The underlayment for tile countertops must be flat, solid and secured well to the cabinets.
It's usually best to start out with new underlayment for a tile countertop, although you can lay tile over most any rigid, solid surface.
We recommend an underlayment of ¾” exterior grade plywood with a layer of ½” backerboard (concrete or fiber cement) on top of it.
Measure the dimensions of the installed cabinet tops. Add a 3/4" overhang for a drip edge on the front and any exposed sides.
Fasten the top to the cabinets. Use screws driven up through the corner braces inside the cabinets. The screws should not be long enough to come up through the top of the plywood. If the cabinets have solid framing pieces you might also be able to drive screws down through the plywood. Use galvanized or deck-grade screws.
Cutting Sink Openings
Cut openings for sinks before installing backerboard.
First find the center of the countertop. Most sinks come with a template to help you cut the opening. The template goes down on the countertop over the center line. Tape it down and trace the outline.
To give your jig saw a starting place, drill a pilot hole inside the outline. Cut the hole for the sink.
As you get to the end of your cut, support the middle cutout piece so it doesn't sag.
Next, cut the backerboard pieces to match the plywood. Before you install the backerboard use the plywood sink cutout to mark the cutout on the underside of the backerboard.
To cut the hole, use a nail punch to follow the outline of the cutout. Then hit it gently with a hammer around the inside of the opening. It should break along the scored line. You can also try to cut the opening by scoring it on both sides with a utility knife or special carbide-tipped knife. However, this is a little more difficult.
Attach the backerboard to the plywood with a layer of thinset mortar. Use the flat side of a notched trowel to spread the thinset out. Then use the notched side of the trowel to comb out the adhesive. Secure the sheets with galvanized roofing nails.