CSLB General Building (B)Finishes and Building EnvelopeMedium
A roofer is installing counter-flashing into a reglet cut in a masonry chimney above existing step flashing on a composition shingle roof. To ensure a watertight seal, the counter-flashing should overlap the step flashing by a minimum of how much?
- A4 inches
- B1 inch
- C2 inches
- D6 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 4 inches
Standard roofing practice and code guidance require counter-flashing to overlap the underlying step flashing by a minimum of 4 inches to prevent wind-driven rain from bypassing the joint. Less overlap risks water infiltration behind the flashing at the chimney interface.
Why the other options are wrong
- B. Far too little overlap to resist wind-driven rain intrusion.
- C. Still insufficient for a reliable watertight lap at a chimney.
- D. Exceeds the typical requirement, though not incorrect, it is not the recognized minimum.
Counter-Flashing Overlap
Counter-flashing set into a masonry reglet must overlap the step flashing below by a minimum distance to maintain a continuous watertight barrier at chimney and wall penetrations.
- Minimum 4-inch overlap over step flashing
- Reglet cut into mortar joint receives the counter-flashing lip
- Prevents water from tracking behind the flashing assembly
Memory trick: 'Four inches keeps the chimney dry' — remember the 4-inch overlap rule.