QUESTIONI 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.
ANSWERANSWER - 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.