Roof flashing is the thin metal (or rubber) material that seals the joints where shingles meet other surfaces — chimneys, walls, valleys, and skylights. Done well, flashing keeps water out for decades. Done poorly, water enters at every joint and rots structure. Most roof leaks trace to bad flashing.
This guide covers the typical flashing locations on residential roofs: step flashing at walls, chimney flashing, valley flashing, vent pipe flashing.
Before you start
Three things to settle:
1. Flashing material matters. Aluminum: most common, durable, paintable. Galvanized steel: cheaper, rusts over years. Copper: premium, lasts 80+ years, expensive. Lead-coated copper: traditional, premium. Match flashing to roof life expectancy.
2. Step flashing on walls (not continuous). At wall-to-roof intersections (e.g., where a wall meets a sloped roof), use step flashing — individual L-shaped pieces, one per shingle course. Never use a long continuous strip — water gets behind, rots wall, leaks roof.
3. Permits often not required for flashing only. Like-for-like flashing replacement: typically no permit. New construction or major changes: roof permit required. Verify with municipality.
What you'll need
Tools
- Tin snips (for cutting flashing)
- Hammer or roofing nailer
- Pry bar (for lifting old flashing)
- Caulk gun
- Utility knife
- Tape measure
Materials
- Step flashing pieces (L-shaped, ~5×7 inches typical) — for walls
- Roll flashing (continuous strip) — for chimneys, valleys
- Vent pipe flashing (for plumbing vents)
- Aluminum flashing rolls (10-12 inch wide × 50 ft)
- Roofing cement (Henry 208 or similar)
- Roofing nails (1.5 inch typical)
- Underlayment (felt or synthetic)
Flashing layered with shingles, not on top
Each piece of step flashing tucks UNDER the shingle course above it.
Water that hits flashing flows down and out under the shingles below.
Flashing on top of shingles: water gets behind, leaks.
Step-by-step instructions
Identify all flashing locations on roof
Wall-to-roof intersections (gable ends meeting walls).
Chimney perimeter.
Valleys (where two roof slopes meet).
Vent pipes (plumbing vent stacks).
Skylights.
Each location requires specific flashing technique.
Step flashing at walls
Step flashing pieces L-shaped, ~5×7 inches.
First piece: at bottom of wall-roof intersection. One leg under first shingle course; other leg up wall, under siding (or kick-out flashing at bottom).
Each subsequent shingle course: another piece of step flashing, overlapping previous by 2 inches, under that course.
Continue all the way up the wall.
Wall caps over step flashing
Top edge of step flashing (against wall): siding tucks over flashing.
If siding is rigid (brick, stucco): flashing's top edge tucks behind a moisture barrier in the wall.
Caulk top edge of flashing to wall — water can't enter wall.
Chimney flashing
Front of chimney (downhill side): apron flashing, large piece tucked under shingles below, over chimney face above.
Sides of chimney: step flashing.
Back of chimney (uphill side): cricket or saddle flashing — diverts water around chimney.
Top of all chimney flashing: counter-flashing tucks INTO chimney mortar joints, sealed with mortar repair caulk.
Valley flashing
Valley = where two roof slopes meet.
Open valley: continuous metal flashing, exposed.
Closed valley: shingles weave across, no exposed flashing.
Closed-cut valley: shingles come over, then trimmed back exposing some flashing.
Apply per chosen design.
Vent pipe flashing
Pre-formed boot for plumbing vent pipes.
Boot fits over pipe.
Flange tucks under shingles above pipe.
Sealed with roofing cement around pipe.
Skylight flashing
Skylight kits include specific flashing pieces.
Step flashing on sides.
Apron at front (downhill side), saddle at back (uphill side).
Per skylight manufacturer's instructions.
Caulk and seal exposed edges
Roofing cement (Henry 208) at exposed flashing edges.
All places where flashing meets wall, chimney, or other surface.
Don't over-apply — too much cement looks unprofessional and isn't more effective.
Verify all joints are watertight
Walk along roof; verify no gaps in flashing.
Check after a heavy rain — no leaks evident in attic.
Roof inspection by professional if uncertain.
Common mistakes that cost you
- Continuous strip instead of step flashing. Water gets behind. Always individual step pieces at walls.
- Flashing on top of shingles. Water gets behind, leaks. Flashing tucks under upper shingles.
- Wrong material for life expectancy. Galvanized steel rusts in 15-20 years; ahead of roof life. Match material.
- Skipping counter-flashing on chimney. Mortar joint leaks over years. Counter-flashing into mortar.
- Generic boot for irregular vents. Doesn't seal. Match boot to specific pipe and angle.
Frequently asked questions
How long does flashing last?
Aluminum: 30-50 years.
Galvanized steel: 15-25 years.
Copper: 70-100+ years.
Match flashing life to roof shingle life (typical asphalt: 25-30 years).
Can I install flashing without removing shingles?
Repair specific flashing leaks: yes, often by tucking new flashing under existing shingles.
Major flashing replacement: easier with shingles removed (often combined with re-roof).
Patch leaks immediately to prevent further water damage.
Can flashing be repainted?
Aluminum: paintable with rust-resistant primer + paint.
Galvanized: paintable.
Copper: develops natural patina; usually unpainted.
Match colour to fascia or trim for clean appearance.
When to call us instead
Roof flashing is critical to weather-tightness. Our team handles roofing repair and replacement as part of exterior renovations.
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