Gallons per square or mil thickness which is better?


Which is it?  Gallons per square or dry mil thickness.  If you ever had a roof coated or coated a roof, you should have heard one or both of these terms. "Gallons per square" and or "mil thickness."  

Let's start by talking about how the coating works. To do that, I am going to use acrylic coating as my "coating." Since it is the most commonly used coating material, often referred to as "elastomeric"  coating, which isn't accurate.  All roofing coatings are elastomeric. That means they "stretch." 

When a contractor writes up a scope of work for a roof coating project, they tend to describe coating application in these terms "apply the coating at a rate of  1 1/2 gallons per 100 square feet." It may sound good, but it does not mean much. The reason manufacturers say 1 1/2 gallons per 100 square feet any thicker and the coating material will "mud crack," creating a weaker and irregular film when cured. 

To summarize this point; 1 1/2 gallons per 100 square feet is the most coating that can be applied without risking "mud cracking."  It has nothing to do with the final project goal how thick the coating should be.  

How thick should the coating be? According to the Spray Foam Alliance, the minimum dry mil thickness should be 25 mils.  I'm going to get just slightly technical now. Acrylic coatings are a mixture of solid and liquid ingredients. Acrylic coatings cure  (dry and Harden) by the evaporation process.  When the material is applied, the liquids evaporate leaving the solids behind. The solids bond together creating a monolithic (no seams) film. The rate of evaporation is essential to achieve a durable bond, which is why the manufactures limit the amount of coating applied at one time. 

When a contractor applies the coating, it's called "a pass"   If you apply  1 1/2 gallons per 100 square feet in one pass, depending on the amount of solids in the coating, you end up with 9-16 mils of dry film thickness is no less than 25 mils.  It may be more depending on the type of warranty the owner wants. 

Better contractors may write in their scope "3 passes of coating at a rating of 1 1/2 gallons per square foot, " or how many ever passes.  Even though this is a better way to describe the work, it still does not tell us what the mil thickness of the final product should be.

On paper, the math may work out.  The number of passes may add up to the desired mil thickness.  However, there are other factors to be taken into consideration.  Such as the texture of the roof material.  The rougher the texture, the more coating required.  There's the wind speed to consider, the humidity, and other various environmental factors.  All of which affect the final mil thickness of the coating. 

So what's the answer?  Forget about gallons per square feet and require the final dry mil thickness instead.  Keep in mind that the " 1 1/2 gallons per 100 square foot" is a quality control issue.  When the job is completed, you want a certain final mil thickness, regardless of the number of passes.  

To summarize;
"1 1/2 gallons per 100 square feet "= a quality control measure. 

The objective is not the number of passes, but the final dry mil thickness of the coating.  

So next time you are having a roof coated, ask the contractor " what is the final mil thickness"? Better yet, tell the contractor what mil thickness you want. 

Remember, you don't have to go it alone.  Many great roofing consultants can help you, and you can always reach out to the "Arizona Roofer" if you have any questions. 

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