Mike Silvers, CPRC, Silvers Systems Inc. and FRSA Director of Technical Services
Self-adhering manufacturer’s product approvals showed relatively low uplift resistance for these underlayment systems. The highest of those we found provided resistance of 45 psf with the safety factor of 2 accounted for. This means the product should have resistance of 90 psf during testing. The resistance stated in these product approvals would not meet the American Society of Civil Engineers (ASCE) 7-16 requirements in many areas of Florida.
FRSA was concerned that this prescriptive application was being used to circumvent the more restrictive ASCE 7-16 compliant requirements of the 6th Edition FRSA-TRI Florida High Wind Concrete and Clay Tile Installation Manual. The manual has prescriptive methods for two ply hot mopped systems that include greatly enhanced fastening for the #30. For all other underlayments you need a product approval that meets the resistance values for your specific job based on the tables in the manual or engineering calculations that are based on ASCE 7-16.
In order to validate our concerns, the FRSA Education and Research Foundation provided funding, donated through an endowment by Bob Ferrante, that allowed us to conduct testing to verify the actual uplift resistance of this system. We began testing at the PRI facility in Tampa in April. Four different Miami-Dade approved ASTM D226 felts and four different self-adhering membranes were tested using TAS 117B for pull-through and ASTM D1876 adhesion peel test. The best performing of each were installed on five
test decks. The #30 with best pull-through resistance (Product D in Table 1) and the one offering the best surface for adhesion
(Product A in Table 1) were nailed using tin tabs/caps and ring shank nails into two decks using the standard pattern of 6” o.c. at the laps and two rows at 12” o.c. staggered in the field (per RAS), with three others using 6” o.c. at the laps and three rows at 8” o.c. staggered in the field. The best performing self-adhering membrane for adhesion (Product 2 in Table 2) was then applied to the two different #30 on all five decks. Approximately thirty days later, we tested them to failure in a bell chamber. The results were even lower than we had anticipated and very concerning. Tables 1-3 will show the test results.
You can see in Table 3, that the Passing Uplift Pressure (psf) column in yellow shows for Specimen No. 1 and 4, which have the prescriptive nailing patterns, the passing pressures are 30 psf. When you apply the required safety factor of 2, it results in a final resistance pressure of 15 psf. This is very low and confirmed our previous concerns. You can also see that with minimally enhanced fastening and, in one specimen, by taping
the joints of the plywood, it doubled the resistance. But when the safety factor of 2 is applied, the 60 psf becomes 30 psf. This is still very low. These values were much lower than known
values for two-ply hot mopped systems, so the next question is why?
The failure mode shown in the green column in Table 3 were fastener pull-through. The only place we experienced fastener pull out was in the backnailing where the self-adhering membrane being nailed through added to the pull-through resistance. The pictures below show the bottom or underside of a tested underlayment and the fasteners that remain in the deck. Notice how the #30 felt is ripped and the tin tabs are deformed. Previously tested two-ply hot mopped underlayment failures were typically fastener pull out. So, there is clearly a difference in how the felt and tin tab interact with self-adhered versus hot mopped systems.
After a great deal of contemplation and discussion, we formed a hypothesis which I will attempt to explain. A mop is used to apply hot asphalt over a #30 and a nail/tin tag combination asphalt runs under and is applied over the tin tag. Then a second layer of compatible asphalt membrane is immediately applied. When the asphalt cools, the tin tag is sandwiched
between these two asphaltic membranes creating a
surrounding bond and, due to the rigidity achieved, helps to spread the fastener loading into the membranes. This bond locks the tin tag in and reinforces its resistance to tin tag deformation, as well as adding pull-through resistance to the interface. When using a self-adhering membrane, the adhesive does not solidify like asphalt, thereby leaving the tin tag #30 interface much weaker and, due to the flexible nature of the completed membranes, susceptible to single fastener loading and pull-through failure mode (see photos below).
Having a better understanding of the low resistance to uplift pressure that these prescriptive #30 and self-adhering membrane underlayments provide and why, we noted that almost all testing was done exclusively with nail/tin tag fastening. This may be one area where a stiffer cap nail may increase performance. Base sheets with better pull-through resistance and surface for better adhesion is another possibility. The vacuum chamber testing performed did not achieve high enough pressures to evaluate the
adhesion properties of the self-adhering membranes. The information available leads one to believe that a D226 #30 will not achieve adequate uplift resistance to be used as the base sheet in a two-ply self-adhered system. There is evidence that with the right base sheet and fastening – a two-ply system that includes a self-adhering top layer – a compliant underlayment system can be achieved. One important concern is the relatively high cost that will come with this option.
Regardless of why these underlayments don’t provide better overall resistance values, it is clear that we need to rectify the problem so that future editions of the Florida Building Code can address the issue. The 6th Edition FRSA-TRI tile manual deals with this issue but unfortunately the Miami-Dade Roofing Application Standards (RAS) do not. The RAS are referenced in the code for use outside of the High Velocity Hurricane Zone (HVHZ Miami-Dade and Broward counties). If we can address the prescriptive underlayment methods included in the RAS, we can rectify this problem. Many contractors, when working outside of the HVHZ, use
underlayment applied direct to deck. These systems provide the highest uplift resistance at a cost that is less than the prescriptive option and even more cost effective when compared to conforming two-ply systems. As many of you know, direct to deck applications and fasteners without tin caps are not permitted for use in the HVHZ. The stance on the direct to deck application exists in conflict with RAS No. 118-20, 119-20 and 120-20 Underlayment Applications, E. Self-Adhered Underlayment (Single Ply). A single-ply underlayment system utilizing any Product approved self-adhered underlayment. The roof cover is terminated at approved metal flashings. Apply one layer of any self-adhered underlayment in compliance with the underlayment manufacturers approved/requirements. As stated earlier, this is a cost effective way to meet the uplift resistance required by the code and should
be acceptable in the HVHZ as well.
With all of this in mind, the FRSA Codes Subcommittee allowed the research project task group, which includes Manny Oyola, Eagle Roofing Products, Greg Keeler, Owens Corning and me to arrange a meeting with officials at Miami-Dade to discuss our
test results and look for ways to deal with the problem. I am very happy to report that our task group met with Jorge Acebo, Jamie Gascon, Alex Tigera and Gaspar Rodriquez of Miami-Dade County in early September. FRSA appreciates their willingness to
openly exchange points of view, concerns and possible solutions. It was a very productive meeting. The Miami-Dade group are currently discussing their options and we agreed to try and work together to find a good resolution. I will report on our progress in
future articles. Keeping the dialogue open, building consensus and forming coalitions with other industry groups is of the upmost importance when proposing and making code changes. We will attempt to do so whenever our interests align.
Mike Silvers, CPRC is owner of Silvers Systems Inc. and is consulting with FRSA as Director of Technical Services. Mike is an FRSA Past President, Life Member and Campanella Award recipient and brings over 45 years of industry knowledge and experience to FRSA’s team.
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