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MinorPentatonic 04-22-2013 10:46 AM

Moisture under membrance ?
Good morning everyone !

Bit of a stormy Monday, and I hope Tuesday's not quite as bad :)

Doing a customer shower in my house and got a little help, but the membrane install was pretty ugly so I decided to tear it out and redo it (didn't wrap around curb, staples below curb height, etc). May have been fine, but I'd rather spend the 30 bucks and do it again the right way.

While I'm tearing it out, I notice that it's wet UNDERNEATH, as in between the membrane and the preslope. I have no idea how water could have gotten under there in the first place, but the whole sheet was moist- not a huge amound of water, but enough to make me think it's condensation. Is it possible that the cement wasn't cured enough and water, instead of evaporating, was getting trapped underneath the pvc membrance ?

Also, the lathe used for the preslope extends beyond just the floor up the side of the curb as well. The curb is not build yet, just framed with 2x4's. should I trim that lathe back before replacing the membrane, or leave it there and have the membrane on top of it ? I'm guessing trim it, and try to avoid any direct lathe-to-membrane contact.

Thank you all.


And, because I know JazMan is going to do the right thing and assemble all the facts before saying anything, here all the details I have :)

- Joist types, size and spacing: unknown ... did the flooring a while ago. There's a bit of deflection in one, I know.

- Floor: 5/8 PLY, not OSB. Glued and screwed to joists and added blocking every 6 inches.

- Shower floor: roof paper on top of the plywood, lathe on top of that, then mud. used quick pitch screed guides.

-membrane: 40 mil PVC, up 8 inches all around, silicon bead *underneath* membrane to bottom drain flange (NOT top flange)

- did not screw the top flange down tight.

- traced all water lines, fittings and plugs, not a drop of water found.
- shower drain connects to toilet drain, connects to drains from bathroom next door.
- there's water in the shower drain trap, but has been the whole time I've been working on this (which is, sadly, months) and has never once spilled or even splashed out.
- there was a bead of silicon between the membrane and the drain, so even if something weird did happen and the water in the trap came up, I think it would have been stopped by the silicon, or have wetted the top of the membrane as well. the ring of the membrane inside the silicon ring around the drain was dry, but the membrane was moist not just in areas closer to the drain, but even the furthest.
- the inside of the drain is dry.
- has not been leak tested yet - the only water to hit this whole thing is what was mixed into the cement.

JazMan 04-24-2013 10:21 PM


Originally Posted by Minor
And, because I know JazMan is going to do the right thing and assemble all the facts before saying anything,

Pretty sure of yourself huh? :wink:

I don't think you mentioned how long after placing the pre-slope the membrane was installed. It's normally done the next day, and therefore some condensation is expected. If everything else is as you stated, there should be no problem. How long ago was this done? How much time between the steps?

That galvanized metal stuff is not a lathe, this is a lathe; it's called lath. :thumbsup:

The lath for the curb in not stapled to the inside or top of the curb, only to the outside of the curb. You shape it to fit, then fasten. Alternatively you could skip the lath and build the curb with concrete board, then apply a couple coats of waterproofing membrane. This is how to build a traditional shower pan and mortar curb.


MinorPentatonic 04-26-2013 06:58 PM

Hey see, you're right, I failed to mention that the membrane went on 24 hours after the preslope was put in. Quite an eye for detail you have there !!

I guess my real question about the lath (not the lathe) is this: My setter, who I learned upon reviewing the work has never built a mud pan before, has the lath used below the preslope extending up and over the curb. He had placed the membrane on top of it (I tore the membrane out for other reasons, like being face-stapled on top of the curb, only extending halfway over the top of the curb, and being stapled along the studs at or below curb height). I don't see any benefit to lath under a membrane - you can't get mud to it anyhow ...and I'm not sure I like the idea of the membrane sitting on lath either. However, if I cut it out, there will be a pretty nasty edge along the preslope to contend with. So, leave it in place ? trim it down and mud over the lip ? Chisel out the preslope and just start all over ? Give up the whole thing and go back to the guitar ?

PS , a sincere "thank you" to all the exceptional people on this board who freely volunteer their time to help others learn from their experience. It's very impressive and genuinely appreciated.

JazMan 04-26-2013 09:22 PM

You need the lath under the pre-slope, but you then install the shower membrane including over the curb stapling only to the outside. Then you shape the lath for the curb, again stapling only to the outside of the curb and build the curb with mortar, not deck mud. Refer to the link to Ontario tile site I gave.

Is the handyman still in the picture? I'm guessing not. You could probably salvage the pre-slope by removing the exposed lath, but it might be better to start over. I can't tell from here.

An alternative would be to switch to a more up to date method like Kerdi. I prefer Kerdi because for the floor you apply the membrane on the surface, plus the walls will be waterproof. Check it out.

The guitar sounds like a good alternative too. :laughing:


PS, you're welcome!

MinorPentatonic 08-31-2014 08:57 AM

Late followup
Hey Jazman,

I was recently perusing the boards on another topic, found this thread and realized I never followed up with you. I'm so sorry - very thoughtless of me. I can't thank you enough for sharing your wisdom ! It's always very inspiration to find folks who have a passion for their craft and share it. I wish you were in my area so I could hire you !!

Thanks again and best wishes.

JazMan 08-31-2014 09:49 AM

Thank you very much for coming back to acknowledge any help I/we are able to give. I, and the 'regulars' appreciate it.:wink:


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