(Also see Substrates and Exterior Concrete Substrates)
Exterior substrates have entirely different performance requirements than interior substrates. They must be able to withstand exposure to various weather elements and outside conditions and still remain structurally sound, solid and stable.
All exterior substrates should be clean, dry and free of dust, dirt, oil, grease, paint, tar, wax, soap, sealers, clear coats, loosely bonded toppings, any primers that are not MAPEI brand, old adhesive residues (unless otherwise recommended by MAPEI), and any other substance that may prevent, reduce or affect adhesion or performance.
Uncovered wood decking that is not over living spaces or storage areas should be covered over with a layer of exterior-grade plywood with a thickness of 5/8″ or 3/4″, followed by cement board and a liquid-applied waterproofing membrane such as MAPEI’s Mapelastic AquaDefense.
If you’re looking to convert your roof into a usable space, most roofing membranes are not designed to receive a direct bond of tile. The roofing membrane must first be covered with a mud bed (mortar bed) that has a proper slope and then covered with a secondary waterproofing membrane such as Mapelastic AquaDefense. MAPEI recommends hiring a professional when tiling over roofing areas. It’s a complex and technically advanced process that requires the skills of a knowledgeable and experienced installer. In addition, building codes vary between regions, so it’s important for your contractor to know the local codes and comply with all requirements. Note: Never nail or screw into a roofing membrane.