CSLB General Building (B)Finishes and Building EnvelopeMedium
A roofer is installing an open valley on a composition shingle roof where two roof planes intersect. Per standard California roofing practice, what is the minimum acceptable width of the corrosion-resistant sheet metal valley flashing?
- A36 inches wide
- B18 inches wide
- C24 inches wide
- D12 inches wide
Show answer & explanationAnswer & explanation
Correct answer: C. 24 inches wide
Standard practice for open metal valleys on composition shingle roofs requires a minimum 24-inch-wide corrosion-resistant sheet metal flashing centered in the valley, providing at least an 11-inch metal exposure on each side of the centerline with a crimped or hemmed water diverter down the middle.
Why the other options are wrong
- A. 36 inches exceeds standard requirements and is not the recognized minimum, though larger valleys sometimes use wider metal.
- B. 18 inches falls short of the standard minimum required for open valley protection.
- D. 12 inches is too narrow to adequately protect the valley from concentrated water flow.
Open Valley Flashing Width
An open valley uses exposed sheet metal flashing, typically a minimum of 24 inches wide, centered where two roof planes meet, to handle concentrated water flow.
- Minimum 24 inches wide, corrosion-resistant metal
- Often has a center crimp/rib to prevent cross-flow of water
- Shingles are trimmed back from the valley centerline, leaving metal exposed
Memory trick: Valleys carry the most water, so give them two feet of metal.