Part 1: BUR (Built-Up Roof)
The roofing industry's workhorse is the built-up roof (BUR) system. Coatings commonly used on BUR systems include solvent- and water-borne asphalt coatings; solvent- and water-borne aluminized asphalt coatings; and water-borne white elastomeric coatings (generally acrylics).
BUR systems can have several surface finishes—they can have bare or glaze-coated top sheets; be flood-coated with mopping asphalt; have a mineral (granulated) cap sheet as a top layer; or have a finish layer of gravel embedded in hot asphalt.
A mineral cap sheet generally is the easiest surface to coat because its small texture allows most coatings to achieve a strong mechanical grip. These minerals also block asphalt bleed. Smooth BUR systems are readily coated with solvent-borne asphalt coatings; they require careful preparation and base coats to accept water-borne coatings. Gravel-covered roofs generally are impractical to coat without removing the gravel, which will affect a roof system's fire rating.
Asphalt coatings, long recognized as effective for prolonging life cycles and restoring aging BUR systems, are applied after a BUR system's initial installation and at regular intervals during the BUR system's life cycle as part of a regular maintenance program.
Traditional solvent-borne bituminous roof coatings are made from cutback bitumen. To make cutback bitumen, distillate bitumen residue is refined to various specific physical properties through vacuum distillation or oxidation. This refined asphalt then is blended with a petroleum solvent, such as mineral spirits. This blend of bituminous resin and solvent is combined with special clay fillers or fibers to create a gel coating. These components' proportions ultimately determine a coating's consistency and properties.
For the past 60 years, water-borne asphalt coatings have been developed and used as alternatives to solvent-borne coatings. The mixture of bitumen, water and bentonite (clay) is referred to as asphalt emulsion because the microscopic asphalt particles are held in suspension by a clay coating.
In an ongoing, independent coating durability test program conducted by the Midwest Roofing Contractors Association it was observed that solvent- and water-borne coatings in all climates perform better on all types of smooth roof surfaces when applied over an asphalt emulsion base coat. One advantage of asphalt emulsions is they generally meet fire ratings of original asphalt BUR systems.
If an original roof system was applied in hot asphalt, an asphalt emulsion coating generally can be applied relatively soon, but if the original roof was cold-applied with an adhesive, it usually is necessary to wait about 30 days before applying an asphalt emulsion.
Aluminum pigments can be used to formulate solvent- and water-borne asphalt coatings to protect against ultraviolet (UV) radiation and provide significant reflectivity. These coatings have small flakes of aluminum pigment dispersed in them. They typically have solar reflectivity values between 0.40 and 0.50 though premium formulations have demonstrated initial solar reflectivities in excess of 70 percent.
Highly reflective white top coats have become more common during the past several years as a means to reduce peak roof surface temperatures. This minimizes peak electrical demand for air conditioning in many buildings and provides ultimate reduction in diurnal temperature cycles, which can stress and fatigue roof systems.
A majority of these white coatings are water-borne acrylic coatings. Because of the range of physical and chemical properties available within the general acrylic category, a particular coating's suitability for a given substrate should be verified with the coating's manufacturer. Most white acrylic coatings intended for use on asphalt roof systems can be applied directly to mineral cap sheets after appropriate surface cleaning (some don't require a barrier layer); smooth asphalt surfaces normally require a specified primer or base coat designed specifically to stop asphalt from bleeding into the top coat.
Asphalt typically contains light oils called exudates, which rise to a membrane's surface soon after application. Exudates often wash off roof systems after a rainstorm. Nevertheless, any exudate that remains on a roof system before coating application should be thoroughly removed. Exudates can cause staining or even debonding of a coating.
An optional coating for a gravel-surfaced BUR system is spray polyurethane foam (SPF). This effectively changes a substrate from gravel-surfaced BUR to SPF. An advantage of this approach is the layer of insulation above the BUR keeps the BUR from being subjected to extreme heat. A white coating over an SPF roof system can result in additional energy savings. Typical coatings for SPF are acrylic, polyurethane and silicone.
Whitening of the existing base of a commercial BUR system is important. Although BUR systems are among the most durable roof systems available, it is possible to extend their life cycles even more with coatings.
Source: Professional Roofing