Tile Underlayment – Where We Are and How We Got Here Mike Silvers, CPRC, Owner of Silvers Systems Inc. and FRSA Director of Technical Services Changes in roofing underlayment requirements have evolved in dramatic ways over the last several decades, particularly when it comes to tile underlayment. Why have we had so many changes and how has it changed? For many years, underlayments were primarily viewed as a dry-in, used to provide temporary protection of the roof sheathing from the elements. For most steep-slope roofs, they were expected to serve this purpose until the roof covering was installed. The code requirements at the time called for underlayments to be “fastened sufficiently to hold in place” or “fastened only as necessary to hold in place.” The thinking was that once a roof covering was fastened through the underlayment, it wouldn’t need a specific number of fasteners and that their placement wouldn’t be critical. In some areas of the country, an underlayment was not even required. In Florida, the advent of the Florida Building Code (FBC) prompted the eventual standardized nailing pattern of 12 inches and shortly afterwards, 6 inches on center at the laps with additional nails spaced 12 inches on center in a staggered pattern between the laps. This fastening pattern was applied to many nailed underlayments including twoply felt (below 2:12 pitch) run shingle fashion, a single layer at and above 4:12, tile underlayment anchor sheets and single layer underlayment – and even lowslope base sheets. These methods have changed to include taped joints, self-adhering underlayment and the use of synthetic underlayments. The introduction of the FBC was one of the early series of changes that had a major impact on how the codes actually worked. The prescriptive measures used in the pre-FBC legacy codes (Southern Building Code Congress International (SBCCI) and South Florida), where the codes described what generic materials to use and how to install them, were giving way to new performance-based standards. This new system required manufacturers to test their materials to meet specific criteria and standards included in the code. These methods were incorporated in the FBC through Florida’s Product Approval (PA) system or the Miami-Dade Notice of Acceptance (NOA) approval system. This change definitely created an additional level of complexity that was meant to achieve better performing roof systems and the structures they protect. Indications are that this approach is actually working with newer roof systems and underlayments fairing much better in recent high velocity wind events. Observations conducted after hurricanes are nearly all in agreement with this conclusion. 18
FLORIDA ROOFING | April 2023
When the 2007 FBC went into effect in March of 2009, the requirements for a secondary water-barrier (SWB) was introduced. These requirements came in response to widespread roof damage from Hurricanes Andrew (1992), Charley, Ivan, Jeanne (2004), Dennis and Wilma (2005). Many roof systems and underlayments performed poorly during these events. Many structures were left with exposed sheathing and suffered severe water damage. It was thought that if we used more wind and water resistant underlayments that we would have some post-storm protection if the primary roof covering failed. Tile roofs were not included in the SWB requirements. The reason why isn’t clear, but pressure continues to have this requirement included for tile underlayment. Tile underlayment has gone through many changes. Historically, some roof tiles were installed over spaced sheathing without underlayment and sometimes without fasteners (see Figures 1 and 2 below).
Figure 1
Figure 2