When we were carpenter-apprentices we saw many homes being built with tile showers where the tile was glued directly to the wallboard. Before then we had only seen tile installed over a layer of mortar. Mortar installations were difficult to do and expensive. We were convinced that this new mortarless installation was a rather simple process. We decided that if we were ever to build a shower that we would do it mortarless. Were we ever wrong. As we learned more about home building, we discovered that gluing tile to gypsum wallboard - even the green water-resistant kind - was a bad idea. In fact, gluing tile to any kind of gypsum was a form of planned shower-wall-obsolescence. As we advanced from carpentry to customer service it became obvious that mortarless- tile installations were partially or completely failing within 3- to 5-years after installation. Not an acceptable life span.

Here's what we learned: Grout, which is supposed to create a watertight seal between tiles, often cracks. When this happens, the surface loses its ability to act as a water seal. Even tiny, almost invisible cracks are big enough to let water through. The moment water gets behind the tile, it comes into contact with the wallboard - and via capillary action - is completely absorbed. The absorbed moisture begins to break down the wallboard until it finally becomes soft enough to push a hand through. Tiles fall off, cracks get bigger and in no time your once-beautiful shower begins to crumble.

This problem of mortarless showers falling apart became so pronounced during the late '70s and early '80s that many building departments outlawed the technique altogether. The new laws had a profound impact on the cost of a typical tile-shower installation. Suddenly there was a resurgence in popularity among non-tile alternatives such as Corian and cultured marble. Tile sales decreased.

Then someone came up with the bright notion that there was a need for a wallboard product that really was waterproof. Not just water-resistant, but fully water-tolerant. Industry experts already knew that cement and sand was the perfect combination for waterproofing behind tile. First, a layer of asphalt-impregnated building paper is installed, then the mortar, then the adhesive and finally the tile and grout. Grout cracks in this configuration don't cause problems because the mortarboard is not easily damaged by moisture.

So, if mortarboard is so great why is finished-in-place mortar still being used? With the latter all irregularities in the substrate (walls, ceilings, floors, etc.) can be eliminated. Bumpy and wavy surfaces become perfectly smooth for tile when mortar is applied by an expert. How can you get a smooth surface when working with mortarboard? Be sure that the surface that the mortarboard is being applied to is flat. This might mean planing bumps and shimming up low spots, and it is cost effective.

Cement mortarboard offers the convenience of drywall and the durability of a real cement mortar bed. Make sure it goes on a perfectly flat surface, and you'll get a great-looking job. If you plan on building your own shower be prepared for a heavy lift. Mortarboard weighs a lot, yet is very easy to cut. Also, it can be installed in the same fashion as drywall - with nails or screws. A special fiberglass joint tape and a special joint compound are used to join seams and connections and to fill indentations caused by nails or screws.

Mortarboard should always be installed over a layer of asphalt-impregnated felt - as an extra measure to ensure a completely waterproof installation. This avoids wood rot. With the mortarboard in place and with all seams and indentations filled, tile adhesive can be applied. An epoxy adhesive is expensive, but works best. Standard thinset adhesive also will work.

Mortarboard is ideal for wet areas, but it might be a waste of money in areas where water is not a problem (some floors, dry sinks, back splashes, etc.). In such locations a good-old-mortarless installation will do the trick.

Finally, in our opinion, mortarboard should not be used for shower floors, kitchen countertops and bathroom countertops, or any other horizontal surface where water is abundant.