TPO sheets 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.
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.
Finding the right coating may be more difficult than you think at first glance. There are lots of factors that play into how you should protect your roof so a bit of research may be required. Thanks for sharing!
What an informative and well explained post. Roof coating has definitely been underutilized in the past but the technological advances are turning the tables and we see them applied more and more on a variety of roofs these days. Great article!