Ceramic Tile: Cement Mortarboard
Back when we were bright-eyed and bushy-tailed carpenter-apprentices we watched hundreds – thousands – of homes being built with tile showers where the tile was simply 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 pretty simple process. We decided that if we were ever to built a shower that we would do it mortarless. Boy, were we wrong! As we learned more about home building we discovered that gluing tile to gypsum wallboard – even the water-resistant kind – was a really bad idea. In fact, we discovered that 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 a very good life span for an item priced in the four-zero ($0,000) category.
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 teeny, tiny, almost invisible cracks are big enough to let water through. The moment water gets behind the tile it immediately comes into contact with the wallboard – and via capillary action – is completely absorbed. The absorbed moisture begins to breakdown the wallboard until it finally becomes soft enough to push your hand through. Tiles fall off, cracks get bigger and in no time your once beautiful shower begins to crumble into a disgusting looking mess that appears as though it had been in a hurricane.
This problem of mortarless showers falling apart became so pronounced during the late seventies and early eighties that many building departments outlawed the technique altogether. Needless to say 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, solid surface alternatives. Tile sales decreased.
Suddenly, someone got the bright idea that there was a need for a wallboard product that really was waterproof. Not just water-resistant, but fully water tolerant. Everyone in the industry 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. Since cementitious mortar board was first introduced, a number of other reliable tile backer waterproofing products have hit the market with great acceptance.
So, if mortarboard is so great, why is finished in place mortar still being used? Good question! With finished in place mortar 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, but it can be pretty pricey….$$$$. Get the picture? Now we bet you want to know how can you get a smooth surface when working with mortarboard? Again, good question! All you have to do is be sure that the surface that the mortarboard is being applied to is flat. This may mean planing down bumps and shimming up low spots – but think about it – it really isn’t rocket science. And it is cost effective.
Cement mortarboard and similar waterproofing backer materials offer the convenience of drywall and the durability of a real cement mortar bed. All you have to do is make sure it goes on a perfectly flat surface. You’ll get a perfectly great looking job. If you plan on building your own shower be prepared for a heavy lift. Mortarboard is really weighs a lot. The good news is that it 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 insure a completely waterproof installation. We don’t want any wood rot now do we? With the mortarboard in place and with all seams and indentations filled tile adhesive can be applied. An epoxy adhesive is most expensive, but works best. Standard thinset adhesive also will work.
Mortarboard is super for wet areas, but it may be a waste of money in areas where water is not a problem (i.e. 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 prevalent.
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