How is step flashing supposed to go where a shingle roof dies into a sidewall with lap siding?

Step flashing at a shingle-to-sidewall transition with lap siding:

Installation sequence (bottom to top):

1. Kickout/diverter flashing at roof edge — directs water into gutter, not behind siding
2. First step flashing piece (typically 8"×8" bent 90°, 4" up wall, 4" on roof) nailed to roof deck only — not to wall
3. Shingle course over step flashing, nailed per manufacturer specs
4. Next step flashing overlapping previous by 2–3", same nailing rule
5. Repeat to top of penetration

Critical details:

• Step flashing goes UNDER siding — siding laps over the vertical leg. Never caulk this joint; it's a drainage plane.
• Housewrap/WRB must lap over step flashing vertical legs (shingle-style). Tape seams.
• Counterflashing (if required by siding mfr) installs over step flashing vertical leg, under WRB.
• Nailing: Roof deck only. Wall nailing prevents movement and tears flashing.
• Kickout flashing mandatory at bottom per IRC R903.2.1 / IBC 1503.2 — prevents rot at sidewall base.

Common failures: Caulking step-to-siding joint (traps water), nailing to wall, missing kickout, WRB behind flashing instead of over.

Code: IRC R905.2.8.3 (step flashing), R903.2.1 (kickout), R703.1 (WRB lapping).

If siding is already installed, you'll need to remove bottom courses to integrate properly. No shortcuts — this detail fails silently and expensively.

Step flashing at a roof-to-sidewall intersection with lap siding must be installed to direct water away from the wall and prevent infiltration behind the siding. Here’s how it’s done correctly, per IRC R903.2 and common best practices:

1. Install step flashing pieces: Use corrosion-resistant metal (typically aluminum or galvanized steel, minimum 0.019" thick). Each piece is L-shaped: one leg extends up the wall (at least 4"), the other lies flat on the roof deck (at least 4" wide, overlapping the shingle below).

1. Integrate with shingles: As you lay shingles up the slope, place one step flashing piece against the sidewall for each course of shingles. The horizontal leg of the flashing goes under the shingle above it and over the shingle below it — shingles lap over the flashing, not under it. This creates a shingle-like drainage plane.

1. Wall leg placement: The vertical leg of the step flashing must go behind the lap siding. Do not nail it to the wall over the siding. Instead: - Remove or lift the bottom edge of the siding course where flashing will sit. - Slide the vertical leg of the flashing behind the siding, so it rests on the sheathing or weather-resistive barrier (WRB). - The siding then laps over the top of the flashing’s vertical leg, shedding water away from the joint.

1. Sealant & counterflashing (optional but recommended): While step flashing alone is code-compliant if properly integrated, many contractors apply a bead of sealant (e.g., polyurethane) behind the siding where it meets the flashing for extra protection. Some add counterflashing (e.g., a metal cap) embedded into a mortar joint or siding groove — but this is rare with lap siding; reliance on proper step flashing and siding lap is standard.

1. Continuity: Ensure flashing extends at least 6" beyond the intersection in both directions (up the wall and along the roof) to prevent water bypass.

Critical: Never rely on sealant alone. The flashing must create a mechanical drainage path. If the siding is vinyl or fiber cement, ensure it’s not nailed too tightly — allow room for expansion and flashing insertion.

If the wall is brick or stucco, counterflashing is required — but with lap siding, proper step flashing behind the siding suffices.

Unsafe if: Flashing is installed over the siding, shingles lap under flashing, or pieces are not integrated course-by-course. This invites leaks.

For complex geometries (e.g., dormers), consult a licensed roofer or refer to NRCA Roofing Manual. But for a standard sidewall, this method is code-compliant and durable.