Leaks arising where a wall meets shingles. This is one common reason why a roof leaks at a wall. This is typically the result of a “Storm Chaser” type of company or rookie, untrained subcontractors that do quick & high volume jobs.
It is extremely important to install a new step flashing where a roof line meets a vertical wall. We will remove siding to get our new large step flashing up against the wall & under every shingle. When installing step flashing, it’s very important to lay the shingle, then put the new step flashing on top of the shingle, then nail it as high & away from the wall as possible. Repeat this all the way up the wall. The reason for this is, when rain or ice hits the side of that wall, water will migrate slightly under the shingle & on to the metal step flashing, & that water can hit a nail if it’s nailed too low on the step. Nailing high will protect from this because the next shingle will be covering any nails. The pictures should explain it better.