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We are making a tile countertop for new kitchen. I’ve read somewhere that you should cover the plywood base layer with a plastic varor barrier before putting down the cement backerboard. Others say to just put down the backerboard onto the plywood with thinset (with latex additive) and screws. I’ve done that before with no apparent problems after a few years. With the plastic, I’m afraid there might be areas that don’t stick, so we would be counting on the screws to hold everything down instead of mortar.

Second question on same project, we will be putting a wood edging on the countertop, so there will inevitably be a hairline crack develop between the wood and the adjacent tile or grout, so we could eventually get moisture behind the edging and maybe into the edge grain of the plywood base. Steam from the dishwasher might also cause problems. We’ll treat the plywood edges with a couple generous coats of shellac. Any ideas to (a) keep water from getting down there, and (b) preventing damage if it does?