By Dustin Brooks
Liquids are, almost literally, flooding the roofing industry these days. It is rare to find a manufacturer or contractor that does not promote a “roof coating” or “liquid membrane.” This is because liquids have obvious benefits for facility managers. Installing a liquid over an old roof helps maximize the original investment and avoid disruptive, expensive, and wasteful tear-offs. Though it is important to note restorations or recovers are not recommended on roofs past a certain part of deterioration or that have saturated insulation.
However, there are vast differences between a coating and a membrane, but it can be hard to draw the line at times. Most roofing experts 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. It can be helpful to draw comparisons to our human cellular structure.
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 multi-ply felt roof systems get excited. Redundancy starts at the cellular level. A waterproofing membrane, which serves as a pliable, long-term barrier between the structure and the elements.
A coating is not a membrane. How so? Let’s go back to the human anatomy. 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 many people, their 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 it is still possible to sweat, and it is applied relatively thin. It does not last very long, so it needs to be re-applied.
Liquid roofing products are not all that dissimilar. Some of them work well as a coating — a sunscreen. They are applied in thin layers — 0.5-0.7 mm — protect from UV degradation, are permeable, cool things down (thanks to reflectivity)— but need to be re-coated and touched 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.
Picture 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 — 1-2 mm — 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 based on past failures in the industry. Liquid waterproofing technologies have improved drastically and there are many to choose from. It’s important for facility managers to understand the specific performance characteristics of different types of both roof membranes and roof coatings.
Decrypt the Technical Jargon
Elongation: the amount of “stretch” a material has before it tears. Considering liquids are fully adhered to the surface, high elongation is important to withstand structural movements or expansion/contraction of underlying materials. Typically, higher elongation equals better puncture, impact, and hail resistance.
Tensile Strength: the amount of force required to tear a material after it stretches. Low tensile strength means the product is more susceptible to damage. It’s important to have a good combination of elongation and tensile strength. This amount of force is commonly measured in Newtons/mm2 or psi (pounds per square inch).
Permeance: how much water vapor (moisture) can travel through the coating or membrane. Essentially it tells us how “breathable” a product is. Higher permeance means the product will not withstand ponding/standing water. It may be water-resistant, but is not truly waterproof. Low permeance (under 1 perm), means it is impermeable and can hold water on top of it forever without moisture transfer.
Volatile Organic Compounds: VOCs represent solvents in a liquid material that are harmful to human health and the environment. Many countries and municipalities have VOC limits for paints and coatings to limit off-gassing.
Adhesion: how well a product sticks to a particular surface. The examples below use a concrete substrate and a value in pounds per square foot (psf). This is how much force or pressure is required to remove the product from the surface. It is often used to calculate wind uplift strength.
Here’s an overview of the applications of some of the most common and fastest growing liquid roof products:
Acrylic is best used as a “sunscreen”, for UV protection of membranes to extend their life. It’s great for reflectivity to reduce surface temperature and to reduce energy costs (depending on insulation in the existing roof). Other pros include that they’re low -cost, come in a variety of colour options, perform as a good sacrificial/wear layer, and are easy to recoat. The cons of acrylic liquid roofing products are that they won’t withstand ponding water, have low strength, and have been inappropriately promoted as a membrane.
Silicone is best used as a top coat over a membrane to prevent premature degradation, extend life, and add reflectivity. Other pros include that it’s reflective, withstands ponding water, resists animal fats, has good tensile strength and experiences minimal degradation. However, it’s very difficult to repair or recoat, as nothing but silicone will adhere to silicone. Other cons include that it’s not water-based, picks up dirt (turns yellow/brown quickly), has poor tear resistance, and is slippery when wet.
Polyurethane foam is best used as insulation material, not as a waterproofing membrane. Its top coat is typically acrylic or silicone and requires constant maintenance to protect the foam from UV exposure. The pros of polyurethane foam are that it provides excellent insulation (high R-value), can be applied to any thickness, and is watertight. The cons of polyurethane foam are that it is not UV stable, as it requires a top coat to protect it from becoming open-cell, is easily punctured and damaged by foot traffic, birds, and insects, has no elongation, and is difficult to install.
Urethane is best used as a waterproofing membrane for plaza decks, floors, carparks, and other industrial applications. It’s suitable for use on certain types of existing roofs, but is difficult to repair or recoat. The pros or urethane are that it’s strong and durable, impermeable, and has high tensile strength. The cons of urethane are that its installation is sensitive, causes toxic fumes, is flammable in liquid form, can be expensive, and has low elongation.
Cold-Process Bitumen Emulsion
Cold-applied asphalt/bitumen emulsion is best used for roof restoration, with a top coat, but preferably in warm climate zones and over roofs in fair condition. The pros of cold-applied bitumen are that it’s low-cost, easy to apply, withstands ponding water, and has a proven track record. The cons of emulsion are that it cracks in freezing temperatures, requires a top coat, has poor strength and elongation, and requires fabric/fleece reinforcement.
Synthetic rubber liquids are best used as a roof restoration membrane, with or without a top coat. The pros of rubber are that is has high elongation, good tensile strength, can be applied any thickness instantly, is impermeable, and has excellent adhesion. The cons of rubber are that it is black in colour, typically requires a top coat, is more expensive than bitumen, and cannot be installed in freezing temperatures.
With many liquid technologies available to protect roof assets, it’s important to choose the right product and right solution for the application. A new membrane may be needed and thin coating would only cause further problems. Or a new membrane may not be needed, only a reflective coating to extend the existing membrane’s life. Or a metal roof that needs to be protected from any possible rust and a permeable coating would only add problems. Never hesitate to ask contractors, consultants, or suppliers for physical material samples, independent lab reports, data sheets, certifications, case studies, or references. Proper due diligence is vital when investing further in existing roof assets.
Dustin Brooks is the Director of Sales and Vice President at Triton Inc. in Cedar Rapids, Iowa, USA. His group provides a variety of roofing products and services to building owners/operators throughout the world. Their focus is on protecting existing structures through proactive restoration solutions. He enjoys visiting with family, traveling, volunteering with non-profits, and speaking engagements. Dustin can be reached at firstname.lastname@example.org or +1 319 861 5233.