Stone Veneer over Existing Stucco 1

[jdgengineer]

We have a potential project where someone is looking to add thin stone veneer over existing stucco on a wood framed single-family residence. The stone veneer is in limited areas around the exterior. Our main effort would be to determine if the additional weight of the stone veneer triggers mandatory seismic upgrades by increasing the DCR ratios by more than 10% for the lateral system. The existing studs are 2x6, but we would also confirm that they meet the L/360 maximum deflections for seismic and wind loading. While we have worked on several projects with the installation of thin veneer we don't typically get into the specifics of the installation of the stone and leave that to the architectural specifications and prescriptive installation requirements of ACI 530.

Questions I have:

1) Can stucco that is approximately 20 years old adequately serve as the scratch coat for the mortar attachment for a thick set attachment?
2) Assuming #1 is yes, we have architectural and structural drawings for the original installation, but I'm not sure how everything was truly installed. For instance metal lath installation / anchorage, etc. Would some destructive verification be warranted?
3) What else do I need to be considering?

 

[BAretired]

Well, I don't really know much about stucco, but I don't like the concept of adding stone veneer to 20 year old stucco. I would be inclined to remove the stucco first, then apply the veneer.

BA

 

[Ron]

In response to your questions by number....

1. Yes. The base for faux stone veneer is a properly applied stucco base coat (scratch coat plus brown coat). It will be necessary to remove any paint coating on the stucco and roughen it or scarify it to receive the stone veneer.

2. Yes. Destructive verification should be done to check the condition and embedment of the lath. If the lath is corroding or not embedded properly in the scratch coat, removal of the existing stucco is indicated. If it's 20 years old, someone did something right, but check it anyway for wood rot in the structure and water resistive barriers behind the stucco. Much has changed in the last 20 years with regard to stucco application.

3. Additional considerations:

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A. The stone veneer is not installed in a thickset application. It is more of a thinset application.

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B. The additional weight of the stone veneer should not impact the seismic considerations.

I see that you are in California. They have some screwy ideas about how stucco is applied in some cases. As an example, stucco is often installed on suspended lath with no sheathing behind it in California. Further, a lot of the stucco subs in California have been erroneously indoctrinated to stucco application by one of the prominent trade associations on the west coast. Their ideas might work if it never rained but that's not likely.

I am a member of ASTM Committee C11...the committee that creates the stucco standards for the country. It's a constant battle to get appropriate standards in place.

 

[BAretired]

Oops! I should have waited for Ron's post.

BA

 

[jdgengineer]

Thanks for the responses Ron. I appreciate your input.

As a follow-up. There is plywood (at least per drawings) around the entire house.

I mentioned thickset as I thought that is what the installation was called. It is thin brick veneer. My understanding "Thin Set" was with the use of a cement backer board without scratch coat. My understanding was "Thick Set" was the use of a scratch coat. Perhaps my terminology is incorrect?

And one more question. You mentioned that it should not have seismic implications. While my hope is that is the case. The exterior walls were only designed initially for the weight of stucco. Wouldn't the addition of thin brick veneer (up to 15 psf in weight, likely not that heavy but still need to confirm) trigger the review of the structure's lateral system? The roof / floor weights are staying the same, but to me this could possibly trigger the 10%?

 

[Ron]

For thin brick veneer applications, the added weight will be more on the order of 6-8 psf. The thinset adhesive will only be about 1/4 to 3/8 inch thick and the thin brick is relatively light.

Your terminology is ok, it's just that you have a hybrid system. Your "thickset" is already in place so the only thing left is adhering the thin brick to the stucco base which acts as your thickset.

Since your thickset is already in place it acts as a cement board for your thinset application.

I would not recommend a cement board application with thinset on an exterior application. An exterior application should be a viable, properly applied stucco system meeting the requirements of ASTM C1063 and ASTM C926, with the thin brick being a substitute for the finish coat of stucco.