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Should I use mechanical anchors to install my 2cm thick granite stone?
QUESTION
I am having a new wood-burning masonry fireplace installed. I want to finish it very simply with a 2cm granite slab surround.This finish will be just 3 slabs ---two legs and a top piece, framing the firebox---all about 14" wide.
No mantle; no other trim. The firebox is 44" wide and 31" tall.
The substrate is 1/2" cement board attached to wood framing.
I was planning on just using a high-quality medium bed mortar to attach the granite to the cement board and face or firebox---same as if it were a big heavy piece of tile.
So, my question is:
Should I also use some type of mechanical fastener to attach the 2cm granite to the wall or is mortar enough by itself.
I read your "Recommended Best Practices" online where it shows a lot of mechanical fasteners for vertical stone installation, but that seemed to apply to much larger pieces of stone.
I would appreciate your comments.
ANSWER
ANSWER - Mechanical anchoring of the stone is always considered the safest method of installation. If you do use mechanical anchors you need to be aware of the capacity of the particular stone you are using to withstand the stress caused by the installation of the type of mechanical anchor you use. As you may weaken the stone from the kerf or holes you cut into the stone.On the other hand, as long as the cement backer board is adequately attached to the framing and the back of the stone does not have contaminates, you can adhere it to the backer board.
An epoxy adhesive can be used for spot bonding, but I would not recommend it if it is near a fireplace. I would use a cementitious ANSI A118.15 thin-set mortar that shouldn't be affected from the heat of the fireplace. It is critical that you achieve full thin-set contact between the back of the stone and the backerboard. What use to be called medium bed mortars are not called Large Heavy Tile mortars. You can not apply the thin-set in a manner where it will be more than 1/2" thick after embedment. It is better to shim the backerboard so it is plumb and not go more than 1/4" thick thin-set after embedment.
Also make sure you install an ASTM C920 sealant at transitions, but make sure you use a type that can take the amount of heat it will be subjected to.
4 thoughts on “Should I use mechanical anchors to install my 2cm thick granite stone?”
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I have a very similar set up as the original poster except that my mantle is antique Eastlake style marble, so, I have two legs and a top that vary in thickness from 1” – 2” depending on carving depth. The legs are 9” wide x 35” high and the top is 13” high x 52” wide.. the backs are flat and they will be mounted to a wood framed wall. The hearth is marble tile on top of a decoupling membrane and plywood subfloor.
After a lot of different suggestions from different installers and suppliers, my current plan is 1/2” cement board (not hardie backer) plus Laticrete multimax light LHT modified mortar with the legs resting directly on the hearth for added vertical support.
However, I’ve yet to find anyone who has given me a confident answer. I’m getting a lot of “we think this will hold” and not a single installer who is confident enough in their answer to actually install them.
So, I have two questions:
1. How would you suggest attaching these pieces to the wall?
2. Should the legs rest directly on the marble tile hearth or should there be an expansion gap?
I will note that I’d rather avoid mechanical anchoring because of the fragile antique nature of the marble.
Thank you in advance for any insight you can provide!
The success of an installation is based on many variables. It depends on the strength of the stone, the configuration of the stone, the material and structural stability of the wall substrate and framing, and then it depends how it is installed and the quality of the installation. So it is reasonable and smart of the tile installers to say “we think in theory it could work.” but are careful not to say it will work. It just depends…
It is always best to mechanically anchor the stone to the wall framing. There are different methods for doing so. Although if you are using a high strength ANSI A118.15 thinset mortar over a properly installed 1/2″ cementitious backer board properly attached to the framing, and achieving full thinset contact between the back of the properly cleaned stone and the properly prepared backer board, it should provided an adequate attachment.
I don’t see a problem with the legs sitting on the marble hearth, so it will support that load. You do need to apply an ASTM C920 sealant at that hearth to leg transition to mitigate the likely movement.
I would like to use granite for my fireplace. It will take 2 full 2 or 2 cm. slabs stacked horizontally. I have been told that this isn’t safe because the glue may not hold. Is there a better way to secure this other than just using glue that could be inconsistent in it’s mixture from the factory? I have seen walls this tall, but I do want it to be very secure because there is bombing/shooting practices from the base I live by that has shattered my large bathroom mirror. Please email me your thoughts. Thanks bunches.
It is always best to mechanically anchor slabs of stone to the framing or structure.
There are special epoxy adhesives like Laticrete Latapoxy 310 for spot bonding slabs of stone. There are limitations on what substrates it can be bonded to. There are special procedures to prepare the slab of stone and the substrate. Of course if it isn’t installed correctly they you can have problems.
Mechanical anchoring it is the safest method for installing the slabs of stone.