QUESTIONI am replacing an old saltillo tile on my patio, which is above our garage. It was installed about 13 years ago and water now leaks into the garage even though a membrane was installed below the tile. I like the look of concrete tiles but will that leak over time too because of the grout? Is there no what to prevent leaking except to use tiles that have no grout at all? I don't want to use those tiles that abut together and look like wood. Is porcelain tile a better option?
ANSWERANSWER - Regardless of what type of tile you install over your garage, none of the installations will be waterproof unless you waterproof under the tile first.
Some tiles are impervious such as glazed ceramic tile or glazed and unglazed porcelain tile, but that won't prevent water getting through the grout joints and traveling under the tile.
All tiles need to have at least a 1/8" wide grout joint. The grout is normally a cementitious grout that is porous and will not stop water passing through. You can use an epoxy grout or an ASTM C920 sealant caulking in the grout joints and in theory they waterproof the grout joint, but the tile installation isn't waterproof unless you waterproof the substrate under the tile.
To waterproof the substrate under the tile you need to have a clean stable surface that is properly sloped 1/4" per foot to a drain to allow water escape and not be trapped under the tiles. The waterproofing should flash up the sides of the adjacent walls at least 3 inches. There are liquid applied waterproof membranes for tile installations that work well and are relatively easy to install.
Although it is normally required that tile applications over occupied space below needs to be waterproofed with a roofing waterproof membrane. There are different types to choose from.