Gallery 1 Old grout in this 2nd floor shower allowed water to infiltrate behind the tiles and rot the backer board.It also allowed water to seep through the ceiling in the bathroom downstairs. We removed the moldy drywall backerboard and plaster down to the lath,before installing new, cement backerboard and laying in new tile.Downstairs we patched the hole with drywall and coated the exposed plaster with PlasterWeld to maximize adhesion, before plastering it,and painting it.