Steep Slope Underlayment Changes in the FBC 8th Edition

Sat, Jul 01, 2023

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. Read more

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