How Step Flashing Actually Works
Unlike a single long strip of bent metal, step flashing uses many individual pieces layered one per shingle course. Each L-shaped piece is bent roughly 90 degrees: one leg lies on the roof deck under a shingle, and the other leg runs up the vertical wall behind the siding or behind counter-flashing.
The pieces overlap like shingles themselves, so water always lands on top of the piece below it and is carried down and out. This "woven" or "stepped" pattern is why a continuous strip is never an acceptable substitute at a sloped wall, and why repairs done with caulk alone almost always fail.
Why Step Flashing Is Critical in the Inland Northwest
Roof-to-wall joints are one of the most common leak points on Spokane and North Idaho homes, and our climate puts extra stress on them:
- Ice dams: When snowmelt refreezes at the eaves and along walls, water can back up under shingles. Properly layered step flashing gives that water a metal path back out instead of into the wall cavity.
- Freeze-thaw cycles: Repeated freezing and thawing works at any caulk or sealant. Step flashing relies on overlapping metal, not sealant, so it keeps protecting the joint even after caulk has aged out.
- Heavy snow load: Deep snow sitting against a dormer or sidewall holds moisture in contact with the joint for weeks, making correct flashing far more important than in dry climates.
- Wind-driven rain: Spokane wind can push water uphill against walls, where only proper stepped overlaps will hold it back.
Signs of Failed or Missing Step Flashing
Watch for water stains on interior walls below a roofline, peeling paint or rot on siding near a roof-wall joint, or visible caulk smeared where metal should be. A frequent culprit is a roofer who reused old, corroded flashing during a re-roof, or who relied on sealant instead of replacing the pieces.
Because step flashing is woven into the shingles and tucked behind siding, it generally cannot be replaced correctly without removing those layers. When flashing has failed across a wall, the lasting fix is often addressed during a full roof replacement, where every piece can be installed new and in the correct order.