Q. Why do I need to water proof the shower stall before tile installation?
The grout joints between tile, and even some tiles themselves, are porous. When you take a shower, most of the water runs down the drain; however, when used day after day water begins to seep through the tile floor into the mortar bed below. If there wasn't a water barrier between the tile and the sub floor the water would eventually seep onto the sub floor damaging any wood (causing dry rot), drywall, or carpet it may come in contact with. The roofing felt and tar prevent the water from reaching the sub floor. The water eventually seeps back into the drain through small "weep" holes.
RIGHT:Over time water seeps through tile and grout settling onto the tar pan underneath.
LEFT:Showing weep holes in top half of a typical cast iron shower drain--nails are used to prevent tar blockage during the hot mopping process, they are removed after the water test.