We had several “Ask the Expert” questions regarding underlayment fastening recently. Below are 2 questions from an attorney and responses from TRI President and Technical Director Rick Olson.
“Do the guidelines in Tables 1A and 1B regarding “Attachment Type of Fasteners” that call for ¾ penetration apply to the underlayment?”
Table 1A and 1B come from the code requirements (these were verbatim the older UBC code tables 15D-2 and 15D-3). Our table 1A is for all tiles. Table 1B is for tiles that have projecting anchor lugs and are installed on roof pitches of 4:12 and greater. Table 1B is the majority of tile installations. The section dealing with fasteners is used for the fastening of the tiles, not the underlayment
“What does the ¾ penetration guideline refer to?”
The statement in the code of “3/4” into or through the thickness of the sheathing or batten which ever is less” is to allow the tile fastener to have the maximum withdrawal resistance. Once a fastener has penetrated 3/4” there is no greater resistance for being more.