Foam curb with mud pan - what to do about membrane? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


View Full Version : Foam curb with mud pan - what to do about membrane?

07-26-2019, 10:11 AM
Greetings! I am in the midst of a master bath remodel which involves a new mud pan shower. After researching curbs and the issues that can arise with wood, I have decided that a foam curb is the better choice, especially since my shower is angled and the curb will have two angled joints.

However, from what I gather, the tile-ready foam curbs (Wedi, Dural Durabase, Kerdi, etc.) don't hold on to screws well. Given this, what is the best way to attach the membrane to the outside of the curb? I know that the foam curb itself doesn't need any further waterproofing but I obviously want the membrane to come out over the curb to keep water from seeping between the curb and liner into the pan. Can it be screwed into the floor outside the shower? Or will screws/staples do an adequate job of holding it to the foam long enough to get it tiled? My contractor has not used this type of curb before so I want to make sure I know how it should be done!

Also, I would appreciate your recommendations on which brand of curb to use. Wedi's curbs are sloped on top and have a notch at the bottom so that you can run your pre-slope material under the lip, which both seem like great ideas. They also happen to be in stock at a local store, which is a plus. I am open to other brands, though, as long as I can get them ordered and delivered in a reasonable time frame. Thanks!

Sponsored Links

07-26-2019, 11:56 AM
Typically, you simply thinset the Kerdi fabric to the curb...on all three sides. Thinset also to set the curb itself to the floor. No fasteners needed or desired. :dance:

07-26-2019, 12:05 PM
Lazarus, thanks for your response. I understand that when using a full Kerdi (or Schluter, or whatever) system, the membrane adheres directly to the foam curb and floor/wall boards. However, we will be doing a traditional mud pan, so our membrane will be Oatey PVC membrane or something similar. Can these membranes also be glued directly to foam curbs?

07-26-2019, 12:55 PM
No, with a rubber membrane such as Oatey, you really cannot glue it to the curb since you cannot tile directly over rubber. The procedure would be to lap the rubber over and down the face. I would say to glue it down except some adhesives will melt the curb. You could use fasteners with washers on the outer face only and then preform and bend expanded lathe to "clasp" over the curb.

At this point, you need to cover it all with brick morter or "fat mud." (floor mud with some lime added to make it sticky.) Then tile.

Frankly, as those curbs a pretty expensive, far better to either screw down 2 or 3 2x4's or, if over slab, I like to use bricks or masonry thinsetted to each other and the floor. Now you can fold the liner over everything and nail or screw it to the wood, or some liquid nails, if brick. Again "overfold" the lathe and, as before, mud everything and tile it. :neesie:

07-26-2019, 12:55 PM

07-26-2019, 01:43 PM
Laz, this is a second floor bathroom, so I'm not sure a brick/masonry curb is appropriate. I have seen many references to NO WOOD, but not a lot of alternatives offered other than rigid foam. I do understand that some adhesives will melt the foam, though, and that you would have to mud over the liner regardless.

07-26-2019, 01:47 PM
The link I posted is made exactly for what you have.

07-26-2019, 01:56 PM
Eric, thank you for that link. I understand how your product works, but it still requires wood underneath, which is what I am hoping to avoid. From what I have read, wood expands and contracts at a different rate than tile/thinset, which is what eventually causes cracks in wood curbs. Does your product allow the wood to expand and contract independently of the tile? And is it made to allow space for a PVC membrane layer in addition to 2x4s underneath?

07-26-2019, 02:29 PM
Its the moisture that is the major issue with the wood. 2X4 curbs wrapped with membrane have been done since the early 60. The curb overlay is sized to allow for the membrane. We have been doing foam curbs for almost 20 years and membrane for more then 50.
Wood curbs and wood substrates ARE a recognized method by the TCNA..

Otherwise lath and mud it.

07-26-2019, 02:42 PM
Thanks so much for the info. Since our shower is an angled shower with a three-sided curb, I was feeling very insecure about the membrane-over-wood-plus-mud construction, but if that is a TCNA-approved method I assume we'll be okay as long as the person doing the work knows what they're doing. The sub that did most of the carpentry just got fired (presumably for not showing up on time; I think he was fairly competent otherwise, at least when it came to basic carpentry), but my GC said he will have his best tile guy doing the shower build and tile. Fingers crossed! :rolleyes:

I actually just talked to one of the retailers who sells your product and he said it is mostly being phased out because everyone is moving to bonded systems like Kerdi, Schluter, etc. Are these really so far superior to the old-fashioned mud bed method? I could probably still switch at this point, although it would mean pulling out the drain - nothing else has been done at this point.

07-26-2019, 02:56 PM
We make both and were the first to do so .Our Bond-able membrane Nobleseal TS came out in 1982.
From a membrane performance stand point the our Chloraloy is the best and longest performing product in the industry. It came out in 1964. I have never seen a failure of the membrane in 38 years. Either method when done correctly works, when not may not!

07-26-2019, 03:22 PM
Bionda.....wood can certainly be used on an upper floor. You DO need to avoid "pressure treated" wood however. As it dries, it tends to twist and cause problems. Use either dried pine or PTKD wood. That is "Pressure Treated Kiln Dried."

Also, E3 has good advice, but again, a bit more in his link, they have both solid curbs and one's that "clad" over 2x4's

07-26-2019, 05:05 PM
Tough enough to find kiln dried treated lumber when you're askin' for the correct stuff. 'Fraid I've never seen any lumber marked PTKD. You can find some marked KDAT (Kiln Dried After Treatment) if you shop at the right lumber yard and each piece will be marked, usually with a little plastic tag on one end. In all of San Antonio, Texas, there has been only one source for many, many years. Might be more than one source by now, but I don't pay much attention these days. Worth looking for if you need it for the right application.

But rotting and termite damage are the things pressure treated lumber best protects from, and in a curb application over SOG concrete that's not what's most likely to be your enemy, moisture vapor coming up through the slab is your enemy there and the KDAT lumber won't help you with that problem. It'll get moist and swell just like regular kiln dried lumber under those conditions.

If you want to avoid that problem (if you actually have it), your best bet is to build the curb from CMUs and put your waterproofing membrane, of whatever type you favor, over that.

For a wood framed floor there is no problem at all with using plain kiln dried lumber to build your curb.However, from what I gather, the tile-ready foam curbs (Wedi, Dural Durabase, Kerdi, etc.) don't hold on to screws well. Matters not at all as you'll never, ever, want to drive any screws through your waterproofing membrane on the curb.

My opinion; worth price charged.

07-26-2019, 07:18 PM
Thanks everyone for your input. Clearly I misinterpreted the warnings against wood curbs. This is a second floor bathroom on wood joists, so it sounds like kiln dried lumber will do just fine. I'll talk to my contractor about the foam overlays E3 suggested - if we can wait for the order to come in I'm willing to pay the extra for peace of mind, but you have all made me feel much better about the original plan.

One final question- cx, you said screws should never be used on the curb. I fully understand that you don't want screws on the side of the curb that is in the shower nor on top, but I thought attaching the membrane to the wood on the outside using screws was fine. If not, how should it be attached?

07-26-2019, 07:40 PM
Doesn't usually need to be attached at all, Bionda. Normally, on a traditional shower receptor, the waterproofing liner laps over the curb and is held in place by the metal lath bent over it to accommodate the fat mud used to finish the curb. But you certainly can, as you suggest, use mechanical fasteners on the outside face of the curb, but a few staples to hold it in place while you installed the dam corners would be adequate for that. I suppose you could use screws, but I can't imagine why you would wanna do that.

In lieu of the lath and mud you could use that foam thing Eric recommended. I favor the lath and mud.

My opinion; worth price charged.

07-26-2019, 08:09 PM
Good to know - makes sense that the liner wouldn't need much more than the lath to hold it in place since gravity should do most of the work if it is properly draped over the curb. Does a 2-angle curb present greater challenges in terms of draping/folding the membrane than a standard single 90 degree curb? My shower is a neo-angle but the curb has three sides, two perpendicular to each other with the angle between. The sub who got fired mentioned cutting the membrane to create even the 90 degree corners, which I know is a big no-no, but I am curious to know how they will manage the 3-part curb without cuts.

07-26-2019, 09:08 PM
Does a 2-angle curb present greater challenges in terms of draping/folding the membrane than a standard single 90 degree curb? Perhaps a photo or a drawing would help. Not sure I understand just what you're working with there.

07-27-2019, 05:34 AM
It's a strange bathroom, for sure. The shower is kind of wedged into a corner due to the presence of a chimney - ignore where it says exposed brick, it has actually been furred out and Durocked. The heavy black lines are the curb.

07-30-2019, 07:51 AM
Little trickier to fold the liner over such curbs, Bionda, but it can be done. Tacking it to the outside face of the wood curb should be plenty to hold it in place while you form your metal lath over it, but the bulk in the folds can become a bit of an issue. You can compensate with the mud you'll use to form the final curb shape.

Don't think Noble Company's foam overlays would work very well in that application, but you'd need to get Eric to address that.

My opinion; worth price charged.

07-30-2019, 07:55 AM
because of the fold ---lath and mud!

08-01-2019, 10:40 AM
Thanks you guys! I really appreciate your expertise. Hopefully the shower installer will be as good as you! He's building the curbs and pre-slope today so we'll see what things look like when I get home.

Tiger Mountain Tile Inc
08-04-2019, 02:20 PM
So it sounds like my comment is probably too late but I would not recommend a mud pan liner with foam board walls.

Wedi's curb would be a good one to use and they have instructions for how to fasten it to the floor (I believe you can use their sealant but you'll have to look it up). You could then use Wedi's subliner membrane and glue it to the curb with sealant.

This would mean that you are installing a mud bed with the waterproof membrane (subliner) on top- not inside like a normal mud pan. Then use Noble's divot to connect to the drain.

It's exactly what I've done here: