DEAR TIM: I am planning a ceramic tile shower for my new bathroom. I would really like a seat in the shower. What is a good size and height for such a seat? How do you install one to eliminate leaking? Are there options other than ceramic tile for the seat surface? Sarah - Chicago, IL

DEAR SARAH: You are going to really enjoy your planned shower seat. I have installed many of them over the years. My customers and my wife find them extremely convenient and attractive. Every ceiling below my jobs is bone dry. Yours will be too.

The first thing you need to consider is the overall size of your ceramic shower. Showers with seats work best if they are rectangular. A width of 36 inches will provide excellent elbow room. The minimum width should be 30 inches. A length of 48 to 54 inches will allow you to comfortably install a 12 inch wide seat. You can survive with a 42 inch long shower, however, it may be fairly cramped. I have found that a shower seat height of 14 inches off the shower floor provides the greatest versatility.

The rough framing for the seat is completed after you frame the walls for the overall size of the shower. Be sure that you leave a one quarter to one half inch gap where the wood studs would meet at each corner. This space will allow you to hide the waterproofing membrane within the wall framing. The extra space also makes it easier to nail the cement board. The nails will be farther away from the edge.

The platform for the shower seat can be a scrap piece of half inch or three quarter inch plywood. Construct a small 2x4 wall 13 and one half inches high. This small wall should be built 11 inches out from the back wall of the shower. A scrap 2x4 nailed horizontally to the back shower wall at the same height of the mini-wall will support the plywood base for the finished seat. Nail the plywood securely to the mini-wall and the horizontal 2x4 cleat.

Waterproofing the seat and the shower floor is accomplished with a PVC membrane. This membrane must be installed on the floor of the shower, up the face of the mini-wall and on top of the seat platform. In addition, the membrane must lap up onto all adjacent walls a minimum of 6 inches. This membrane should, if possible be installed in one continuous sheet. There is a solvent which can be used to chemically weld one piece to another if it becomes necessary. Water can and does leak through horizontal surfaces in a ceramic shower. The water is captured by the membrane and directed into the shower drain.

The membrane is not cut at inside corners. Fold the excess material and slide it through the gap in the framing. Nail it high on the fold to the back of one of the corner 2x4s. Failure to nail in this manner may cause a leak to develop at a later date.

There are numerous options for your seat material. You can use ceramic tile, marble, granite, cultured marble, or solid surface countertop material. If you use ceramic tile, you must first install a piece of cementitious board over the membrane. Do not nail the board through the membrane! Simply mix one part cement and one part sand. Make the mixture fairly wet. Apply a one quarter to one half inch thick layer of this cement paste on top of the membrane. Set the cut piece of cement board directly in the mixture. Use a level to check the seat for level left to right as you face the seat. A slight pitch towards the center of the shower will allow water to drain off the seat.

If you use one of the other materials for the seat, you can eliminate the cement board. The finish seat material is set in the same mixture of sand and cement. Be sure that you install the seat before you install any ceramic tile. The wall tile should sit on top of your seat just as it would sit on the top edge of a bathtub. Make sure the seat projects one half inch beyond the ceramic tile on the face of the mini-wall.