Greg Keeler, Technical Services Leader, Owens Corning Science & Technology
If you’re a roofing contractor in South Florida, you’re certainly aware of the quandary that is posed when confronted with underlayment options. This is especially true when it comes to use of a self-adhering underlayment.
The recent landfall of Hurricane Ian in Southwest Florida was a stark reminder of the perils that face homeowners in South Florida. We’ve all witnessed scenes of widespread damage to roof coverings of all types: tile, metal panels, asphalt shingles, etc. Thus, the topic of how to protect the roof deck and the building below it when the roof covering is damaged or blown off becomes critical. One of the best ways to provide protection from water intrusion into the structure is to install a self-adhering underlayment directly to the roof sheathing. After all, these products are designed and tested to be adhered directly to wood sheathing, not to #30 felt or synthetic underlayments.
Let’s jump into the advantages and disadvantages of adhering the underlayment directly to the deck. The table below was developed in cooperation with multiple industry stakeholders, including a Florida code enforcement agency. I’m sure we didn’t capture everything, but it’s a fairly comprehensive list.
As the table below indicates, the advantages far outweigh the disadvantages. In fact, a couple of the suggested “disadvantages” essentially conflict with each other: if the products being installed on a deck that isn’t 100 percent clean and dry was really an issue (and how often is “dry” even possible in South Florida?), they wouldn’t also be difficult or impossible to remove when reroofing.
There has been a lot of testing done in the past few years to determine if a prescriptive #30 felt mechanically fastened base sheet with a self-adhering underlayment installed over it can provide any meaningful uplift resistance. What we found is that such a
system, even when installed with the greatest attention to detail, provides far less uplift resistance than that which would be required by the Florida Building Code (FBC) and the FRSA-TRI Florida High Wind Concrete and Clay Tile Installation Manual.
Self-Adhering Example
Let’s take a quick look at one hypothetical scenario. Project Description:
■ Single family home
■ 160 mph wind zone
■ 20’ Mean roof height
■ Exposure Category C
■ Adhered concrete tile
■ Self-adhering underlayment approved for use under tile
If this home is in the High Velocity Hurricane Zone (HVHZ), the underlayment requirements are prescriptive and would require that the underlayment meet the requirements of TAS 103. Section 7 of the TAS test protocol stipulates that (4) 8 foot by 8 foot test decks are constructed and underlayment is applied directly to the plywood (which doesn’t reflect permissible conditions in the HVHZ). Then a 5 foot by 5 foot chamber is placed over the underlayment and negative pressure (suction) is applied to the deck, starting at 15 psf. The 15 psf pressure is maintained for one minute, then it is increased by 15 psf and held for one minute. This
process is repeated until the pressure reaches 90 psf. If the underlayment is still adhered after one minute at 90 psf, the test deck passes. If all four decks pass this test, the product is considered compliant with the uplift testing requirements.
Now contrast the TAS 103 uplift requirements with the requirements from the FRSA-TRI Florida High Wind Concrete and Clay Tile Installation Manual. According to Table 1A of the tile manual, for a roof in even the lowest wind speed area of the HVHZ with
the same characteristics, the underlayment would be required to attain an uplift resistance of 102.8 psf. This uplift resistance rating would be achieved by testing to failure using one of two testing protocols: FM 4474 or UL 1897. These protocols utilize the same procedure of starting with a negative pressure of 15 psf and holding for one minute, then increasing by 15 psf increments
and holding for one minute each increment until the product fails. A 2:1 safety factor is then applied to the last pressure at which the product passed. Thus, a product that meets the 102.8 psf uplift requirements would need to pass at a minimum 205.6 psf pressure. That is more than double the pressure that is required by Section 7 of TAS 103.
One of the primary arguments I hear regarding adhering underlayment directly to the deck is related to reroofing. As was mentioned above, once these products are adhered to the deck, they are virtually impossible to remove without damaging the sheathing. In that situation, one option is to install a mechanically fastened slip or anchor sheet over the existing underlayment, then install (if desired) a new layer of self-adhered underlayment over it. In this scenario, in the event the new slip or anchor
sheet blows off the roof, the roof deck is still protected by the layer of self-adhering underlayment that couldn’t be removed.
I know I’ve thrown a lot of numbers and technical information at you, but I urge you to answer this fundamental question: If a hurricane is bearing down on your home, would you feel more comfortable with a self-adhered underlayment adhered directly to your roof sheathing or a self-adhered underlayment adhered to #30 felt that is nailed to your roof sheathing? I know for me the choice is clear.
FRM
Greg Keeler is the Technical Services Leader for Owens Corning Roofing and Asphalt, LLC. He has been with Owens Corning for 11 years and has 36 years of experience in architecture and spent 24 years as a building official. He serves on and chairs several task groups within ASTM D08, Committee on Roofing and Waterproofing and the Asphalt Roofing Manufacturer’s Association. He also serves on the UL Standards Technical Panels for UL 790, UL 2218 and UL 580/1897 and on FRSA’s Codes Committee and Codes Subcommittee.
