" gap or a bit less.

03-11-2007, 07:35 AM
Hi Joe,

Muskymike is a moderator and a pro - listen to him.

You don't have to seal the poly seams when you overlap them.

I think it is a good idea to replace as many of the drywall screws as you can, especially near the bottom, or at least add galvanized screws in adjacent locations.

Your shower without redgard or kerdi will be fine, especially if you can remove moisture from the shower after use (bathroom fan helps tremendously, so does wiping the shower down but few people really do that). As far as the jets go, just use care to fill any holes with 100% silicone and you will be fine. The intention of the poly is to keep mass moisture migration from the walls, but small holes here and there (ie. nail holes or jet cutouts) will not be a problem.

I'm with muskymike on the 2". There will be little to no gap after the mud bed.

Good luck!

03-11-2007, 09:39 AM
Hi Joe.

I'd like to zero in on that copper pan because these types of installations can be problematic.

Just for starters, does the pan have a pre-slope built into it? Then tell us what you know about the plans to finish building that shower floor. You going to do that, or the tile guy?


03-11-2007, 11:06 AM
The copper pan is not pre-sloped the pro installer is going to do that shower. Again after reading this forum I realized that the copper pan was a waste of money and will not be as good as a kerdi install or a membrane. I wish I found this site before I let the boarders and plumber at this project. Around here the copper pan is presented as the only "real" option. Also the plumber did not secure it to the floor is that necessary. I would think that minimally some thinset would have been nice. Regardless it's all done. I hate to rip that shower apart also. However if you guys think it is in my best interest I think I will follow your advice.

03-11-2007, 12:32 PM
Sorry to hear about that Joe although I'm not surprised. Flat copper pans seem to be a New England thing.

As I'm sure you are aware by now, water will find it's way down through the mud bed and onto the pan below. With no pre-slope to open weep holes, not all of that water will be channeled down the drain. In time this water that collects down there will become brackish and an excellent food source for mold.

So the tough choice you get to make is to live with the consequences of all that or have the pan removed and reconstructed the right way.


03-12-2007, 07:45 AM
Hi Joe,

I'm glad Mike caught the copper pan - I overlooked it. Frankly I'm not familiar with their durability, but more concerned as Mike is that there is no preslope. As Mike states you have to decide whether or not you want to keep it or consider removing it. I will tell you that if you're concerned about recuding mold and odor as much as possible, this pan will play a more dominant role than how you waterproof your shower walls.

I had a shower with a chronic mold and odor problem. At first I thought the odor was coming from the drain. Later I demoed the shower to find a lead pan with no preslope covered by very smelly mortar (the drain was also removed, but simply to upgrade plumbing - it had no odor.) I rebuilt the shower using the methods displayed in the liberry (PVC liner on preslope) and poly behind CBU on the walls. No Kerdi or redgard was used. I used the shower for a few years before selling the house, and in that time never had a problem with mold or odor.

I would give this some thought before moving forward. I have been in this situation many times before and you always have to ask yourself how critical the issue is, am I nitpicking, etc. The reality is, the shower pan may not leak but without a preslope there will eventually be a mold and odor problem.

Good luck!