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What is “Wet Film Thickness” and why should I care?

Dan Marvin, Director, Technical Services, MAPEI Americas

Membranes that are applied wet and allowed to dry are everywhere in the tile world for waterproofing, crack isolation and more. But how do you know if you need one coat or two, or more? There are lots of rules of thumb – ‘If you can see through it you need another coat’ is a common one – but the only true way of knowing for sure is to measure it.

To determine if you have applied enough liquid membrane, you need to know how much the manufacturer recommends. Almost all technical data sheets for liquid-applied membranes list a target film thickness. It will typically be listed in the form of “mils” of thickness; each mil is one one-thousandth of an inch (1/1000” or 0.001”). So, a 30 mil wet film thickness (you will often see this abbreviated WFT) means 0.030” of membrane. You will also see dry film thickness (DFT) referenced. To determine the dry film thickness, multiply the wet film thickness by the percentage of solids in the product (30 mils of a 50% solids membrane is .030” x 0.5 = 0.015” DFT).

As manufacturers, when we measure the properties of these membranes, we assume that you are using the correct amount. If we tell you that water will not penetrate the membrane, we mean water will not penetrate the specified thickness of the membrane. If it is extremely thin in some areas, or has holes in it from nails or pinholes during application, it will not be waterproof. Likewise, if we say that a crack isolation membrane can protect your floor from a 1/4” substrate crack, the size of crack the membrane protects against goes down as the thickness of the membrane goes down. 

This is particularly important in steam showers. The thicker the membrane, the more it resists high temperature/high pressure water from moving through it (the ‘permeance’ of the product). The TCNA Handbook for Ceramic, Glass and Stone Tile Installation requires no more than 0.5 perms from a membrane being used in a steam shower. The liquid products that meet this requirement typically do so at 30 mils or more, much thicker than you would apply with one or sometimes even two coats. If the steam shower fails – and you did not apply enough waterproof membrane – that is on you.

Luckily, measuring wet film thickness is very straightforward. There are a variety of gauges available to manually measure the thickness, so make sure that you are using one that measures in the range you need. Sometimes you will hear them referred to as ‘combs’ or ‘rakes’ because of their shape with numbered teeth that show the film thickness. 

 We are including such a MAPEI wet film thickness gauge in select TileLetter issues. If you did not get one, contact your MAPEI sales rep.

Once you have applied your wet membrane (and while it is still liquid), place the wet film thickness gauge straight down into the membrane. Do not wiggle or move the gauge, just make sure that it goes all the way to touch the substrate. Remove the gauge and begin looking at the bottom of the teeth. Teeth that are longer than the membrane is thick will have membrane on them. At some point, you will find a tooth that has no membrane on it. The thickness is between the number on that tooth and the one next to it that has membrane on it. That’s it! Try it in several places to make sure you are seeing consistent results and wipe off your gauge quickly after every test – you do not want buildup on the teeth.

Once you have performed the test a few times, it becomes a quick double-check to ensure you are getting enough membrane applied. It can also show if you are applying too much. 

 If you ever need help determining the required thickness of a MAPEI membrane or have questions about the process, feel free to call 1-800-992-6273 to speak with a Product Support Representative.