Installation of Step Flashing
Each course of shingles is placed with the bottom edge just covering the step flashing. The act of stacking these items as they move up the hill is called "shingling"
If shingles were installed without step flashing water could easily flow into the gap at the intersection between the chimney and the shingles.
Step flashing directs water down the roof and away from the open channel between the vertical plane of the chimney and the edge of the shingles
The first shingle in the course is installed over the step flashing. This is followed by a card of step flashing. Then a course of shingles
Next the top of the vertical leg of the step flashing needs to be protected
This is done with counterflashing
which will be inserted into the reglets (ground slots in the horizontal joints of the mortar)