What is Z flashing on a metal roof?
Short answer. Z flashing on a metal roof, often called a Z closure or Z bar, is a strip of metal bent into a Z shape that is fastened across the panel ribs, usually over foam closures and sealant, to seal the panel ends and give endwall flashing, ridge caps and transition trims a solid place to attach. On walls, a similar Z flashing covers horizontal joints in siding and trim to shed water outward.
Where it shows up on metal roofs
- Endwalls and headwalls: the Z sits across the top of the panels, and the endwall flashing hooks or fastens to it.
- Ridges and hips on standing seam: Z closures fastened to the panels anchor the ridge cap without screws through the cap's face.
- Transitions: at a pitch change, a Z gives the transition flashing a fastening line.
- Valleys: on some systems, a Z closure holds the panel ends off the valley pan.
How it works
- Closures shaped to the panel profile fill the ribs.
- Butyl tape or sealant goes under the Z and the closures.
- The Z is fastened through the closures into the panel.
- The trim above, such as a ridge cap or endwall flashing, attaches to the Z's upper leg, so its fasteners stay out of the water path.
Because the trim attaches to the Z instead of straight through its face, the system tolerates expansion and contraction better, which matters in Georgia's big daily temperature swings.
Z flashing on walls
Outside roofing, Z flashing is the strip that covers horizontal joints in panel siding or trim, such as above a window head or between two sheets of plywood siding. Its upper leg goes behind the siding above and its lower leg laps over the siding below.
Problems to watch
- Missing sealant under the Z, letting wind-driven rain through at the panel ends.
- Closures that do not match the panel profile.
- Fasteners driven through the trim face instead of into the Z.