The surface must be clean, leveled, and dry to attach granite slabs to walls. Use a high-quality adhesive, such as epoxy, and apply it in a thin, even layer. Be sure to apply enough adhesive to create a strong bond, but not so much that it oozes out. Press the slab firmly into place and allow it to dry according to the adhesive manufacturer’s instructions. We recommend seeking a professional installer’s guidance for the best results.