By Fred Wolfe
Selecting the proper reroofing material for a given structure after an existing assembly has reached the end of its useful life can be a daunting decision. Affordability, among other technical and performance considerations, remains at the top of the list of most building owners’ concerns. In the pursuit of a healthy balance between cost and quality, thermoplastic polyolefin (TPO) membranes and roof coatings are two categories that continue to gain market share among the many roofing options.
Coatings have long been perceived as a last resort—a bandage to buy the owner another few years until a permanent solution can be afforded. Initially, TPO was also not without its own perceived issues, mostly due to performance. Recently, that material’s reputation has improved, and it is now considered a long-lasting solution for cost-conscious owners.
Similarly, roof coatings have improved due to advances in technology and experience. This improvement is helping change roof coatings’ reputation, and the old stereotypes are no longer relevant. Such products may surprise many with their ability to provide waterproofing, reflection of heat, easy installation, and reduced overall lifecycle costs.
Certain roof coatings, especially those with a reinforcing layer, can provide an extra level of waterproofing protection as a continuous top surfacing/layer—similar to the approach used by roofing membranes. However, the ‘human component’ of installation is a big factor in the performance of these two approaches.
Installation errors can be a possible source of leaks. These can lead to major issues in food processing, electronics manufacturing, and pharmaceutical facilities, and can mean loss of profit and possible temporary shutdown of the facility.
When installing TPO seams, for example, one poorly installed field or flashing seam resulting in a small ‘fish mouth’ or ‘mole run’ could be the source of water. With a coating installation, there are no seams. The system is fairly simple because the small number of components reduces the possibility of mistakes. Regardless of the system, an experienced and conscientious contractor is important for proper installation and performance.
Installation of a roof coating system can be very quiet, especially when the product is roller-applied. Often, the loudest noise is the low hum of a pressure-washer. This is beneficial to office personnel who require minimal noise levels during work hours.
Additionally, night tie-ins are easily executed, with disruptions to facility operations minimized during coating application. All roof coating systems are fully adhered and self-terminating—meaning stopping for weather or at the end of the day can be much easier with a coating system. Additionally, facilities such as hospitals, research centers and other odor-sensitive buildings may require a low-odor option or a material low in volatile organic compounds (VOCs). This concern arises because coating fumes are often heavier than air and may drift into the building or be drawn in through an HVAC intake.
Fewer penetrations are better in a roofing system, and if there is a breach in the membrane, fewer fasteners penetrating the roof deck provides less opportunity for water to find its way to the interior. Some re-cover roof systems, depending on the decking material, require mechanical attachments. Coatings take advantage of the existing fasteners by adhering to the existing membrane, and do not compound potential failures through additional penetrations in the system.
TPO is a polymer comprised of ethylene propylene rubber and polypropylene, and the roofing membrane includes a reinforcing scrim. ASTM D6878, Standard Specification for Thermoplastic Polyolefin-based Sheet Roofing is the standard by which TPO is manufactured. The material is installed as a sheet, heat-welded at the seams, and may be mechanically attached or fully adhered with a compatible adhesive.
Coatings are polymers or resinous compounds that bond to the surface of an existing membrane. They are not typically reinforced, but may have a scrim incorporated on installation. Usually, coatings are applied by roller or sprayer in liquid form. They cure through various mechanisms in order to form a protective layer with waterproofing characteristics above the existing roof assembly. When a reinforcement is included with a coating installation, the system has more membrane-like characteristics.