Why Would Anyone Use Mastic for Tile Showers? And Why You Should NOT!

November 8th, 2009 by Grout Guy

Ugly peeling mastic on back of tile

The ugly peeling mastic on the back of this tile came from a shower that was less than 2 years old. It was bulging and grout lines were cracking from the damp swollen sheetrock behind the tile. Black mold was growing also, which was a concern to the upset homeowner. This shower could have lasted a century if thinset cement was used instead. The total replacement that was necessary cost the homeowner totally unnecessary money, money she had already spent once and now had to spend again.

If mastic is so evil, why would anyone use the stuff anymore?

To recap earlier posts, mastic is a tile adhesive used to hold up tile on vertical surfaces like walls. But in a shower, it weakens over time as the moisture from the shower makes its way to the back of the tile. This leads to a LOT of tile failures and the need for tile repair.

If tile repair were cheap and easy, it wouldn’t be much of an issue…but it’s not so easy and cheap.

So, why is mastic still sold?

It’s very easy to use. It is sticky, so it grabs wall tile and won’t allow it to slide down the wall. The better material, thinset cement, doesn’t have instant grab. Instead, you have to actually build a small support of the wall tile so they don’t slide down and fall off while the cement is wet. This is called a batten board, and it takes maybe 10 minutes to put one on the wall. So if the shower has 3 walls, it might take a total of 30 extra minutes to tile the shower. Maybe a bit more, counting the time to mix the thinset, but not much more. So for half an hour’s savings, tilesetters make your showers ‘guaranteed to fail’. Ridiculous!

The only legitimate use for mastic is on a wall surface that you know will never get wet!

Outside a shower, you might use it for bathroom walls or kitchen walls. For backsplashes, I’d still be nervous if the sink splashed near the wall tile. I’ve even seen some backsplashes ruined by mastic installation.
Fireplaces? Nope! Mastic also weakens during exposure to heat as well, not just water.

So, to summarize, the stuff is only useful if you know the tile will never get wet, or hot, which kind of rules out almost every tile installation I can think of.

Just say no to the stuff….say yes to real thinset cement!