Mike Silvers, CPRC, Owner, Silvers Systems Inc and FRSA Director of Technical Services
Most of the underlayment requirements outlined in the new 8th Edition of the Florida Building Code (2023), other than those in the HVHZ (High Velocity Hurricane Zone – Miami-Dade and Broward Counties), haven’t undergone many substantive changes. Some of the language has, however, been reworked, particularly the description of the two-ply nailed application for underlayment in section 3 (below) of the new code. The overlap descriptions used for this option in the previous edition of the FBC, caused inconsistent underlayment exposures that often necessitated chalking lines even when using an underlayment with ply lines specifically designed for a two-ply application.
In both the new code and the previous version of the code, the first course is based on using a strip of underlayment that is half the width of a full sheet, thereby reducing waste and labor cost when compared to the method used in older versions of the code. These required an additional inch of width for the strip, making the remaining piece unusable for the first course. The new language not only allows for all types of approved underlayment materials but also the use of any width sheet. I’ll review the actual lap changes, but first let’s look at the newly condensed language. Based on changes made in the code, HVHZ underlayment applications should be very similar to those described here except the use of tin caps/tabs will still be required. The drafts of the actual code have not been published as of this writing, so the sections shown below are based on the supplements developed from the actual modifications. They are accurate other than any possible editorial or correlation changes.
CHAPTER 15 ROOF ASSEMBLIES AND ROOFTOP STRUCTURES CHAPTER 9 ROOF ASSEMBLIES SECTION 1507 & R905 REQUIREMENTS FOR ROOF COVERINGS
1. The entire roof deck shall be covered with an approved self-adhering polymer modified bitumen underlayment complying with ASTM D1970 installed in accordance with both the underlayment manufacturers and roof covering manufacturer’s installation instructions for the deck material, roof ventilation configuration and climate exposure for the roof covering to be installed.
Exceptions:
1. This method is not permitted for wood shingles or shakes.
2. An existing self-adhering modified bitumen underlayment that has been previously installed over the roof decking and, where it is required, renailing of the roof sheathing in accordance with Section 706.7.1 of the Florida Building Code, Existing Building can be confirmed or verified. An approved underlayment in accordance with Table 1507.1.1.1 for the applicable roof covering shall be applied over the entire roof over the existing self-adhered modified bitumen underlayment.
2. A minimum 3-3/4-inch-wide (102 96 mm) strip of self-adhering polymer-modified bitumen membrane complying with ASTM D1970 or self-adhering flexible flashing tape complying with AAMA 711, Level 3 [for exposure up to 176°F (80°C)], installed in accordance with the manufacturer’s instructions for the deck material, shall be applied over all joints in the roof decking. An approved underlayment in accordance with Table 1507.1.1.1 for the applicable roof covering shall be applied over the entire roof over the membrane strips.
3. Two layers of ASTM D226 Type II or ASTM D4869 Type III, Type IV, or ASTM D8257 underlayment shall be installed as follows: Apply a strip of underlayment for the first course that is half the width of a full sheet parallel to and starting at the eaves, fastened sufficiently to hold in place. Starting at the eave, apply a full sheet of reinforced underlayment for the second course. Apply the third course of underlayment overlapping the second course successive sheets half the width of a full sheet plus 2 inches. Overlap
all successive courses half the width of a full sheet plus 1 inch. End laps shall be 6 inches (152 mm) and shall be offset by 6 feet (1829 mm). Underlayment shall be attached to a nailable deck with corrosion-resistant fasteners with a maximum fastener spacing measured horizontally and vertically of 12 inches (305 mm) o.c. between side laps, and one row at the end and side laps fastened 6 inches (152 mm) o.c. Underlayment shall be attached using annular ring or deformed shank nails with metal or plastic caps with a nominal cap diameter of not less than 1 inch (25.4 mm). Metal caps are required where the ultimate design wind speed, Vult, equals or exceeds 170 mph. Metal caps shall have a thickness of not less than 32-guage sheet metal. The minimum thickness of the outside edge of plastic caps shall be 0.035 inch (0.889 mm). The cap nail shank shall be not less than 0.083 inch (2.1082 mm) for ring shank cap nails. The cap nail shank shall have a length sufficient to penetrate through the roof sheathing of not less than 3/4 inches (19.05 mm) into the roof sheathing.
Exception:
1. Use of ASTM D8257 underlayment is not permitted for wood shingles or shakes. The diagrams shown on pages 52 and 54 are for installation with three different width rolls. The laps
shown on the right are as described in option 3. More importantly for roofers, are the exposures between the headlaps. You will notice that the first exposure above the eave is two inches less the width of a half sheet. All succeeding exposures are one inch less than the width of a half sheet. For a 36 inch roll, the half sheet is 18 inches wide. So, the first exposure would be 16 inches with all succeeding exposures being 17 inches. Using the exposures rather than overlap will
make this change easier to understand and to teach installers.
One important item to note is wider width underlayment will require additional rows of fasteners in order to maintain the maximum fastener spacing measured horizontally and vertically. For underlayment used for asphalt shingles there are some additional changes. All nailed underlayment must be installed under the drip edge, both at the rakes and at the eaves. The reason for this change is to reduce the uplift effects on the underlayment at the roof’s edges during high velocity wind events.
When using a self-adhering underlayment, you have the option for the underlayment to go under or over the drip edge. However, if you install the underlayment over the drip edge,
the metal flange must be primed with a primer that is compatible with the underlayment. For all asphalt shingle applications, a 4-inch width of roof cement must be placed between the drip edge flange or the self-adhering underlayment
and the shingles, both at the eaves and the rakes as described in the code section below and shown in the diagram.
CHAPTER 9 ROOF ASSEMBLIES SECTION 1507 & R905
REQUIREMENTS FOR ROOF COVERINGS CHAPTER 15 ROOF ASSEMBLIES AND ROOFTOP STRUCTURES
1507.2.9.3 (R905.2.8.5)
Drip edge. Provide drip edge at eaves and gables of shingle roofs. Overlap is to be a minimum of 3 inches (76 mm). Eave drip edges shall extend 1/2 inch (13 mm) below sheathing and
extend back on the roof a minimum of 2 inches (51 mm). Self-adhering, ASTM D1970 underlayment may be installed over a primed drip edge flange. There shall be a minimum 4 inches
(51 mm) width of roof cement installed over the drip edge flange or the self-adhering underlayment. The drip edge shall be mechanically fastened a maximum of 12 inches (305 mm) on center. Where the Vasd, as determined in accordance with Section 1609.3.1, is 110 mph (177 km/h) or greater or the mean roof height exceeds 33 feet (10 058 mm), drip edges shall be mechanically fastened a maximum of 4 inches (102 mm) on center.
Hopefully, this information will help prepare you for the code change that will go into effect on December 31, 2023. Watch for other important changes in the code in future editions of this column. FRSA members can also access previous articles available on the Members Only section of the website, www.floridaroof.com.
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