I have seen several brochures and references that show a 1-inch air space or cavity behind random ashlar stone-veneer walls. My question is, what is wrong with a mud backup? What is the reason for the air space? Fire hazards? Insulation? Having been a stone mason for more than 13 years, I understand the difference in difficulty between laying rubble and random ashlar. I think most masons back up their stone with mud and chips regardless of whether the stone is 4, 6, or 8 inches thick.
Stone walls with a mud backup can work effectively. These walls must be thick and the joints well-filled in order to prevent water leakage problems. If the walls are thin and the joints are not well-filled with mortar, water can penetrate into and through these masonry walls. This is particularly a problem above windows, where the water travelling downward could enter into the interior. Cavities sometimes are installed in stone walls for the same reason they are provided in clay masonry namely, to provide a drainage space to collect water that penetrates the wall; flashing and weepholes at the base of the wall direct water back to the exterior. Cavities are particularly useful for ashlar veneer walls in low-rise buildings located in open areas or near coastal regions. (In buildings over four stories tall, panel systems generally are used, not random ashlar systems.) For a cavity to be effective, it must be free of mortar droppings that permit water to cross the cavity. Typically, it is difficult for a mason to keep a 1-inch cavity clean; therefore, the cavity should be 2 inches wide. Solid stone walls sometimes are used on low-rise structures, especially those having overhangs. In these cases, the joints between the stones should be well-filled with mortar and adequately tooled. Since most of the water is stopped at the exterior face, the small quantity of water that does penetrate the veneer can be absorbed by the masonry. Flashing still is needed at penetrations, such as windows and doors, in order to prevent water from dripping on the top of these elements.