Selecting a Roof System: The Case for TPO
There are no “one type fits all” roof systems. So, when faced with a new construction project or re-roofing an existing facility, owners should be fully informed as to the type of roof system best suited for the scope of work. Along with a pre-bid evaluation, selection of the proper roof system is one of the most important steps owners can take to help avoid future, long-term problems.
The two types of roof systems that are most prominent in the market today are:
- Modified Bitumen, Multi-Ply
- TPO, Single-Ply
While there are other roof system types available today including PVC, EPDM, Built-Up Asphalt, etc., TPO and Modified Bitumen are far and away the most commonly-specified and selected systems. However, TPO and Modified Bitumen are vastly different systems. You can read a brief description of Modified Bitumen here and continue reading for a brief overview of TPO roof systems.
TPO stands for Thermoplastic Polyolefin and is a “single-ply” roof system consisting of a top and bottom sheet of TPO membrane with polyester reinforcing fabric in between with all three layers laminated together by heat at the factory. In the adhered application of a TPO roof, the system includes the insulation, cover board, roof membrane, flashing, and accessories. The primary manufacturers of TPO single-ply membrane roof systems are Firestone Building Products, Carlisle Syntec and GAF. TPO single-ply roofing systems are typically white in color. However, gray and tan are typically also available as well as specialized colors (for an up-charge). The TPO membrane comes in a thickness of 45 mil, 60 mil & 80 mil, with 60 mil most-commonly selected. Though specialized, thicker TPO membrane can be ordered (for an up-charge).
TPO membrane systems have been installed on a variety of building types; however, despite the wide array of applications of TPO systems, this single-ply roofing system comes with limitations. TPO roof systems are NOT generally a good selection for roofs with multiple penetrations, exposure to certain chemicals or heavy rooftop foot traffic.
Think of heavy industrial manufacturing examples including pulp/paper, chemical, textile, steel, and even paint shops of automotive manufacturing plants. Roof systems on these types of facilities are subject to higher levels of chemical exposure and potentially heavier foot traffic from electrical and other specialty trades accessing equipment located on the roof. Healthcare facilities with multiple roof penetrations and an abundance of mechanical equipment on the roof are also examples that are not generally a good fit for TPO systems.
However, TPO is well-suited for many other types of roof applications, especially ones which have limited roof penetrations, limited rooftop HVAC and other equipment and have limited foot traffic exposure. So, TPO is typically a good fit for warehouses, areas of manufacturing facilities with few roof penetrations or rooftop equipment and multi-story office buildings with limited rooftop equipment. When the application is a good fit, TPO systems are also a good option because they provide a layer of building insulation to the top of the building envelope while the entire system is relatively lightweight, allowing it to be installed directly on top of many existing roof systems - which can provide several advantages when re-roofing an existing facility. In a “recovery” scenario, the underlying roof system must be inspected to confirm no serious moisture is in the existing roof.
Existing metal standing seam roof systems are generally good candidates for TPO “retro-fit” roof systems. Such applications involve the installation of the roof insulation layer between the existing standing seams, followed by a hardcover board (DensDeck and Securock are popular cover board options). These first two components are mechanically attached to the existing metal roof or into structural steel joists if the existing metal roof does not provide for proper pull-out resistance from high winds. The pull-out resistance of a metal roof can be tested to determine resistance. The insulation and cover board are followed by the installation of a 60 mil or 80 mil TPO roof membrane system, either mechanically attached or fully adhered.
On concrete decks, the insulation in the TPO system is adhered with low-rise spray adhesive, specifically formulated for adhering insulation to the concrete roof deck.
Certain applications of lightweight insulating concrete can receive a mechanically-attached version of the TPO system, subject to acceptable uplift resistance testing of the lightweight’s underlying metal pan. On other existing roof systems with lightweight insulating concrete, adding a venting base layer mechanically attached to the lightweight insulating concrete system followed by an adhered application of the TPO system is an option.
In replacing existing asphalt or coal tar pitch roof systems with a new TPO system, certain inherent drawbacks present themselves. The seams of the TPO membrane, which are joined together by a robotic heat-welding machine in the field, can be contaminated by dust or debris generated during the demolition of the existing bituminous roof. Also, pieces of asphalt or coal tar tend to lodge themselves in the boots of roofing crew members. This can lead to the white TPO membrane being “scarred” by foot-traffic during the installation of the new TPO roof. Cleaning the TPO surface of these scars is tedious and—at times—nearly impossible to accomplish. Also, gravel from an existing built-up roof can puncture the TPO during the replacement process. Use of boot “pull-overs”, similar to those used in hospitals, can help control the conditions. If such a project is undertaken, the owner is best-served by choosing the 80 mil version of the TPO membrane. The 80 mil membrane is more durable than the 60 mil version. Application of the 45 mil TPO membrane has waned in recent years due to its “less-than durable” nature.
Once TPO roof applications are complete, they are inspected by the manufacturer’s technical representative. Subsequent to a passing inspection grade from the technical inspector, the manufacturer provides the owner with a 5, 10, 15, or 20-year warranty covering the material (roof membrane, roof insulation, accessories) as well as the labor for installation.
The four components (listed below in the order of importance) which lead to a successful TPO roof installation are:
- Selecting the proper TPO roof system for the specific conditions
- Selecting a prequalified, manufacturer-approved contractor
- Periodic “in-progress” inspections during installation of the roof system by the manufacturer’s technical inspector
- Weekly inspection of the roof work by the owner’s professional representative – generally, their roof consultant or architect
While this information is of a general nature, it includes practical recommendations based on professional experience. If you would like to leverage our many years of commercial roofing and building envelope experience, please contact us with questions or budget information for your anticipated project.