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    Gang Chen

    “Mastic is an adhesive that, along with thin-set mortar, is used to stick tile to wall or floor surfaces before grouting. While mastic does have strong points, such as great adhesive properties and adaptability to many substrata, high performance in wet areas is not one of its high points.”

    Gang Chen, Author, AIA, LEED AP BD+C (

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    Michael Ermann

    Brendan, let's answer your question with a question from Amber Book. . .


    Identify the best location for an application of mastic in the section detail above. scroll way down for the answer.













    Answer C-3





    We don’t want rainwater or condensation that has formed just outboard of the enclosure (CMU) to slip behind the flashing as gravity takes it downward. The mastic (a catchall industry word for gooey, often black, robust sealant) caps the flashing’s top edge at the backup wall.

    An obviously better solution for new construction laps the wall membrane over the flashing (see below or click <<here>>)











    In a CMU backup wall, again for obvious reasons, you’ll want the flashing into the mortar joint if you can (click <<here>>).


    To watch an Amber Book : 40 Minutes of Competence video on this topic click <<here>>.

    Strangely. . . 

    Mastic and weep holes are also in shower assemblies, which can be confusing, because those terms are most often used with flashing in cavity walls. Both the outdoor flashing application and the indoor shower drain application are very much in play when studying for these exams.

    Weep holes are found in shower drains as alternative paths for water to move down the drain. They allow under-floor water that has seeped through the tile grout and joints to find its way back to the drain. See this video.

    Tile mastic and thinset are the two most common options for affixing tile. Mastic, originally an organic resin from a Mediterranean plant, is now a catch-all term for sticky stuff that dries quickly (useful for affixing vertical tile so it doesn't sag while drying). Mastic, as applied to tile laying, is a term that has largely been phased out and replaced with "fast-grabbing tile adhesive," "no-sag tile adhesive," or just "tile adhesive."

    Thinset is a cement product. It takes longer to set, which can be a good thing, or a bad thing, depending on whether you want the tile to stay put without having to hold it for a while, or you want the flexibility to reposition the tile later. Thinset can also be used to lay vertical tile, but is always used to set horizontal tile in showers. Unlike mastic, which can get damp and still hold, thinset can remain submerged in water and still perform. So, to get the terms straight. . .

    "Thinset" and "mortar" and "thinset mortar" are interchangeable terms. Like the stickier "mastic," which is interchangeable with "adhesive" they are what hold the tile to the floor, ceiling, wall, or backsplash. Grout is the stuff that fills in the spaces between tiles. Grout, which is also a cement product, but one with a different chemical mixture than thinset mortar, provides additional tile bonding and protects the edges of the tile from chipping. It is used always between the tiles, regardless of whether the tile was affixed with thinset mortar or mastic adhesive.

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