Why Chimney Flashing Fails

Chimneys are the most complex roof penetration. They're large, immovable, subject to thermal expansion, and water flows around them from multiple directions. 90% of chimney leaks are caused by improper flashing, not chimney damage.

Common Failure Modes:

• Missing counter flashing: Water runs behind step flashing
• Caulk instead of proper flashing: Caulk fails in 3-5 years
• No cricket on upslope side: Debris and water accumulate
• Corroded flashing: Rust holes in galvanized steel
• Improper step flashing overlap: Water infiltrates between pieces

Before You Start

Skill Level: Expert (Professional STRONGLY recommended)

Time Required: 1-2 days

Cost: $200-400 in materials (DIY) vs. $800-1,500 (professional installation)

Required Tools

Flashing Tools:

• Tin snips (straight, left, right cuts)
• Metal brake (for bending flashing)
• Hammer
• Roofing nailer or hammer
• Caulk gun

Masonry Tools:

• Angle grinder with masonry blade
• Cold chisel
• Pointing trowel
• Wire brush

Measurement Tools:

• Tape measure
• Square
• Level
• Chalk line

Materials Needed

• Step flashing (pre-made or custom-fabricated, 8x10 inch pieces)
• Base flashing (for chimney base)
• Counter flashing (to embed in mortar joints)
• Cricket flashing (for upslope side, if chimney is 30+ inches wide)
• Ice & water shield
• Roofing cement
• Butyl tape sealant
• Mortar (Type N or S)
• Roofing nails

Understanding Chimney Flashing Components

1. Base Flashing (Apron)

Covers the downslope side of chimney. Water flows over this flashing.

2. Step Flashing

Individual L-shaped pieces installed along chimney sides. One piece per shingle course. Water flows over each piece onto the piece below.

3. Counter Flashing

Embedded in chimney mortar joints. Overlaps step flashing. Prevents water from running behind step flashing.

4. Cricket (Saddle)

Peaked structure on upslope side of chimney. Diverts water around chimney. Required for chimneys wider than 30 inches.

Step 1: Remove Old Flashing

Process:

1. Remove shingles: Around chimney perimeter (12 inches from chimney on all sides)
2. Remove old flashing: Step flashing, base flashing, counter flashing
3. Remove old mortar: From joints where counter flashing was embedded
4. Clean chimney surface: Wire brush to remove debris and old sealant
5. Inspect chimney: Check for cracks, loose bricks, deteriorated mortar

Step 2: Apply Ice & Water Shield

Why This Matters:

Ice & water shield provides a waterproof backup layer. If flashing fails, ice & water shield prevents leaks.

Installation:

1. Apply to deck: 12 inches beyond chimney on all sides
2. Run up chimney sides: 6-8 inches up chimney face
3. Overlap seams: 6 inches where pieces meet
4. Press firmly: Ensure complete adhesion

Step 3: Install Base Flashing (Downslope Side)

Installation:

1. Measure chimney width: Cut flashing to extend 6 inches beyond chimney on each side
2. Bend flashing: 90-degree bend to fit against chimney face (4-6 inches up chimney)
3. Position flashing: Center on chimney base
4. Nail to deck: Along bottom edge (will be covered by shingles)
5. Seal to chimney: Apply roofing cement where flashing meets chimney

Step 4: Install Step Flashing (Both Sides)

Critical Details:

Step flashing is the most important component. Each piece must overlap correctly or water will infiltrate.

Step Flashing Dimensions:

• Typically 8 inches wide x 10 inches long
• Bent 90 degrees in middle (5 inches on roof, 5 inches up chimney)
• One piece per shingle course

Installation (Each Side):

1. Start at bottom corner: First piece overlaps base flashing by 3 inches
2. Position first piece:
   • 5 inches on roof deck
   • 5 inches up chimney face
   • Aligned with bottom of shingle course
3. Nail to deck: Two nails in deck portion (high on flashing, will be covered by next shingle)
4. DO NOT nail to chimney: Chimney and roof move independently
5. Install shingle: Over step flashing (flashing goes under shingle above, over shingle below)
6. Install next step flashing: Overlaps previous piece by 3 inches
7. Repeat up chimney side: One piece per shingle course
8. Seal to chimney: Apply roofing cement where flashing meets chimney

Step 5: Build and Install Cricket (If Required)

Cricket Construction:

1. Frame cricket:
   • Build peaked structure from 2x4s
   • Peak height: 1/3 of chimney width
   • Base: chimney width
   • Slopes away from chimney at 45 degrees
2. Sheath with plywood: 1/2" plywood over frame
3. Apply ice & water shield: Over entire cricket surface
4. Install cricket flashing:
   • Metal flashing covers cricket
   • Extends 6 inches up chimney
   • Extends onto roof deck 12 inches
5. Install step flashing: Along cricket sides (same process as chimney sides)

Step 6: Cut Mortar Joints for Counter Flashing

Process:

1. Select mortar joints: Horizontal joints 4-6 inches above step flashing
2. Mark cut line: Chalk line along selected joint
3. Cut joint:
   • Use angle grinder with masonry blade
   • Cut 1-1/2 inches deep
   • Cut full width of chimney side
4. Clean joint: Remove all debris with wire brush and compressed air

Step 7: Install Counter Flashing

Why This Matters:

Counter flashing is embedded in mortar joints and overlaps step flashing. This prevents water from running behind step flashing. Without counter flashing, leaks are inevitable.

Installation:

1. Fabricate counter flashing:
   • L-shaped pieces
   • Top leg: 1-1/2 inches (embeds in mortar)
   • Bottom leg: 4-5 inches (overlaps step flashing by 3 inches)
2. Insert into joint: Top leg goes into cut mortar joint
3. Overlap step flashing: Bottom leg overlaps step flashing by 3 inches minimum
4. Overlap pieces: Counter flashing pieces overlap each other by 2 inches
5. Pack mortar: Fill joint with mortar, packing tightly around flashing
6. Tool joint: Smooth mortar to match existing joints
7. Seal bottom edge: Apply small bead of sealant where counter flashing meets chimney (not required but recommended)

Step 8: Reinstall Shingles

Process:

1. Install shingles around chimney: Cut to fit against chimney
2. Maintain proper overlap: Shingles over step flashing
3. Seal cut edges: Dab of roofing cement under cut edges
4. Ensure water flow: Water should flow over shingles, over step flashing, away from chimney

Step 9: Final Sealing and Inspection

Final Steps:

1. Inspect all flashing: Ensure proper overlap and positioning
2. Check counter flashing: Verify mortar is solid and flashing is secure
3. Seal any gaps: Small gaps can be sealed with roofing cement
4. Clean chimney: Remove all debris and excess sealant
5. Water test: Run hose water over chimney, check for leaks from interior

Common Mistakes to Avoid

1. Using Caulk Instead of Counter Flashing

Caulk fails in 3-5 years. Counter flashing lasts 30+ years. This is the #1 chimney flashing mistake.

2. Nailing Step Flashing to Chimney

Chimney and roof move independently (thermal expansion, settling). Nailing flashing to chimney causes tears and leaks.

3. Skipping the Cricket

Without a cricket, debris accumulates on upslope side, water backs up, and leaks occur.

4. Improper Step Flashing Overlap

Each piece must overlap the piece below by 3 inches minimum. Insufficient overlap allows water infiltration.

5. Shallow Counter Flashing Cuts

Counter flashing must be embedded 1-1/2 inches deep. Shallow cuts allow flashing to pull out.

The Bottom Line

Chimney flashing is the most complex roofing detail. It requires roofing AND masonry skills. Mistakes cause expensive water damage to interior walls, ceilings, and belongings.

At RJ Roofing Bros, we've flashed hundreds of chimneys. We know the details that prevent leaks: proper step flashing overlap, correctly embedded counter flashing, and properly constructed crickets. We warranty our work. The $800-1,500 professional installation cost is cheap insurance against $5,000-15,000 in water damage.

Chimney leaking? Call (604) 997-1292 for expert flashing repair.