Does roof flashing go under or over shingles?

Roof flashing goes both under and over shingles depending on its location and type, following the principle of water shedding where each layer overlaps the one below to direct water downward. The general rule is that flashing is integrated with shingles in a layered, overlapping manner to prevent water from getting underneath. Step flashing along sidewalls and chimneys is installed in a alternating pattern where each piece of L-shaped step flashing is placed under a shingle course and over the shingle course below, interwoven with shingles, so water running down the wall goes onto the flashing and then onto the shingle below, not behind it. Valley flashing typically goes under shingles on both sides in a closed valley, or under shingles with metal exposed in an open valley, with shingles overlapping the metal. Drip edge at eaves goes under underlayment and shingles at the eave edge to direct water into gutters. Vent pipe boot flashing goes over shingles on the lower side and under shingles on the upper side, with the top edge tucked under the shingle above to prevent water entry. Counter flashing on chimneys goes over step flashing and is inserted into mortar joints. The key principle is that flashing should never be installed where water can run behind it, and proper installation requires shingles to overlap flashing on the upper side while flashing overlaps shingles on the lower side.