home made chimney flashing

This is a 7 year old garden home. Whoever flashed the chimney used a piece of galvanized valley metal to make their wall flashing from scratch. They installed it a little short too as you can see in the pic. It leaked a little at a time along the way every since it was brand new. Of course it's OSB decking on the roof and when I raised the shingles up, I just barely put my hand on the OSB and it caved right in. A 7 foot long section of soffit is also rotted out underneath the overhang below the chimney. I will be repairing the mess one day this coming week and painting the new soffit.