QUESTION

Installing Porcelain Tile with Mastic - I came across your website and hope that you might be able to provide some advice, or point me in the right direction to get some. I'm renovating a small bathroom (two walls exterior facing). I spent the weekend applying porcelain floor tiles to the walls and tub surround (using proper tile backerboard - 1/2 inch)... the tiles are 12" x 12" and 16" x 16" inch - floor tile. I used mastic instead of mortar for the job, and I'm having serious second thoughts. My concern is that: a) the mastic may not be strong enough to support the weight. While installing, I used a couple of nails to support each tile. I'm thinking of leaving them in place and grouting them right in. The other concern is that moisture in the tub surround area may eventually cause the mastic to emulsify and release prematurely.I read a bit about this on various forums, and see conflicting reports about the suitability of mastic to wet areas - and or heavy tiles - I used a 1/4" x 1/4" notched trowel. Got very good adhesion while setting the tiles... but I'm worried. The grout will be 3/16"... will that grout transmit some moisture to the back of those tiles, and would that or any condensation on the back of the tiles caused by a hot shower on a cold day cause the mastic to emulsify and eventually release? hope you can provide some insight here, and keeping my fingers crossed that you'll say it should be fine. Thanks in advance for any wisdom you may provide.

ANSWER

ANSWER - Organic mastic is not considered a long lasting product, and depending on the type of mastic you used (ANSI A136.1 type 1 versus type 2, and the quality of the product) it can be moisture or alkaline sensitive and deteriorate over time, or become brittle over time.

Mastic can last many years under the right conditions if it is a good quality mastic.  You apparently used a mastic that was not non-sag.  Some mastics are designed to keep larger tiles from sliding down the wall during the installation.  Whether you used the type 2 mastic for prolong moisture exposure or not, you will have to look at the product data sheet or container label.

Since you used in on walls and the closest area to water is the tub surround, then as long as it is well attached to the wall and water is kept from it, it should be ok.  Mastic is water resistant to initial contact, but if subjected to prolong exposure it can deteriorate depending on the type.

Make sure the transition joints within the tile (inside wall joints) and where it butts into the tub are filled with a caulking rated as meeting ASTM C920 which is a pure silicone or urethane sealant, and not a latex or acrylic caulk.  Good luck.