Do I need additional waterproofing?

NO! Better-Bench is designed to mount in a manner that does not require additional waterproofing membranes, nor will it compromise the membranes in the wall. Waterproofing is accomplished by injecting a small amount of caulk in the screw locations prior to their insertion. Additionally, you will apply a bead of caulk over the top most edge of the bench unit where it meets the wall. This will divert any moisture that has penetrated the grout anywhere above the bench line, into the bench itself, and away from the screw locations. Caulk may be added around the fastener heads inside the bench for additional assurance against water penetration at the screw locations.

All caulking is performed prior to applying the mortar!