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Unread 07-08-2021, 01:45 PM   #1
makethatkerdistick
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Efflorescence and spalling of slab foundation under Kerdi shower

I noticed some efflorescence and spalling on my foundation. Directly behind the walls is my Kerdi shower, so I am somewhat concerned that there might be a problem of greater magnitude.

The first two pictures show the exterior wall behind which is the shower. The third picture is from inside the garage. Behind that wall is the far end of the Kerdi shower.

The space before the slab has had water pooling after heavy rains. I need to fill that in to improve it, I know. However, this is a decades-old problem on this house. Only after I had put in the Kerdi shower did efflorescence start to manifest itself.
I am not sure whether the spalling originated during our extreme freeze in February. It is a possibility as the efflorescence in the freeze-protected garage is not accompanied by any spalling.

I checked all supply lines inside the wall, and there is definitely no leak. My water lines are in the wall/attic, not under the slab. I also put a hygrometer inside a wall pocket behind the shower, and there is no increased humidity at all.

I have a bunch of theories:

1.
Vapor makes its way through the slab from when it pools during rain and finds some calcium salts from the new drypack mudbed under the shower. As it evaporates back out, it brings those salts to the surface.

2. There somehow is a constant leak under the house (which could only be waste plumbing) which adds moisture to the slab, driving it eventually out, leaving those salts behind.

3. There is a leak in the Kerdi fabric somehow, adding moisture to the mudbed underneath, eventually evaporating through the slab.


I can't see how 3.) is very likely. I meticulously kerdied that shower. 2.) seems also rather unlikely. Yes, we have old cast iron plumbing under the slab, but there have not been any other problems indicative of leakage (such as consistently moist soil even in dry weather.).

Am I missing something? Should I worry? I was hoping to clean the spalling, apply a bonding agent and put a cementitious top coat on the foundation to prevent further erosion. Is efflorescence under Kerdi bathrooms on slabs not that uncommon? The thing is that I have another Kerdi shower in the house. In that spot there is absolutely no trace of efflorescence at all. I am a bit puzzled here.

PS: Ignore the light-colored mortar patch in the exterior wall. This was from moving a hose bib around.
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Unread 07-08-2021, 03:02 PM   #2
jadnashua
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Is the problem only where the shower is, or does it extend out onto the rest of the foundation?

Elsewhere in the slab, unless there's a waterproof membrane, any moisture that might come up through the slab won't tend to accumulate, it will tend to evaporate into the house...under the shower, any that comes up through the slab will tend to accumulate, and migrate out towards the edges. Entropy just works that way...if there's enough moisture, it will carry some of the salts with it.
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Unread 07-08-2021, 03:19 PM   #3
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What are we seeing in the third photo, Wolfgang?

Did you do a rudimentary MVER test on your slab in that bathroom before you finished the shower and floor?
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Unread 07-09-2021, 01:54 PM   #4
makethatkerdistick
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Hi Jim, yes, the problem seems to be mainly associated with the shower area. The entire bathroom beyond the shower is also equipped with Ditra. Thus there is a reasonably large area that prevents evaporation through the floor.

Hi CX, no I did not perform such a test. Are you talking about taping a piece of plastic to the slab and watching for moisture accumulation? I suppose I didn't worry as there is a plastic vapor barrier under the slab. However, in recent years we've had unusually high rain amounts.

What you see in the third picture is the far end of the shower seen from the adjacent garage. The shower abuts the exterior wall as well as the garage.

Since there was no such accumulation of efflorescence before (the slab is 50 years old, and I've owned the house for seven years before I installed this bathroom in 2018), I wonder if the efflorescence stems from drypack material that gets moisture through the slab and then the salts show up on the foundation.

I am concerned about the spalling also. Not sure if it's from the February freeze or not. Does spalling accompany efflorescence in non-freezing conditions?
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Unread 07-09-2021, 04:17 PM   #5
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Difficult to make any judgements from only a few photos, Wolfgang, but in general I'm inclined to think it's moisture from above the slab making that upper edge somewhat water soaked and the recent freeze has caused the spalling. The spalled residue is visible on the ground? The spalling is only in that area of the foundation?

No possibility the moisture could be from the joint where the hose bib usta be?

The shower being immediately behind the wall is certainly of concern as the source, but we don't wanna believe that, eh?

No fenestrations in the wall above that area?

Nothing different in the roof in that area?

I see no weep holes at the bottom of the brick, but the photo is not an extensive area. You have adequate weeps?
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Unread 07-09-2021, 05:49 PM   #6
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Have you checked your meter? My water meter has a spinner that spins with any water that goes thru it. I don't know if your meter has that but you might shut everything off in the house and check it. That might help in your investigation.
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Unread 07-09-2021, 10:51 PM   #7
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Hi CX,
Yes, the spalling is only in that area, and the spalled residue popped off when I cleaned it with a wire brush.

There is a small window at the top of that exterior wall, sitting right under a two foot overhang. If water intrusion is your concern, absolutely zero chance. Roof is not leaking, I am sure of that, too.

No, I don't have any weeps. Better make myself some weeps there. It hasn't been a noticeable problem so far, but who knows what's going on behind the brick veneer. When I redid that exterior wall with insulation, I did not notice any signs of moisture damage to the coated gypsum sheathing (yes, that's what they used in 1968).

The abandoned copper pipe is from my under-slab supply system. I wasn't able to cap it (due to it being buried deep behind the brick. I squirted a bunch of silicone into it. However, the whole system is disconnected at the source. There cannot be any pressure buildup.
The supply line to the new hose bib is through the shower service wall. I have access to that location from another room. All dry in there and not a single trace of moisture or dampness.

Hi Davy,
I recently checked the water meter for another unrelated reason. Absolutely no movement of the needle. I also reached into accessible wall pockets on two sides of the shower. It's bone dry in there. All the copper piping is brand new. No leaks, no soft/wet baseplate, no musty odor, no water spots on the few sections of Hardiebacker I can see at the bottom. Nothing.

The shower trap keeps a proper water level. Another possible leakage point might be the flange of the Kerdi drain which is secured to the PVC pipe with a flexible Fernco coupling. However, I used a torque wrench and followed installation guidelines. Seems rather unlikely that that would have failed.

By the by, here is the construction thread for that shower:
https://www.johnbridge.com/vbulletin...d.php?t=125450

I attach two pictures. As you can see, I made a weep hole by the Kerdi drain. I drilled at a very shallow angle through the grout and through the side of the drain grate to a depth of approx. 3/8 in. In that location, the floor tile alone is 3/8, and there is at least 3/8 in of thinset below. Thus, I am 99.9% certain that the Kerdi channel and its flange were not pierced at all. I have been thinking whether that might be a possible source (in the unlikely event that it had been pierced by drilling said weep hole. I created this because some water didn't drain away from that section after shower use.

Now, on that exterior slab section you saw in the first section, I do have a rain water problem. After very heavy rains water will sit there for 10-12 hours up to three or four inches. I wonder if indeed that is enough water to wick up, migrate up the slab and into the mud bed below, and upon drying to carry out soluble salts from the drypack. The rain pooling problem is decades old and didn't seem to cause any issues before, but maybe now with the Kerdi and Ditra in place this has changed by forcing evaporation out to the side?

I will have to fill in that area and raise it as to deflect water away from that portion of the house. Would this problem be enough to cause the action I described?
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Unread 07-14-2021, 10:40 AM   #8
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Still looking for some tips (if you have any).

I called my local Schluter rep to see if he is willing to take a look at this.

In the meantime, I wonder whether I should do a flood test. Surely, if there were a shower leak that can cause such efflorescence, then I'd somehow see some form of moisture show up around the foundation after, say, 24 hours.

Deep down I am still very hopeful this is not a leak issue. My pride in my Kerdi workmanship would be deeply hurt.
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Unread 07-19-2021, 10:05 AM   #9
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Good news and bad news.

The good news: My shower isn't leaking. I did a 24h flood test and observed closely. 100% sure this shower is tight!

The bad news: I found efflorescence also in another part of the garage where a rug had been placed for a couple of years. There was some efflorescence underneath. Definitely a slab moisture issue. Perhaps not a gigantic one, but it's there.

Now here is my problem. All sill plates in this 1968 home were regular construction wood placed directly on the concrete without a sill seal or other capillary break. When I did the bathroom, the sill was in great shape. Leaky construction and warmer/dryer climate permitted enough drying, I suppose. I have introduced airtight foam insulation in my remodel. At the time it didn't occur to me that I should replace the sill plate to introduce a capillary break.

In the garage, which faces the bathroom, I noticed one small damp spot where the sill plate meets the concrete. I cut back the drywall at the bottom, exposing approx. one inch of the sill plate completely, so fresh air can get to it. I then reinstalled the baseboard plate. Now I have an air gap that permits additional drying of the sill plate and is cosmetically invisible. I presume that might be enough to keep the sill's moisture content to an acceptable level to prevent long-term damage.

I am concerned about the exterior wall, though. The sill plate is sandwiched between the shower assembly and the brick veneer. I am afraid that excessive moisture could accumulate over time in the sill and destroy it.

I am currently removing one brick at the bottom of the exterior wall to take a look at the wall assembly and see if in fact that is so. My thinking was to cut the exterior sheathing (which is a coated gypsum board) back at the bottom to expose the sill plate behind the brick veneer and then put in some sort of vent assembly in lieu of that brick to introduce increased drying ability. That way I may extend the life of that sill plate. Since I can only remove a brick at a time, this is somewhat of a patchwork, but it might improve the situation.

I am concerned about this. After all, the house lived with this poor standard for a good five decades, but certainly it is less than optimal.

Are there ways to retrofit a sill plate with a capillary brake without disassembling the whole wall assembly? I was thinking of trying to make a small gap with my oscillating tool and then squeeze a piece of aluminum flashing between slab and sill plate. Not sure if that'll work.
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Unread 07-19-2021, 11:04 AM   #10
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You're certain there is no SilFoam or roofing felt under the exterior sole plates, Wolfgang? It was common to use such material under only the exterior wall plates and not the interior plates. Or the exterior plates would be pressure treated wood and, again, not the interior plates.

And again, some weep holes at the bottom of your brick veneer should have been left when the brick was laid. And there should have been roofing felt or similar over the exterior sheething and down onto the rock lug before the brickwork was done.

My opinion; worth price charged.
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Unread 07-19-2021, 11:22 AM   #11
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Hi CX,
I will soon be able to tell about the presence of any such material once I get that brick out. Not there yet. Slow drilling and chiseling of surrounding mortar joints to convince it to come out.

I've done other sill plate repairs in this house on exterior walls (owed to termite damage caused by old leaking shower). At that time, upon removal of the rotted sill plate, I could detect no such material as you describe. But that was performed from the interior, and thus I wouldn't really see what was going on between the sheathing and brick. All I saw at the time that there was no vapor barrier under the sill plate. At the time, of course, wherever I reinstalled, I used a vapor barrier under pressure-treated lumber. This was in another part of the house, but I don't expect my current location to be different. I'll update with a picture once I get the brick out.

No weep holes at all. And as far as I know, just drilling into the mortar joints won't really create working weep holes either.

Now, whether the wood was pressure-treated, how could one tell after so many decades? I looked for stamps and such but didn't see any. Looks like plain lumber to me. Would treated lumber appear different in color after such a long time?

Was pressure-treated lumber routinely used in 1968? Was it even prescribed by building codes then? There's a lot on my house that SHOULD have been done when it was built. Oddly, though, I do have a polyethylene vapor barrier under the slab. How complete or carefully installed, I don't know. I just know of its presence from some drain relocation work.
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Unread 07-21-2021, 11:51 AM   #12
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Again, good news and bad news.

The bad news: No sill seal, no flashing material!
The good news: the sill plate is in good shape!

The gypsum exterior sheathing in the first picture originally extended all the way down onto the brick lug. I cut it off in this space to expose the sill wood. There is a slight musty smell but not very much. The gypsum certainly inhibits drying ability of the sill towards the exterior.

My thinking was to pry away the sheathing in this area as best I can to introduce air drying capacity and then to replace the brick with something that is potentially load-bearing and has a vent opening built in. Is anyone aware of a particular venting product that can be used in this area? If not, I could also slightly cut the brick, reinstall and insert a slightly smaller vent.
I plan to introduce perhaps another two such openings along this 5 foot section of affected area. That would further improve ventilation along the entire section of the sill.

Again, I am concerned about this in particular because a.) I somehow doubt that pressure-treated lumber was used in 1968 on my home. and b.) I beefed up the insulation on this wall, greatly reducing previous drying ability of the sill when mineral wool filled the leaky cavities above. In addition to what you see in picture 3, there is now another 1/2 foam board in place (to reduce thermal bridging), then the backer board and the Kerdi assembly. This is airtight with no ability to dry towards the interior.

Right now, I sense a nice steady air draft where I took out the brick, moving up the space between brick and sheathing and into the attic (and expelled out the ridge vent). That seems to be good insurance against possible future sill deterioration.

If it can't dry, it'll die, right? Does anyone see a problem with my thinking.
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Unread 07-21-2021, 03:20 PM   #13
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In my experience, Wolfgang, if you had treated wood sole plates, they would have been the old CCA type and likely still have some green tint. I've seen such wood of that age that could still be indentified by its color. Not so the newer ACQ treated wood that I've seen. But the plate area you've exposed certainly doesn't appear in poor condition at all.

The only "ventilation" we've ever put in our rock or brick veneers would be the weep holes in the bottom between rock and concrete foundation. Never have made any larger openings for the purpose.

My opinion; worth price charged.
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Unread 07-24-2021, 04:44 PM   #14
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I've heard of that chromium copper arsenic stuff. Probably effective yet toxic.

I think it'll be cheap insurance to have ventilation on this wall. I introduced airtight insulation and might create problems down the road if I don't ventilate the sill plate. Maybe not now after two years, but who knows what would happen in ten years. I really don't ever want to tear down this wall. I am going to look for some vent covers that can be integrated and will go from there. I'll post a final picture of what I can come up with.

Seems like making weepholes retroactively is useless as there is no flashing behind the brick. Note to self: Introduce capillary break in each future exterior wall remodel!
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Unread 07-24-2021, 05:25 PM   #15
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If your sheathing extends down to the bottom of your plates, Wolfgang, I think the weep holes would still be somewhat useful. But as usual:

My opinion; worth price charged.
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