Interpretation #9462
Published December 01, 2025
Original Request
- Code Version
- 2023
- Code
- RESIDENT
- Code Description
- Chapter
- 9
- Section
- 905.1.1 Exception (2)
- Topic
- Zip System
Question
Code specifies "refer to manufacturer's install guide". Zip System specs states 1 layer of Zip deck with tape equals 1 layer of underlayment, which they say is sufficient for roof pitches over 4/12 pitch. Does a 5/12 pitch using Zip System still need a 2nd layer of underlayment, or is the manufacturer's specifications of just 1 layer sufficient?
Answer Published 11/21/2025 at 8:45 AM
The request references Section: 905.1.1 Exception (2) which only addresses an existing self-adhering modified bitumen underlayment during roof replacement. It has no relevance to Zip systems except during a roof replacement where a self-adhering underlayment has been previously installed over zip sheathing on the entire roof. The zip system isn't treated differently than other sheathing in this scenario. An AHJ should treat underlayment on "roof slope of 4:12 and greater" the same per Table R905.1.1.1 and that section.
Commentary
None.