QUESTION

Outdoor Counter Top Re-Installation - I have an outdoor counter topped with 6x6 ceramic tiles, but the installer failed to ensure that the wood was properly protected. After five years, the plywood underlayment has rotted. I have been able to remove most of the now unmatchable tile, but it still has the mastic and the "wonderboard" adhering to it. What is the best method for removing this backing so that I may reconstruct the counter?

ANSWER

ANSWER - I would not try to save the wood underneath the cementitious backer board.  The fact that moisture reached the wood either through the top, sides or underneath is the reason it failed, so it isn't suitable for reuse.

Assuming you want to do a similar installation as the old installation (always best to do a full mortar bed installation), I would remove the existing wood top, and then start by placing a 15 lb building paper down first with 3/4" exterior grade plywood on top, assuming the underneath and sides are protected from coming into direct contact with moisture.  Then use the appropriate multipurpose thin-set and screws for attaching the new cement backerboard (CBB) to the wood.  Then waterproof the CBB and edges of the wood with one of the appropriate liquid applied or trowel applied waterproof membranes meeting ANSI A118.10 for ceramic tile.  Then thin-set your tile directly to the waterproof membrane, and grout.

Good Luck!