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Roof Performance: Coatings vs. Membranes

By Dustin Brooks

There is a difference between a coating and a membrane, but it can be hard to draw the line at times. I consider a roofing membrane to be a primary long-term, durable, and flexible barrier to prevent water and chemicals from damaging insulation and entering an institutional or commercial building.

Going beyond roofing, consider the definition of a membrane — a "pliable sheet-like structure acting as a boundary, lining, or partition in an organism." Cells in the human body have membranes that separate the inside from the outside and protect vital interior elements from damage. These cellular membranes are two layers — lipid bilayer — working together. (This is where all fans of two-, four-, and six-ply roof systems start cheering. Redundancy starts at the cellular level.) They are not all that different from a waterproofing membrane, which serves as a pliable, long-term barrier between the structure and the elements.

What do I mean when I say a coating is not a membrane? Let's go back to the human anatomy analogy. The skin is a membrane. It is tough, flexible, and virtually impermeable. A person can sit in the bathtub for hours and get wrinkly, but the body does not blow up like a balloon. Skin is only permeable to lipids and fat solvents. It protects the underlying tissues and organs from the outside.

But for those who are naturally pale, like me, the skin is damaged by staying in the sun too long. Sunscreen is a coating. Its purpose is to protect the skin — the membrane — from damage. It is permeable, so humans can still sweat, and it is applied relatively thin. It does not last very long, so re-application is important.

Where is this analogy going? Liquid roofing products are not all that dissimilar. Some of them work well as a coating — a sunscreen. They are applied in thin layers — 20-30 mils — protect from ultraviolet degradation, are permeable, cool down the body — reflective — but they require re-coating and touching up often. They do not perform like membranes. They are not very durable, tough, flexible, or long-lasting, but they work well for protecting membranes.

Think about it: What if human cells were only protected with a couple layers of acrylic paint? Other liquid products work well 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, and some are multi-layered, but their intended purpose is the same. No seams, fully-adhered, and self-terminating. These liquid-applied, seamless membranes should act more like skin than sunscreen.

Too often, any material sold in a bucket or drum is labeled a coating and given the same negative stereotype. But they don't all perform poorly. With so many to choose from and filter through, the field can be crowded and selection can be confusing.

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 several 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 with Triton Inc.

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posted on 11/29/2017