Modern roofing technology has undergone the most explosive proliferation of new technologies, alternative systems, advancements, improvements, and a plethora of "latest and greatest" in its history. With these new technologies comes the responsibility of the roofing professional to familiarize him or herself with the current state-of-the-art.
For roofing jobs on existing buildings, the question faced by the roofing professional still remains the same: should an expensive new roof be installed or is there a retrofit alternative which will prolong the life of the existing roof? The answer may be a liquid-applied roof coating over the existing aged roof.
Roofing technologies include the traditional built-up roof (BUR), plus newer systems, such as various types of single ply and modified bitumen sheets and spray-applied polyurethane foam with a protective coating.
Since the 1980's, coatings are firmly established as an integral and required part of the urethane foam roofing system. The type of polyurethane used to make the foam is not durable if left uncoated and will begin to powder and degrade within weeks of application. Moreover, polyurethane foam is rather brittle and its skin can be cracked from impact with falling debris, heavy foot traffic, or severe hail storms. Coatings protect the foam from these potential mishaps.
Liquid-applied coatings have also proven their utility over other substrates, and can be used to protect, restore, and prolong the life of aged existing built-up or other conventional roofing systems. Unlike re-roofing alternatives, they are extremely lightweight, adding virtually nothing to the load on the roof deck or supporting structural members. In fact, roofs may be re-coated several times without contributing significantly to the load.
Because these coatings can be formulated white, they can be effectively used to impart reflectivity to the roof. This not only improves aesthetics, but also cools the roofing surface, contributing to longer service life. Since the surface of the roof is cooler, an additional benefit is a reduction in a demand for air conditioning. This has been well documented by National Coatings Corporation. The economic advantages of longer roof service life and reduced energy costs for air conditioning are the economic driving forces for reflective roof coatings. However, if aesthetics is a key objective, coatings can also be color tinted.
The Differences in Coatings
Technically, these coatings are much more than paints. By definition, they are liquid-applied, fully-adhered, elastomeric functional membranes formed in-situ on the existing roof. In the sense of being adherent to the substrate to which they are applied-in this case, the existing roof-they are paint-like. However, their elastomeric feature, which distinguishes them from paints, means they are also flexible. This flexibility, or elasticity, must exist not only at one temperature but over the temperature range existing in any particular building's climate. Elasticity enables the coating to tolerate a roof's movement as it responds to the stresses caused by temperature changes, settling of the foundation, weight loads caused by ice and snow, and seasonal expansion and contraction of the roofing envelope. These coatings are normally applied at film thicknesses of 20 to 30 dry mils (.020 inch to .030 inch) using a roller, spray, or brush. By comparison, paints are applied at two to three mils, thus making these roof coatings approximately ten times thicker than traditional paints.
Classes of Coatings
Roof coatings can be classified into three types: solvent, water-based, and 100 percent solids. Solvent systems are those containing an organic solvent as carrier for the coating. In this case, solvent evaporation causes the coating to form the membrane on the roof. Water-based coatings employ water as this carrier. The third class, albeit less common, is the solvent-free coating where two liquid components are premixed in a specially designed airless spray unit; a chemical reaction causes some of these two-part coatings to dry almost immediately. Each of these three coating classes has specific advantages and disadvantages relating to the type and presence or absence of solvent.
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