Evaluating Real-World Performance of Field Aged TPO Roofs

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Sponsored by GAF
Presented by Jennifer Keegan, AAIA

Aged Membrane Repairability

New TPO membrane that was commensurate in type and thickness of the existing membrane, was welded to the aged membrane roof samples to evaluate the ability to repair older roofs. Repairs with new membrane welded down onto the cap of the aged membrane—also called a top down repair—were evaluated, with Figure 5 indicating the general process. While this repair process is the most common, in some instances it is necessary to weld repair membrane to the core (the underside) of the aged roof, also called a bottom up repair, as indicated in Figure 6. Both approaches were evaluated in this study. Note that weld strength to the core was not evaluated for cases of adhered membrane roof samples due to remnants of adhesive and/or facer from the insulation or cover board. Given the remnants attached to the underside of adhered membranes, repairs to the core would not be reliable.

Photo courtesy of WSRCA

Figure 5. Repair of aged TPO membrane with a new patch welded to the aged cap.


Photo courtesy of WSRCA

Figure 6. Repair of aged TPO membrane with a new patch welded to the aged core.

In both cases, industry-standard cleaning protocols were followed in preparation of the test specimens used to measure weld strength and film-tearing bond. For consistency and to eliminate variables, a robotic welder was used at 12.1 feet per minute at 1148F.

Aged Membrane Solar Reflectance

The solar reflectance of the aged membrane samples was measured according to ASTM C1549. While the test method allows for sample rinsing, that was not carried out in this study. Thus, this study is indicative of actual solar reflectance that is experienced by the roofs. Therefore, long-term adherence to energy efficiency and/or HVAC equipment sizing assumptions by the building designer can be checked.

Results

As located in Figure 1, samples were taken from 20 different roofs across the U.S. Collectively, these roofs have seen over 1,000 days of hail, almost 400 feet of rain, nearly 4,000 days of over 90F and wind gusts of up to 120 mph. On average, a typical roof membrane in this study was installed between 2005 and 2006 has been exposed to 157 days of hail, over 50 feet of rain, over 500 days of over 90F and wind gusts of up to 92 mph.

To put this into perspective, the roof membrane from Orlando, Fla. has been exposed to the elements for over 17 years and has weathered 32 hurricanes, 56 feet of rain, 1,660 days of over 90F and wind gusts of up to 105 mph. Given this real world weathering by mother nature, all artificial aging requirements for testing were waived.

Membrane Thickness and Thickness of Coating over Scrim

TOS is a critical characteristic of single-ply membranes, because it is a measurement of the quantity of the weathering layer, which provides the UV and heat stabilization properties of the membrane. Erosion of the membrane down to the reinforcing scrim layer may compromise the weather tightness of the membrane, indicate failure, and require repair or replacement as shown in Figure 7.

Photo courtesy of Rene Dupuis

Figure 7: Premature failure as cap erodes and exposes scrim.

The membrane thickness over scrim (TOS) data for the field samples are shown in Figure 8. With one exception, all of the membranes tested show TOS values above the ASTM D6878-19 specification. For 60-mil membrane, this is 18-mil and for 45-mil membrane, it is 13.5-mil. The results suggest that from the perspective of erosion, all of the membranes have significant remaining life.

Figure 8. Membrane TOS 60-mil samples in blue, 45-mil in gray, with their respective ASTM D6878 minimum specification. Note that for Project 5, 60-mil membrane was installed at the perimeter and 45-mil membrane was installed in the field.

As indicated in Figure 8, sample 3 is 1 mil below the current ASTM minimum for TOS. However, this sample would have complied with the published ASTM requirements at the time of manufacture.

Total sample thicknesses for the field samples are shown in Figure 9.

Figure 9. Membrane thickness, nominal 45-mil in gray, nominal 60-mil in blue. Samples taken within the field of the roof. Note projects 1 and 2 used a 50-mil membrane and comply with the ASTM thickness requirements.

ASTM standard D6878-19 requires the as-produced membrane to be within +15 percent, -10 percent of stated thickness, and not less than 39-mils. Even after 12 or more years of aging, most of the 45 and 60-mil membrane samples complied with the current ASTM requirements for newly manufactured TPO membranes. Total membrane thickness in Projects 17 (an 18-year field aged membrane) is 2 mils shy of the requirement for new ASTM membranes while Projects 18-19 are 1 mil shy of the requirement.

It is somewhat in conflict with the TOS values, which are generally at or above the minimum. The apparent discrepancy between the two measurements suggests that the products included in this study were manufactured with a significant weathering layer, and after many years in service have maintained the critical thickness over scrim.

Samples taken from the perimeter might be expected to exhibit increased erosion of the weathering layer and therefore, have reduced TOS versus the field samples. However, as indicated in Figure 10, this was not observed.

Figure 10. TOS for the field versus perimeter samples. The diagonal line would be the result if no differences in weathering were observed. Samples in the field and perimeter were permitted on 9 of the roofs in this study.

As seen in Figure 10, at least for the roofs where permission to obtain field and perimeter samples was granted, no consistent trend in terms of weathering were identified. This is an indication that the data are within the error tolerances of the measurement technique.

 

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Originally published in September 2020

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