I'll differ from ants a little bit. While I'd prefer a neater hole, there are a few considerations...

First, I'll say refer to the valve's installation instructions. Some valve bodies get caulked/sealed into the wall and thus need a closer tolerance so they can get sealed to the hydroban. Some don't. It doesn't look like your valve is one of those valves that does.

Plus it's not a steam shower so that negates the need for a continuous vapor seal. If it was, you wouldn't be using Hydroban.

If you are using large tile, they could overhang the rough edge of the cement board so the edge of the tile will be closer to the valve, and thus you could get away with that raggedy hole.

If you're using a small mosaic tile on the walls, then the small mosaic would be flapping in the breeze without more cement board behind them. The board would definitely have to be repaired or replaced.

Sometimes gaps can be filled with mortar and reinforcement tape, but with your gap it'll usually be faster, less expensive, and you're more reassured of getting a better quality end product (especially if you are using smaller tile) by doing what ants wrote and reinstalling a new piece of cement board.

I will be using 12x12 or larger tile around this valve so I might be okay with the gap as-is. Also realized that the trim plate for the valve has a rubber gasket which will be outside of this gap. And, if I apply a small bead of caulk around the outside of the trim plate, seems like I would be okay.

If I still decide to patch it, I will follow antss recommendation and re-install a piece of durock. I presume a drywall knife doesn't cut through durock. What's the best tool to remove this small durock section?

I used a cheap hole-saw to cut my durock though that valve looks to be pretty large and a hole saw that size might be pricey. Still, you'll have the tool in the future.

Even if it's not absolutely necessary to redo the durock, for this DIY'er, the extra work to make something right is always worth it - that way I don't have to worry in the future. Sometimes it's difficult for a DIY'er to know if something is good enough and won't cause problems and therefore has to err on the side of caution.

bill g web:

First off, thanks for the excellent pictorial you put together detailing your Hydroban Schluter drain shower. I am doing the exact same thing in my shower and have been referencing your blog frequently.

As far as the hole-saw, that would work fine for the new piece of durock that I install, but what tool do I use to cutout the existing durock. I'm thinking something along the lines of a drywall saw, but made for cement board -- don't know if this exists.

mongoct:

The Schluter drain I have came with a "Waterproofing Mixing Valve Seal for Tiled Showers", but very little in terms of instructions for using this. It appears to be a seal that goes around the valve, but I'm not sure it will work in my situation since I'm doing Hydroban, not Kerdi. I'll call Schluter to get more details.

Are you doing a hybrid shower? Sloped mud deck with a Kerdi Drain set in it, cement board on the walls, then Hydrobanning everything the walls and the sloped mud?

mongoct:

Yes, my shower is being built the way you described -- one sloped layer of 3701 mud with Kerdi drain, durock up the walls and wrapped around the curb, Hydroban will be applied to sloped mud, walls, and curb.

In that case, using the Schluter valve membrane will help with potential water intrusion around the valve. But it still won't help with tile support behind the too-large hole, and as-is, a bit of the the Kerdi valve membrane will be sort of flapping in the breeze a bit as it floats over the gap.

I'd dry fit the valve membrane over the valve and see how much unsupported Kerdi membrane there is. If the neoprene ring on the membrane fills a good portion of that gap, you could pack the remainder of the gap with stiff thinset, then pull more thinset out on the surface of the cement board, comb it out, then embed the valve membrane in the thinset, carefully smoothing everything as you go. Don't goop so much in the gap that it drips behind and gets on the copper plumbing or the brass valve body.

The cured thinset will provide support to the previously "flapping kerdi", and the Kerdi will provide support for the thinset, tying it in to the cement board. So to speak. A little mutual reinforcement and synergy. So to speak.

But again, if using larger tiles, it's not that huge of a deal. The edge of the tile could span that gap.

Regardless, since you have it I'd use the valve membrane and thinset it to the cement board. Then when you Hydroban the walls, HB over the cement board and onto the fleece of the valve membrane. Just like the floor, where you'll HB down the mud slope and onto the fleece of the Kerdi Drain.

Off-topic: Don't forget fasteners. Make sure you have adequate screws through the cement board and into the wall studs.

Do realize that thinset has "thin" in its name. It's designed to be installed in "thin", not "thick" amounts. Reason being that it shrinks as it cures. The thicker the application the more prevalent the shrinkage.

As to the Schluter membrane for the valve...they are primarily for protecting drywall cutouts when Kerdi is installed over drywll. But they can be used on cement board too.

There's a neoprene ring in the center of the membrane. The thicker part of the ring (it sticks out about 1/2" from the surface of the Kerdi) goes IN the wall, the 1/2" thickness of neoprene essentially protects the 1/2" raw edge of the backer board from potential water intrusion.

If the valve membrane fits your valve body, you can certainly use it, even though you have cement board.

mongoct:

You should consider changing your user name to PSYCHIC_tilesetter. Not only did you answer my initial questions, but you answered questions I was planning to ask before I asked them. Thanks for your detailed explanation which now clears things up tremendously.

Off-topic followup regarding fasteners: I'm using hardibacker screws on 1/2" durock. How far apart should the screws be to be "adequate"?

Fasten 8" on center.

Instructions

Glad to have helped!

I can manage screws 8" on center in the vertical direction, but in the horizontal direction, the studs behind the durock are 16" on center, so that's the best I can do. I thought typical stud separation is 16" on center so how does anyone space durock screws closer than that horizontally?

I just reread Durock instructions which state,

"Fasten to studs spaced 16" o.c.
Space fasteners 8" o.c. for walls,
6" o.c. for ceilings. Fit ends and
edges of panels closely but not
forced together."

Sounds like 16" horizontal spacing is fine with 8" vertical spacing. Am I reading this correctly?

Maybe Mongo will chime in but I interpreted those instructions as fasteners placed 8" oc all around, which means you need backing between the studs, positioned across the studs, horozontally, where the CBU edges are. Like in my blog: http://www.billbblog.com/Diy/33/Build-Shower.

Maybe that's overkill, especially the way I did the blocking. Since your CBU is up already and you don't have blocking, maybe that's still fine. The shower I removed had cracks in the tile at the CBU edges and there was no blocking, but also, the installer neglected to tape the CBU, which may have been the real reason for the cracking.

Bill:

I recall reading about the blocking you did in your blog, but after asking around a bit, couldn't find anyone who had used blocking or thought it was absolutely necessary. So, I went without.

I'm sure you're fine but I wouldn't skip the joint taping. One of my books says "it's a good idea to add blocking between studs where the backer boards come together", so... It's really not that much work for peace of mind. But you're right, most of what I'd read didn't mention the need for blocking. See what mongo says. I dont' think I'd bother to take everything down at this point.

Horizontal blocking is not required behind the panel's horizontal seams, but you do need the vertical seams to fall on a stud.

Nothing wrong with adding optional horizontal blocking if you choose.

So the maximum 8" oc fastener spacing is just for screwing into the studs. That's a requirement.

If you add optional horizontal blocking behind the horizontal seams, then you can do 8" screw spacing there too.