My contractor says the gap between tile and shower pan should be left open and not filled with material like silicone when you using natural stone like marble as they are porous. He claims water will seep through the stone - the open will help that water to drain into the pan. He coated the dry wall with the water resistant red material. If the stone is stuck to the walls how will the wall trickle down the back? Doesn’t not make sense. Please help