I have a 48 inch wide by 17 inches deep garden window and want to put granite tile on the inside. I estimate the combined weight of tile, board and cement to be 50 to 60 lb. This excludes any plants or other stuff we may put in it once it is finished. Is there a guideline as to how much weight one can put in these windows? My conrtractor says no problem, but I would like to be sure.