Leaking metal roof where two pitches meet: how to flash a pitch transition properly

A new homeowner posted on Reddit's r/Roofing this week about a workshop the previous owner built onto a garage. The metal roof leaks heavily where the two slopes meet, the joint is covered only by a strip of metal, and the sheathing under it has rotted. That deck comes first: Mid Florida Metal Roofing Supply says soft decking or sagging framing has to be fixed before new metal goes on.

Western States Metal Roofing's guide for corrugated panels shows a standard detail. Transition trim, also sold as pitch break trim, sells for between $1.50 and $2.50 a lineal foot, and its bend lines up with where the panels end, not with the framing break. On the lower panels, foam closures sit on double-sided butyl tape, and the trim is screwed through the high ribs into the closures 12 inches on center. An 8-inch strip of self-adhered underlayment goes 3 inches above the trim, followed by butyl and closures for the upper panels. Trim pieces overlap at least 6 inches with two lines of sealant. On its SL-16 concealed-fastener panel, ABC Metal Roofing seals a Z-trim between the ribs, rivets the transition trim to it and hooks the upper panels into an offset cleat.

Lap direction decides whether the joint sheds water. Mid Florida's guide says the lower flange carries water onto the lower plane and the upper flange tucks under the upper panel, with laps formed downhill. Where the two planes form an inside corner that gathers water, the joint behaves like a valley and takes valley flashing instead. Flat sheet without formed bends cannot hold the angle, and caulk smeared over a leak hides it without fixing the water path.

Check the slope as well. IRC Section R905.10.2 sets 3:12 as the minimum for lapped, exposed-fastener panels without lap sealant, 1/2:12 with applied lap sealant and 1/4:12 for standing seam, according to roof consultant Jack Gray's summary of the code. An add-on roof flatter than 3:12 can leak at the panel laps even with a correct transition.