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I am doing a master bath remodel and installing a Schluter system for my shower. How do I install the glass surround on the Kerdi curb? The curb is more or less Styrofoam and I don't see how it can hold a screw. I am also worried about penetrating the membrane, but I understand I can waterproof the intrusions with Kerdifix.

I imagine that this sort of thing is done all the time (I can't see spending the money for Schluter and then using a shower curtain). It must be ok to screw into the Schluter curb, right?

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    You should add pictures to increase clarity, but the products I'm seem to be foam core plastic strut. Is that the case you're asking about? Plastics can be quite strong and hold the correct type of screw well. Plastic screws mostly have a wide sharp thread and are screwed into pre-drilled untapped holes. I assume the product you want to affix is a groove to hold the glass sheet with screw holes in the middle where the glass will hide them. If this is the case, no additional sealing on your screw holes should be required as you should not allow water to enter the rail anyway.
    – K H
    Apr 11, 2021 at 3:52
  • Does this appear to describe your case? If you edit your question to add a little clarity, preferably with pictures of the work area and the product you want to install I can write you an answer.
    – K H
    Apr 11, 2021 at 3:54
  • K H - I added few links that hopefully help clarify my question. Let me know if it does not.
    – Cory S.
    Apr 12, 2021 at 15:43

2 Answers 2

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The kerdi curb won't hold your screws. You can try putting a screw on a piece of scrap and you should be able to pull the screw out by hand.

It is designed to be used at the bottom of the shower door just to hold the tiles and good enough for a person to step on it. The side hinge of the shower door must be mounted to a stud.

Kerdi curb and boards are not substitute for the framing. They are like drywall+kerdi membrane all in one to reduce installation time.

To seal the screw holes, drive the screw in half way, then squeeze some kerdi fix in there, and drive it all the way.

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  • Above is the answer to your question. If you wanted to know how the bottom curb must be done, you attach a couple of 2x4 to your floor. That is the foundation for your curb. You waterproof it with the same material you acquire for the walls.
    – Quoc Vu
    Apr 13, 2021 at 4:21
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Ok I just read their installation manual. They mention that every penetration must be sealed and do not provide any advice on how to do deal with mounting screws. . They do however come off as a company who would be happy to tell you why their product is best for everything and how to use it.

I've also read the installation manual for the shower wall and door. The wall and base are intended to be installed with 5/16 inserts which are provided. This type of fastener implies to me that even if the Kerdi stuff isn't as strong as lumber, once it has tile on it for the rigid wall of the shower you should be fine, just use silicone to seal when you put the clip down and possibly a touch under the screw head before you finish driving it. The inserts work by expanding when the screw is driven through them, so to pull out, either they have to break or they have to pull out a section of drywall and tile significantly larger than the screw. If the Kerdi wallboard is rigid like and as strong as drywall, the inserts should be fine, but from a video of the Kerdi curb, it appears to be foam, and if it's not adequately rigid, you'd be relying on the pullout strength of just the tile so you should add wood backing, forego the inserts, and use longer screws if necessary. At the bottom you don't need much pullout strength, mostly shear strength, but if you can't trust the full strength of the insert, you need a better fastener, so you should inset wood into the KerdiKerb as suggested by Quoc Vu or build the curb the traditional way to make it more solid. Also as suggested by Quoc Vu, you can test the pullout strength of one of the inserts with a cutoff piece of the curb. Drill a 5/16 hole and pound in an insert, drive the screw in and pry it out with a prybar to see what breaks. It should still be much harder to pull out the insert than to pull out just a screw. If the insert and screw come out, it means the insert couldn't expand. If it does damage and widens the hole, it expanded but the material isn't strong enough. If it tears out a huge chunk it's probably adequate. If it is hard to pull out it might be fine. You can pull one out of ordinary drywall and out of wood to compare. Wood doesn't allow the insert to properly expand. If you are screwing into wood, don't use the provided inserts and if you use the Kerdi curb, get your own longer screws as the foam is quite thick. If you don't mind installing wood backing, it is significantly stronger than using inserts, but the people who designed your shower door designed it for inserts, which shows a lower expectation. They probably designed it to work with inserts because they wanted the option of installation in concrete. The inserts provided in your product look like this:

enter image description here

They look something like this in real life:

enter image description here

This is a type best for thin or semisoft materials like drywall. The split allows the sides to spread out and spread force well. If you do a pull out test and you find that the material was hard enough to prevent the insert expanding, you can use an insert specifically for hard materials like this:

enter image description here

For the shower door it's a bit more complicated. They specifically state twice in the manual that the shower door is heavy and must be screwed into stud or a 2x6" board mounted behind the wallboard for the purpose. They then go on to describe drilling holes and mounting the same inserts used for the rigid wall and screwing in to them, completely disregarding the stud requirement. Sounds like an extra requirement from a smart proactive person, and then a procedure slapped together by someone less diligent. I would suggest you trust the guy who wrote the extra requirement and acquire screws with the same head that are long enough to drive into the stud. There is a chance they actually decided the inserts would be OK and used them intentionally, and you could call them to find out, but it may be easier to just go with the stronger requirement.

Edit: Made some edits after Quoc Vu caused me to reconsider and further investigate. The advertising materials call the Kerdi curb load bearing, but it's foam so I'm concerned it could have too much give even with wood backing. If the bottom mounting brackets concentrate enough force to bend or compress the Kerdi curb, the tile or grout will crack and you would have been better off with wood framing and only 1/2" of more vulnerable material in the form of drywall.

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    This is incorrect. Whether it has tile on it or not is irrelevant. The tile does not provide extra material for the screw to bit on. We typically drill a hole in the tile first, and the hole is bigger than the screw so it is only grabbing to whatever is behind the tile.
    – Quoc Vu
    Apr 13, 2021 at 4:13
  • @QuocVu if you read the manuals for the shower door, they cover how to correctly drill and mount the required inserts, which are intended for use with any normal walls and provide the material for the screw to bite. They do rely on the customer to choose the correct drill type. For a product like Kerdi board, the sealing and membranes give it stability, but the added layer of tile by the time the door and wall are installed increases rigidity and total strength. Most forces will be horizontal so the shear strength not the pullout strength of the bottom screws is relevant.
    – K H
    Apr 13, 2021 at 4:31
  • The pullout strength of the inserts on the wall mount rail should be fine as well as the glass is only held in by a rubber strip and can't pull hard on them. The instruction manual makes no assumption that any of the screws will be driven into something as strong as lumber, other than a specific cryptic warning about and due to the weight and mounting of the shower door. They've chosen rather puny screws, but my guess is the shear strength just exceeds or matches the force that will bend and shatter the glass, which is fine for the wall, but I personally wouldn't trust with a heavy door.
    – K H
    Apr 13, 2021 at 4:34
  • while the wall is rigid, it still does not help hold the screw. What matters is the material the screw can grab onto. The op seems to purely rely on the kerdi curb which is insufficient for screws holding weight. If you look at the pictures the mounting of the glass shown by Schluter and the shower model, they use different fastener at the bottom. I have done multiple showers with Schluter and highly recommend buying john bridge's book which cover many details. Unfortunately, the manual and their employees can only tell us what it can do, but it won't say want it can't do.
    – Quoc Vu
    Apr 13, 2021 at 4:56
  • @QuocVu The insert "holds" the screw. That's what I'm talking about with pullout strength, the weak point of the inserts they've chosen. Are you not clear on what an insert is? The inserts they've used have the same or better strength in drywall and tile as they do in a stud because the stud won't allow them to expand enough to work properly, so in a stud you'd be better off with just a screw. Tiles and drywall(Kerdi board is just fancy drywall) don't have great pullout strength even with an insert and they've still chosen inserts to mount their product anyway.
    – K H
    Apr 13, 2021 at 5:06

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