Adequate bath ventilation to insure moisture reduction from shower steam is code required in all areas of the country. Install a powered fan unit (Panasonic makes a top rated quiet one) vented to the exterior. Use it religiously. The material you use for cabinet construction is not involved in this question. Moisture attacks the finish first, compromising the paint, lacquer or polyurethane finish. Peeling, flaking and cloudy finish are the signs that you have too much moisture in the room. Teak won't hold paint...too oily. Cypress will but is rarely used for cabinetry as it is a softwood. Many rot resistant woods are softwoods...think cedar and redwood. Some exotic hardwoods are more rot resistant. Mahogany is a common wood for cabinets. It is also used extensively for exterior doors and trim on custom homes because it is rot resistant.

There are a few lumber yards nearby but I'm fearing they will be super expensive compared to HD/Lowes, but I will give them a call. Thanks for the wood suggestions. What type of wood would be 'exterior grade' plywood? I saw douglas fir sanded plywood at Lowes for $33/sheet, and the label said that it can be used in areas that are "weather-protected as well as exterior projects when properly finished"

thanks gals. Bev,,,,no need to go through all of that. heres how we did a few of ours. go get some large lag bolts (or carriage bolts) from HD. mount them into the studs on either side of the opening. If you 8" bolts that should allow a good 5 inches or so protrusion. now cut off the bolt part from the end. hold your board up to it and mark where the lags are on the wood. bore out two holes in the back. you can do three if you want. then just slip the holes onto the bolts (obviously you notice that your measurements from bolt to hole have to be exact!) We did this with are large, thick slab of mantle for the fireplace in the same manner. in fact, you can see it above in the pic I posted! basically it looks like this before you cut off the end. we laid the tile, bored out the holes in the teak I finished, and set them on the bolts.

1. I would look for a wood mantel that matches or is similar in color to the floor, I personally feel like it just looks more pleasing to the eye. 2. A solid stone piece would look perfect for the top of the hearth.