Drip edge (R905.2.8.5)
2018 IRC R905.2.8.5 opens: a drip edge shall be provided at eaves and rake edges of shingle roofs. Georgia made no amendment to it.
Traps:
- Eaves only or rakes only each leaves out one of the two edges the section names.
- There is no slope condition. "Only below 4:12" borrows the OSHA low-slope boundary from 29 CFR 1926.500.
- Drip edge is not optional. Some older IRC editions did not require it, which is where that common error comes from.
Remember the drip edge's own lap is 2 inches; that figure turns up elsewhere as a distractor.
Open valley linings (R905.2.8.2)
R905.2.8.2 gives two ways to line an open valley on an asphalt shingle roof.
- Item 1, metal: a corrosion-resistant metal lining from Table R905.2.8.2, not less than 24 inches (610 mm) wide.
- Item 2, mineral-surfaced roll roofing: two plies. The code reads: the bottom layer shall be 18 inches (457 mm) and the top layer not less than 36 inches (914 mm) wide.
A 12-inch figure is not a valley lining width anywhere in R905.2.8.2.
The named errors
- Swapping the plies. 36 in. at the bottom and 18 in. on top is backwards. The narrow ply goes down first, the wide ply covers it: 18 bottom, 36 top.
- Using 24 in. for roll roofing. 24 in. is the metal width. Two 24-inch plies, or a single 24-inch ply, of roll roofing does not comply.
- Borrowing Appendix U numbers. A 19-inch and 36-inch pairing is the Appendix U secondary water barrier (19-inch starter, 36-inch sheets), not a valley lining.
Checking a valley in the field
Run each lining against its own item:
- 24-inch metal from the table: meets Item 1.
- 30-inch metal from the table: exceeds the not-less-than-24-inch rule, so it complies.
- 18-inch bottom ply plus 36-inch top ply of roll roofing: exactly Item 2.
- A single ply of 24-inch roll roofing: fails. Item 2 allows roll roofing only as two plies, and 24 in. is borrowed from the metal rule.