Roofs leak where things poke through them
The middle of a shingle plane is the easy part of a roof. Courses overlap, water runs downhill, and there is nothing to get wrong that a competent crew would get wrong. Look at where roofs actually fail and the failures cluster in three places: penetrations, where something comes up through the roof — flues, plumbing stacks, bath and kitchen exhausts; transitions, where one surface runs into another — valleys, roof-to-wall lines, chimneys; and terminations, where the roof stops — the eaves, the rakes and the ridge. On this house, the photographs are basically a list of them.
There is a reason for the pattern. Everywhere else, waterproofing is geometry: overlap the courses and gravity does the rest. At a penetration or a transition, geometry stops working and the roof relies instead on a piece of formed metal, a rubber collar, or a sealed joint. Those are all shorter-lived than the shingles around them. A rubber pipe collar hardens and splits in sunlight well before the field is worn out. Galvanised step flashing rusts through at the fold, on the inside, where nobody can see it. Sealant is a maintenance item that gets treated as a permanent one. Twenty years in, the field can be perfectly serviceable and every detail in it can be at the end of its life.
Which is why what matters in a tear-off is what happens at those points, not what happens in the field. At each flue and stack, the old collar and flashing come off with the shingles and new ones go in, laid into the courses so the shingle above laps the metal rather than the other way round. Where the roof meets the brick wall, step flashing goes in piece by piece as the courses go up — one piece per course, woven in, which is why it cannot be added later without lifting shingles — and is covered by counter flashing at the masonry. At the ridge, the vent and cap are new. At the eave, the roof gets new drip edge and gutter apron so water leaves the deck and lands in the gutter instead of behind it. The cheap version of a tear-off skips most of that: new shingles laid up to old metal, old nail holes, old sealant. From the driveway the two roofs are identical. From the attic, three winters later, they are not.
If you are comparing quotes for your own house, this is the line where they differ. Two proposals for the same shingle, at prices that look close, can be describing two different jobs entirely once you read what each one says about the flashings, the boots, the valleys and the edge metal. A quote that is quiet about penetrations and transitions is quiet about the only places your roof is likely to leak.
