What is transition flashing on a metal roof?
Short answer. Transition flashing is the formed metal piece that seals the line where a metal roof changes pitch, such as a steep main roof flowing into a lower porch roof, or the break on a gambrel or mansard roof. The upper panels lap over it and the lower panels run under it, so water always flows downhill onto the next surface, with closures and sealant filling the panel ribs at the joint.
Two kinds of pitch change
- Steep to low, like a main roof flowing into a shallower porch roof. Water speeds down the steep section and hits the flatter one, so the flashing must carry it smoothly across without backing up.
- Low to steep, like the break on a gambrel or mansard. The flashing wraps over the bend, with the lower, steeper panels tucked under it.
How it is built
- Underlayment: self-adhering membrane bridges the deck at the bend, lapped upper over lower.
- Lower panels go on first and stop just short of the bend.
- Closures fill the voids under the lower panel ribs at their top ends, set in sealant or butyl tape.
- Transition flashing, formed to the angle of the pitch change, is fastened through the closures into the lower panels.
- Upper panels lap down over the transition flashing, with their own closures or hemmed ends, so water runs off onto the flashing.
On standing seam systems, panels may be bent on site to follow a gentle transition, or the joint may use an offset cleat and a hemmed panel end, following the manufacturer's detail.
Common problems
- Flashing bent to the wrong angle, leaving gaps under the upper panels.
- Missing closures, letting wind-driven rain blow up under the transition.
- Fasteners placed in the low point of the bend, where water collects.
- A lower section built flatter than the panel system's minimum slope.
Shingle to metal transitions
Where a shingle roof meets a metal section, the same principle applies: the upper material laps over the lower, with flashing between and membrane underneath. See metal roofing in Georgia and roof flashing explained.