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Roof Membranes, Roof Coatings, and Roof Performance

By Dustin Brooks

There's a difference between a coating and a membrane, and it can be hard to draw the line at times. Going beyond roofing, let's look at the definition of a membrane. A membrane is a "pliable sheet-like structure acting as a boundary, lining, or partition in an organism."

The cells in your body have membranes separating the inside from the outside, protecting vital interior elements from damage. These cellular membranes are two layers (lipid bilayer) working together — not all that different from a waterproofing membrane, serving as a pliable, long-term barrier between the structure and the elements.

So what do I mean when I say a coating is not a membrane? Let's go back to the human anatomy analogy. Your skin is a membrane. It's tough, flexible, and virtually impermeable. You can sit in the bathtub for hours and get wrinkly, but your body doesn't blow up like a balloon; skin is only permeable to lipids and fat solvents.

But skin can be damaged when you're in the sun too long. Sunscreen is a coating. The purpose of sunscreen is to protect your skin, the membrane, from damage. It's permeable, so you can still sweat, and it's applied relatively thin. It doesn't last very long, so you'll need to re-apply.

Liquid roofing products are not all that dissimilar. You have those that work great as a coating: applied in thin layers (20-30 mils), protect from UV degradation, permeable, cool you down (reflective), but need re-coated and touched up often. They do not perform like a membrane. They are not very durable, tough, flexible, or long-lasting. But they work great for protecting membranes.

Then you have other liquid products that work great as a membrane and end up like those made into rolls at a factory. Thick (50-80 mils), durable, flexible, strong, and impermeable. They serve as a primary barrier separating the structure from the elements. Some are reinforced, some are multi-layered, but the intended purpose is the same. No seams, fully-adhered, and self-terminating. These liquid-applied, seamless membranes should act more like your skin than your sunscreen.

It’s important for facility managers to understand the specific performance characteristics of different types of both roof membranes and roof coatings. Here are a couple charts comparing the properties of both coatings and membranes, including the VOC content of each. Which ones would you rather have acting as a skin protecting your structure?

Dustin Brooks is director of sales at Triton Inc.