How to Seal Your Roof Around a Chimney

Why Chimneys Are the Most Common Leak Source

A chimney penetrates the roof at a large, rectangular opening where four different transition points meet the roofing surface. Each side presents a different water-management challenge:

• Upslope side: Water flowing down the roof hits the chimney face and must be diverted around it. Without a cricket (saddle), water pools and works under shingles.
• Downslope side: The base apron flashing must direct water over the shingles below without gaps.
• Left and right sides: Step flashing must seal the joint at every shingle course as water flows laterally around the chimney.

Add to this the constant thermal cycling — the chimney heats up when the fireplace runs and cools when it stops — which expands and contracts flashing joints repeatedly. Most chimney flashing fails at the mortar-to-metal junction where counter-flashing is embedded in the masonry.

Anatomy of Proper Chimney Flashing

• Base apron flashing: A continuous L-shaped piece at the downslope chimney face. One leg goes up the chimney 4–6 inches, the other extends over the shingles below.
• Step flashing: Individual L-shaped pieces (5×7 inches is standard) installed at each shingle course along the chimney sides. Each piece overlaps the one below by at least 2 inches. The vertical leg goes up the chimney; the horizontal leg extends under the shingle.
• Counter-flashing: A second layer of metal embedded 1–1.5 inches into the chimney mortar joints (reglet-cut) that folds down over the step flashing. This prevents water from getting behind the step flashing. Counter-flashing is what makes chimney flashing weatherproof — without it, water runs down the brick face and behind the step flashing.
• Cricket (saddle): A small peaked structure on the upslope side of chimneys wider than 30 inches. It diverts water around the chimney like a miniature roof. Made from metal or framed wood covered with metal flashing.

Repair vs. Replace: How to Decide

Repair (re-seal) when:

• The metal flashing is intact but sealant or mortar around the counter-flashing has cracked or separated.
• A single step flashing piece has lifted or a nail has backed out.
• The leak is small and traceable to one specific gap.

Repair involves removing old caulk, cleaning the joint, and applying high-quality polyurethane roofing sealant. Cost: $300–$800.

Replace when:

• Flashing is corroded, bent, or was installed with roofing cement instead of proper counter-flashing.
• The chimney has been re-pointed (mortar replaced) without re-embedding the counter-flashing.
• Multiple leak points exist or the flashing has been repeatedly patched.
• The roof is being replaced — always install new chimney flashing during a re-roof.

Full replacement involves removing old flashing, cutting reglets in the mortar, installing new step flashing woven into the shingles, embedding new counter-flashing, and building or replacing the cricket if needed. Cost: $800–$2,500.

Why Caulk Alone Doesn't Work

Smearing roofing cement or caulk over flashing joints is the most common "repair" and the one that fails fastest. Sealant is a secondary barrier — it fills small gaps in an otherwise solid flashing system. When used as the primary barrier (replacing missing counter-flashing or bridging large gaps), it cracks from thermal cycling within 1–3 years. Every time you re-caulk instead of properly re-flashing, you add another layer of material that traps moisture and makes the eventual proper repair more expensive.