Roof coatings and reroofing projects

by Katie Daniel | October 2, 2015 10:56 am

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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.

Waterproofing protection
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.

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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.

System traits
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.

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TPO sheets[4] are heat-welded together at the seams. However, these seams are potential weak points, reducing the overall break strength of approximately 1112 to 1668 N (250 to 375 lbf) in the field of the sheet to about 177 to 355 N (40 to 80 lbf) at the seams. In contrast, roof coatings are seamless, and when installed correctly, result in tensile strength ratings of 1723 to 10,342 kPa (250 to 1500 psi) in the field, depending on the coating composition.

Most roof coatings use the same polymeric material for flashing, priming, and field installation, providing fewer components for ease of installation. They incorporate fibers or high-build rheological traits to provide seamless flashing details at existing roof seams, curbs, fasteners, or other penetrations and angle changes. Some coatings may utilize a scrim for increased toughness.

Flashing materials bond to the existing substrate, providing a continuous barrier at the most critical elements of the roof system. The field coating is applied over these details to form a seamless system, sometimes incorporating a scrim or fabric for additional tensile strength and toughness. Since the flashing and field materials are of the same polymer technology, they form a chemical bond that yields a seamless, monolithic film without weak points at leak prone areas. This bond is permanent, and when a re-coat of the material is required, an additional layer chemically bonds to the first, providing another seamless system.

Movement considerations
While some buildings experience more movement than others, all buildings move—and this puts stress on the integrity of the construction. Sometimes, movement in the building will cause enough stress on the roof and the membrane can split, crack, or dislodge from the deck.

Elongation and tensile strength help overcome the forces imparted to the roof system by building movement. Elongation is the ability of a material to stretch beyond its original length without breaking. Tensile strength is the ability of a material to resist breaking under tension, usually measured in roofing as pounds per square inch. These qualities are important to roofing products because their function is to maintain a monolithic barrier to the weather.

When a building moves, the roofing material must be able to move with it using its elongation. If the re-covered roof substrate flexes or heaves, causing a split in a seam or a crack, the covering material would ideally have sufficient elongation to bridge the crack. TPO has relatively low elongation, with elongation tests producing failures at 25 percent. TPO relies on its tensile strength to overcome induced stresses in order to protect the property beneath. TPO membrane issues can occur at seams, as they can come apart from environmental stresses.

Roof coatings, on the other hand, are highly elastomeric—some roof coating data sheets provide elongation ratings of 200 to 1200 percent. This allows them to bridge cracks and remain monolithic under harsh conditions.

TPO incorporates several different components when it is installed. With roofing and waterproofing, uniformity is important, and it is good practice to use as few differing components as possible. This approach offers more surety of performance due to known compatibility, fewer unknowns, or weak points in the system. Metal fasteners, especially in metal decking, are thermal bridges allowing transfer of heat, which creates inefficiencies in the insulation layer. Under the right conditions, thermal bridges can be locations for condensation within the roof system. Issues of rot, rust, and biological growth can be the long-term result. Fasteners can potentially back out of roof decks and create punctures or be locations for membrane degradation in TPO and other single-ply systems.

Flashing details at penetrations in a TPO roof system are individually installed. Seams must be meticulously fitted with a hot-air seaming device. The material is heated to the appropriate welding temperature and pressure-rolled, creating heat-welded seams at critical flashings and junctures in the roof system. High temperatures and UV radiation are key factors in the aging of TPO roofing[5].

Cost comparison
The cost of installing TPO roofing can vary by the installing contractor, brand of material, sheet thickness, attachment method, and insulation type. An owner must also be ready to address any unforeseen circumstances that may occur when tear-off begins. It is best to have a conversation about how to address the issues beforehand with the installing contractor.

For the benefit of this comparison, a common roofing system will be used. This includes a popular thickness of TPO with 50 mm (2 in.) of polyisocyanurate (polyiso) insulation and a 13-mm (1/2-in.) coverboard. This 1.14-mm (45-mil) TPO roof system is mechanically attached on a metal deck. Depending on the area of the country and other variables, this type of system will typically cost between $6.50 to $11.50 per square foot.

When this roof reaches the end of its useful life, the building owner may choose to re-cover it. If the roof has been re-covered, the International Building Code (IBC) does not allow for another re-cover to be performed. Building codes allow a maximum of two roofs on a building. If a building already has two roofs, both must be torn off and replaced with a new roof. If, on the other hand, the current roof has yet to be re-covered, the owner may elect to install a re-cover insulation board and another layer of TPO. Using the same variables as the above re-roof scenario, an owner might expect to pay approximately $2.75 to $4.50 per square foot.

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Determining the initial cost of a coating installation can be tricky due to the wide range of coating technologies, brands, application methods, weather conditions, and status of the existing roof. For example, in the original roof from the example in this article, if the building owner were to consider a coating as an option, he or she might find it difficult to decide which technology is best suited. The roof coating manufacturer or a consultant can be a vital part of this selection process. Manufacturers and consultants draw on their knowledge and past experiences to offer viable solutions to suit an owner’s needs. On average, a building owner might expect to pay a quality contractor between $2.50 and $5.50 per square foot to coat an existing roof. Size and specification may decrease or increase this average.

During the warranted life of these two systems, an owner can expect certain maintenance costs to keep the system in optimal condition. These costs are normally in the range of 15 to 25 cents per square foot per year.

Most roof coatings can be re-coated at a cost lower than the original price. As long as the roof substrate is in sound condition, this process can be repeated many times. Recoating a roof can provide annualized savings over traditional roofing methods and over the life of a building, and these savings can be significant.

Conclusion
While TPO has been a low-cost alternative to built-up or modified asphaltic roof systems, roof coatings also provide a viable alternative. When considering the purpose of a roof replacement or traditional re-cover, coatings prove to be lower-cost technologies that, in many aspects, can outperform TPO.

When properly applied, roof coatings help shed water and protect existing roofs from the elements—meeting a building owner’s most foundational need in roofing and reroofing. In addition to performance and cost savings, roof-coating users benefit from less disruption, and the reduction of landfill waste from the lack of existing materials removal. Coatings are now an integral part of the roofing industry, and these types of products are becoming accepted as a financially responsible and indispensable component of a building owner’s investments.

The cost analysis of how much money and time is needed to prepare a surface for coating is something to take into account. In some circumstances, it may be more cost-effective to install a new membrane. For example, TPO can prove a better choice than a coating when the existing roof is deteriorated to the point a coating will not adhere or be able to perform over time.

Fred Wolfe has worked in roof coating manufacturing and installation for more than 12 years. He is the executive director at Roof Control Services. A NACE-certified coating inspector, Wolfe has installed millions of square feet of roof coatings including acrylics, urethanes, silicone, and various specialty systems. He can be reached at fwolfe@roofcontrol.com[7].

Source URL: https://www.constructionspecifier.com/roof-coatings-and-reroofing-projects/

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