Roof shingle over step flashing.
How to flash chimney going thru peak of roof.
Set the flashing aside.
Available in 304l stainless steel and galvalume.
Choose the 304l stainless steel flashing for ultimate longevity.
If you do not wish to form your own corner flashing you can buy pre bent pieces and cut them to size or use a piece of corrugated aluminum which is easier to bend.
Fabricate them together at the appropriate roof pitch fab up a cylinder to enclose them seams put a cap on it and install and flash it.
Step flashing on roof peak.
Place a small drop of caulk where the roof and chimney meet at the chimney corner.
Place the step flashing over the caulk and onto the chimney.
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Bend the corner flashing tightly around the corner.
Not fun but you want to be geometrically correct i guess.
Buy two roof angle flanges cut them less than half way through leave something past a center line.
Fit an 8 inch 20 3 cm piece of square step flashing over the chimney s front corner.