How a Roof Works
Lesson 5. Flashing, the Part That Fails First
At a Glance
- Step flashing is one piece per shingle course, woven in
- One long bent strip plus caulk is the wrong way and it is common
- Pipe boots split around year twelve
- Reusing old flashing on a new roof is a false economy
The Kinds You Have
| Kind | Where | How It Fails |
|---|---|---|
| Step flashing | Roof against a wall | Installed as one strip with caulk instead of stepped |
| Counter flashing | Chimneys | Surface sealed to the brick instead of cut into a joint |
| Valley metal | Where two slopes meet | Corrodes, or debris dams up on it |
| Pipe boot | Around vent pipes | Rubber collar splits from sun exposure |
| Drip edge | Roof edges | Omitted entirely, or it works loose in wind |
| Apron flashing | Below a wall or chimney front | Too short, so wind driven rain gets behind it |
Why Caulk Is Not Flashing
Caulk has a service life. In Davenport a joint goes from below zero in January to over a hundred and forty degrees on a dark roof in July. That is a lot of expansion and contraction every year, and sealant eventually cracks.
Properly built flashing works by shape. Water hits metal and gets directed somewhere safe. Sealant is there to help the shape, not to be the shape. A joint that only works because it is sealed is a joint on a timer.
The Reuse Question
Some estimates include new flashing. Some reuse what is there. The second is cheaper and it is sometimes reasonable, for instance on valley metal that is still sound.
It is not reasonable on step flashing or pipe boots. Those are inexpensive parts, the roof is already open, and doing it later means cutting a new roof apart. If a bid reuses them, that is worth asking about before you compare prices.