(16 mm) thickness minimum 34 in. (19 mm). apa spanrated plywood or osb with an exposure 1 classiﬁ cation or better. cover base with usg durock™ brand tile membrane, 15 lb. (6.8 kg) felt or 4mil polyethylene and attach with 14 in. (6 mm) galvanized staples. walls and ceilings 12 in. (12.7 mm) and 58 in. (16 mm) thickness only maximum.