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Unread 07-16-2022, 01:49 PM   #1
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Outdoor Stone Veneer

This is pretty close to a tile job, so I thought I would ask a few questions here. This forum helped me out quite a bit on my tile and Kerdi work in the past.

I have an outdoor concrete block wall. I faced it with some stone veneer. The pieces are 16" (full size, 1/3 are 1/2 size) by 4" by about 1" thick. Maybe 1 pound each. I'm not much for skipping steps anymore, so I read and re-read the manufacturer's instructions. They called for thinset, not mortar, so that's what I used. I also checked a competitor's product, and they also called for thinset.

I back buttered each piece. I did not apply thinset to the CMU wall directly, which I'm hoping is my problem. I did make sure I was getting squeeze out along the edges, and making sure I had plenty of contact behind the stone. The stones vary quite a bit. Some are concave, and some are convex.

My problem is I'm getting a lot of failures. The thinset adheres to the stone veneer very well. I have to grind it off to reuse the stone. It did not adhere to the block wall very well. I had pressure washed the wall prior to installation, and the weather conditions varied but all were within parameters. Failures have been at the top and bottom of the wall, at areas below grade, and also at areas well above grade. I do have a cap block that I now wish had been even wider. There is some moisture getting down onto the stone veneer, but it should hold up to that even without a cap block.

I'm putting pieces back in. This time I'm cutting a few channels into the CMU and pressing thin set into the CMU and channels. I'm also adding a lot more thin set to the back side of the stone. My questions are:

Have you experienced this type of failure on exterior?
Have you used mortar for this type of product instead of thinset?
Is there some type of primer or product that I should be applying to the back side of the CMU wall?
Is there something that I'm missing in my installation that would prevent this?
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Unread 07-16-2022, 08:30 PM   #2
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Hello, Loyd.

"This is pretty close to a tile job"

I'll try to give you pretty close to an accurate answer.

A couple of things are probably contributing to your failure. They're both evident by the lack of bond to the blocks.

First, when you apply fresh mortar to the stone and immediately stick it to the block, that block is going to draw the moisture right out of the mortar and dry it prematurely. Hence the weak bond.

Second, you want to apply fresh mortar to fresh mortar. What that means it you want mortar on both surfaces before sticking them together. You can skim one surface with the flat side of the trowel and comb the other. Doesn't matter which one gets what, as long as you get near 100% contact between the two.

You'll be able to tell if you've got 100% bond by occasionally removing a stone immediately after it's set to check for full contact.

Also, you'll want to dampen the surface of the block just before applying mortar. This will prevent the premature drying. You don't want water running over the surface, which would also weaken the bond, but just enough to keep it damp. A pump sprayer might work best for that.

Nothing wrong with cutting a few grooves in your block, but they're usually porous enough to not need it.

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Unread 07-17-2022, 08:56 AM   #3
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Thanks. The blocks being dry may be one aspect I didn't address. I did a larger size stone on our front entry with mortar, and I did spray with a handheld spray bottle to keep the scratch coat a little damp. No failures on that wall at all. I got in a jam on a shower and had to create a border by sticking tile to the glazed face of the tile, and that worked great, so I was shocked that I didn't get adhesion to the CMU.
I'm in New Mexico, and the wall is on the south side of the house, so it does get cooked and it is likely pretty dry. This time around I did make sure I did press thinset into the block and onto the stone, so I'm hoping that eventually they will stop falling off and I will have all of the loose ones repaired. Fitting them back in is a pain in the ass because they aren't quite uniform. I have to make a map of where each one came out of, and even then I have to trim an edge sometimes to get them back into place. Ugh.
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Unread 07-17-2022, 09:37 AM   #4
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Kevin's comments mirror what I would speculate. Moisture is what draws the cement in once it starts forming a bonding matrix.
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Unread 07-17-2022, 05:11 PM   #5
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Excessive heat can be a problem when setting tile. Check the suitable temperature range on the mortar you're using. Once it has cured, normally it's not a problem. NM can have some significant temperature swings to include freezing, so the type of thinset you choose can make a difference, too. THe cement requires the moisture to cure...drying before that happens prevents the chemical reaction from completing, and a poor bond strength.

With the back of the stone not flat, to get full coverage once on the wall, you may need more thinset than you think, so pulling some off to double-check is important. Plus, you want to ensure the thinset you select is suitable for that thickness.
Jim DeBruycker
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