QUESTION

Can I install Slate over Pine Stair Risers? - Can I bond slate to pine stair risers with a flexible adhesive via a caulk gun such as silicone or flexible construction adhesive? Seems this would eliminate concerns for cracking due to thermal expansion of the dimensional lumber.

ANSWER

ANSWER - Installing stone over pine isn't recommended because it isn't a good material for achieving a reliable bond, and might have too much deflection. It is possible to bond the slate to the risers since it is a vertical application applied to a relatively small surface area, but it has to be compatible with the pine for bonding. Silicone is known for bonding well to a properly cleaned back slate, although some sealants can cause staining in the stone. The silicone does well in mitigating movement and heat.

Assuming these are interior stairs, I would take some 1/4" thick Hardibacker board and glue and screw it to the risers, and then bond the stone to the backerboard with a polymer modified thin-set mortar leaving a minimum 1/8" wide joint around the perimeter of the risers, and then fill the perimeter joints of the risers with something like Dow Corning 795 sealant over a foam backer rod.