I will be tiling a tub/shower area using 8x10 ceramic tile with glass mosaic tile accent rows.

I see about half a dozen varieties of thin-set on the market:

Weights: - 25lb - 50lb

Types: - fortified - standard - flexbond

About every combination of the weights and types you can imagine is available.

What is the right thinset for my installation?

  • mosaics are difficult and experiment if you have not done this before. To much thinset and or to thin will ooze right thru. Even a dry mix will ooze. So build up and then use as little as will adhere the mosaic to the surface. Trush me, take one mosaic and test on a board or something. You can hose the test piece down later to reuse it. You need to see it for yourself to understand.
    – user12686
    Commented Apr 24, 2013 at 17:21
  • My shower was tiled with mastic back in the day. The tiles recently started falling off the wall. Never use mastic in a shower.
    – user51847
    Commented Mar 23, 2016 at 22:48

4 Answers 4



This is the easy one. If you're going to use dark grout, you can use grey thin-set. If your grout is a light color, or you're installing glass tile or a porous stone, use white thin-set. If you use grey thin-set with light grout, glass, or porous stone, the color may show through.

To modify, or not to modify

In most cases, you'll want to use some form of modified thin-set. Unmodified thin-sets are typically only used by professionals, and are modified at the time of mixing (using the pros secret recipe).


This is the economy thin-set. It will work well in most situations, and is a bit easier on the wallet. It should not be used with impervious materials such as glass, and porcelain.


A fortified thinset like VersaBond®, is an "all-purpose" thin-set and an excellent choice for DIYers. It will provide a good bond between just about any tile and substrate you can use, and it's good for floors, walls, and counter tops.


Flexbond is a polymer modified thin-set. It is a good choice when working with porcelain or glass tile, or when tiling over a plywood subfloor. It provides a bit of extra flexibility, which can prevent cracking in some situations.

How much to buy

First you'll have to determine the square footage, of the area you'll be tiling. I'll use a tub surround with 2 walls 3' wide, one wall 5' wide, and tile 6' up each wall as an example.

The total length = 3' + 3' + 5' = 11'
height = 6'
sqft = 11' * 6' = 66 sqft.

I usually estimate that a 50lbs bag of thin-set will cover about 60sqft, so in my example I'll need 1 50lbs and 1 25lbs bag of thin-set. Don't worry, if you overbuy you can always return the 25lbs bag when you're done (just don't open it, unless you need it).

How much to mix

The bag of thin-set should list the Pot Life, which tells you how long the thin-set is good for once it's mixed and in a bucket. You want to mix only enough thin-set that you'll be able to use in this amount of time. This will depend on how fast you work, so you'll have to experiment to figure out what works best for you.

How much to spread

The bag should also list the Open Time, and Adjustment Time. Open time, is how long the thin-set can sit on the substrate after being troweled. Adjustment time is the amount of time after laying the tile, that the tile can be adjusted. In most cases you'll want to ignore these numbers, and simply spread only what you can use in 5-10 minutes. Again, this will depend on how fast you work, so you'll have to figure it out as you go.

Trowel Size

You want to make sure you get 100% coverage under tiles, which may require back-buttering on larger tiles. You also want to use a bit thicker mortar bed when setting floor tiles, since floor tiles may be subject to more abuse. This trowel guide from JAMO Inc., is a good baseline for which trowel to use.

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Though you may find that you have to make slight adjustments, depending on the thin-set and tile you're working with.

  • So with a mix of ceramic and mosaic glass/stone tiles you would use Flexbond for the whole thing or flexbond just for the mosaic part and Fortified for the ceramic? Also with ceramic 8x10 and mosaic 12x probably anywhere from 3" to 9" (we'll be trimming it), a single square notch 1/4"x3/8" will do? Commented Oct 12, 2012 at 13:35
  • Flexbond and Fortified are the same thing. They're polymer-modified thinsets, generally referred to as "modified" thinsets as opposed to "unmodified" cementitious thinsets. You generally want to use modified unless you have a specific reason not to, like DITRA or KERDI or large-format tiles. For the mosiac, which is usually thinner than body tile, you'll want to tile above and below the area where you're using the mosaic and use thinset to "build up" a little bit, let it harden, and then apply the mosaic to the built up area with the same thinset. Commented Oct 12, 2012 at 14:21
  • @KarlKatzke Flexbond and Fortified are different, sort of. They are both polymer-modified thin-sets, but the recipes are a bit different giving the Flexbond a bit more flexibility.
    – Tester101
    Commented Oct 12, 2012 at 14:34
  • @TheEvilGreebo I'd use Fortified for the whole thing, unless you're tiling over plywood. And yes, I'd start with a 1/4"x3/8" trowel (but don't forget you might have to adjust depending on the coverage you're getting).
    – Tester101
    Commented Oct 12, 2012 at 14:39

First off, you generally don't want standard mortar; the fortified ones are superior in most applications (though there are some cases where they don't work).

As for which fortified one, I'd spent time looking on the website information. If you can't decide, a polymer-modified thinset would work well. If you are going with glass tile accents, you probably want a white thinset because the color shows through.

You could also take your tiles to a tile store and ask them to sell you an appropriate thinset.

  • Can you elaborate at all on in what situations the fortified mortars don't work? What do fortified and flexbond mean? "Look on the website" and "ask a rep" isn't really much of an answer. Commented Oct 11, 2012 at 12:43

We always use mastic on walls and yes it sometimes takes 3 days to dry but it's the best. When using wet saw to cut the tiles you have to dry the back of them before installing. Thin set (motars), if you clean the joints really good and wash your tiles with a damp sponge it will never show through grout. I always do this so there is no gray or white thinset showing. Haze left behind for those who wants to know; you can use haze chemicals for grout, white vinegar, cheese cloth works great, or even the micro. towels they sell. For the decos; reason you have oozing coming out is from too much mastic ot thin set but if you it dry and come back to take the time to detail every deco you won't have thin set showing.

  • In what way is mastic "the best"?
    – isherwood
    Commented Jan 14, 2016 at 4:18

Don't use thin set on a wall tile installation it isn't necessary. They make a very good adhesive premixed that has excellent bond and is much easier to apply to walls than thin-set. It's called mastic or tile mastic it comes in either one gallon or three and one half gallon buckets. Make sure to use a good backed board as well. Either hardi-backer or something comparable.

  • Please, when installing ANY tile on a wall or floor in a shower setting (wet application) never ever use mastic. Mastic never completely dries especially in wet settings, it re-emulsifies itself. It's like sticking a tile up with a piece of gum, expecting it to stay in place. Personally, I never sell customers mastic, ever. I advise against it, even when in an area that may not get wet. Also, if you're using a waterproofing membrane such as Schluter-Kerdi and you use mastic, you will void all warranties that the company makes. There's a reason why!
    – user19858
    Commented Feb 12, 2014 at 17:28
  • Mastic is not appropriate for any area subject to moisture.
    – user47765
    Commented Jan 13, 2016 at 22:46

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