National Coatings Blog

Determining Dry Film Thickness for Roof Coating Using a Wet Mil Gauge

Posted by Nicole Gale on Mon, Jan 16, 2012 @ 05:35 PM


  • Press gauge into wet elastomeric roof coating.

  • Withdraw vertically and note deepest tooth having paint on it and the next higher tooth that is not coated with paint.

  • The true wet film thickness lies between these two readings. Clean gauge after each use.

Elastomeric Roof Coating Wet Mil Gauge


Many roofing specifications and warranty policies call out for a certain number of dry film thickness mils to be required on a job.  The wet mil gauge gives you one of the numbers needed in the formula to determining the dry film thickness.  The other number you will need to know is the volume solids of the elastomeric roof coating.  Here is the formula:

 Wet Film Thickness x Percent Volume Solids = Dry Film Thickness (DFT)

Let’s use a specification from National Coatings as an example.  The specification calls for 24 mils DFT of elastomeric roof coating AcryShield A503 base coat & AcryShield A400 top coat.  Let’s first take the AcryShield A503 base coat. It has a 54% solids material – the contractor will need to apply 1 ½ gallons (these quantities are based on perfect conditions, perfect spray equipment, no wind, etc…see "Things to Consider” below) per square to achieve approximately 24 wet mils. Using the formula above we will take 24 and multiply it by 0.54 to find out the DFT equals 12.96.  When the base coat is dry, the AcryShield A400 top coat will also need to be sprayed at 1 ½ gallons per square to reach 24 wet mils. This top coat has a 52% volume solids content, so we will take 24 mils and multiply it by 0.52 to find out that the DFT equals 12.48. When you add the 12.96 DFT of base coat and 12.48 DFT of top coat, you are left with a roof membrane that is 25.44 mils DFT which slightly exceeds the specifications requirement of 24 mils DFT. 


  • When measuring the wet film thickness, you must do so immediately after the application of the roof coating. If the coating has a high concentration of volatile solvents (liquids that vaporize at room temperature), they will evaporate immediately giving a false reading.

  • Wind Loss:  Winds up to 20 miles per hour, you may lose 20%-30% of the coating required.

  • Miscellaneous Loss:  Add between 3% to 10% to the total coating required (extra coating required due to lapping of the coating between passes and coating trapped in the lines and in the empty pail/drum)

Cool Roof

Tags: Roof Coating, Building Owner, Facility Manager, Contractor, Specifications