Chimney Leaks: Flashing, Counterflashing, Crowns, and Caps

A chimney leak starts in one of two systems: the metal flashing where the chimney passes through the roof, or the masonry, crown and cap above it. The Chimney Safety Institute of America says water does more damage to masonry chimneys than fire, so find out which system failed before you choose between a roofer and a mason.

The two systems fail in different ways and are fixed by different trades, yet their leaks can show up in the same spot on your ceiling. That overlap can send the first call to the wrong trade, and a few clues gathered from the ground and the attic can save a wasted visit.

Two Systems Keep Water Out at a Chimney

The roof side is sheet metal. Step flashing runs up each side of the chimney, one piece woven in with each course of shingles. A front piece, sometimes called apron flashing, covers the downhill face. Behind the chimney, a back flashing or a small ridge called a cricket turns water around the uphill face. Counterflashing, set into the masonry, laps down over all of it so water running down the brick lands on the outside of the metal. This set is one part of the flashing family described in roof flashing types and how each fails, and the roof anatomy guide shows where each piece sits among the other components.

The masonry side is everything above the roofline. The crown is the sloped top that seals the masonry around the flue. The cap covers the flue opening. The brick and mortar joints shed water down the outside of the chimney.

The Chimney Safety Institute of America (CSIA) makes the stakes plain in its June 2010 article Four Low-Cost Recommendations to Prevent Major Water Damage: “water causes more damage to masonry chimneys than fire.” A leak can become a structural problem for the chimney, not only a stain on the ceiling.

Step Flashing and Counterflashing: How the Roof Side Fails

The roof side works by laps. Each step flashing piece sits under the shingle above it and over the shingle below, so water moving down the slope is always handed to the outside of the next layer. Code requires this joint to be flashed: section R903.2.1 of the 2021 International Residential Code (IRC), in the version printed in Seattle’s residential code, Chapter 9, says flashings “shall be installed at wall and roof intersections, wherever there is a change in roof slope or direction and around roof openings.” Your building department can tell you which edition and local changes apply to your house.

That arrangement fails at predictable points:

• Counterflashing pulled out of the masonry. It is set into the mortar joints. When mortar erodes or the metal works loose, the top edge opens and water runs behind the step flashing.
• Sealant standing in for metal. A bead of caulk or roofing cement where counterflashing should be can look fine at first, then crack and let water straight in. Why sealant cannot replace flashing is explained in where sealant belongs and where it hides problems.
• One long piece instead of steps. A single bent strip nailed along the side of the chimney has no laps with the shingles, so water that gets under a shingle has nowhere to go but in.
• Corroded or damaged metal. Rust holes, bent pieces and old nail holes let water through even when the laps are right.
• The uphill face. Water and debris pile up behind the chimney on the uphill side. A missing or failed back flashing or cricket turns that spot into the wettest part of the chimney. Whether your chimney’s width calls for a cricket is covered in when a chimney needs a cricket.

Crowns, Caps and Brick: How the Masonry Side Fails

The masonry side fails from the top down. A crown that cracks lets water into the chimney’s core. CSIA’s 2010 recommendation is specific: “Crowns should project beyond all sides by two inches to help prevent erosion of the brick and mortar.” A crown that ends flush with the brick sheds water straight down the faces instead of throwing it clear.

The cap is the flue’s roof. Without one, rain falls straight down the flue, and the water shows up in the firebox, on the damper, or in the masonry around the flue.

Brick and mortar absorb water. When absorbed water freezes, it expands, and repeated cycles break off the brick’s face and open the mortar joints. Once joints open, water reaches the inside of the chimney and can travel down to the roof line, where it shows up next to the flashing even though the flashing is sound.

That last point is the trap. A masonry leak can look exactly like a flashing leak from inside the house, because both deliver water to the same place. Replacing good flashing will not stop water that is entering through the crown.

Clues You Can Check From the Ground and the Attic

Clues from the ground and the attic can sort a chimney leak into roof side or masonry side without anyone going on the roof. Home inspectors work the same way: the InterNACHI Standards of Practice, last revised October 2022, let an inspector inspect the roof “from ground level or the eaves” and do not require walking on it. Start with these checks:

• From the ground with binoculars, check each face of the chimney. Note counterflashing that has pulled away from the brick, sealant smeared over joints, rust streaks, cracked or flush crown edges, a missing cap, crumbling mortar, and brick faces that have broken away.
• Look for white staining on the brick. Mineral deposits left by water moving through masonry point to the masonry side.
• After a rain, check the attic with a flashlight. If the chimney brick itself is wet or stained below the point where it passes through the roof deck, water is moving through or down the masonry: the crown, cap and mortar joints are the first suspects, along with counterflashing that has pulled loose from the brick. If water shows on the deck or framing beside the chimney while the brick stays dry, look first at the step flashing and back flashing.
• Check the fireplace. Water in the firebox or on the damper after rain points to the cap, crown or flue. If water has reached the flue, ask the chimney specialist to confirm in writing that the flue is safe to use before the next fire.
• Photograph everything, with the date, and note how heavy the rain was and how long after the rain you looked.

If the clues point nowhere clear, a step-by-step method for following water back to its source is in how to trace a roof leak from stain to source.

Which Trade Fixes What

Match the failed component to the trade before you call:

Reflash, Repair the Masonry, or Both?

If the visible failure is on one side, call that trade with your photos and attic notes: a roofer for lifted, rusted or caulked flashing, a mason or chimney specialist for a cracked crown, missing cap or open mortar. If the clues are mixed or nothing is visible, book one inspection that covers both systems, and ask the inspector to state in writing which system is failing and why.

Before work starts on an older chimney, settle one safety question. EPA lists roofing shingles among building materials that may contain asbestos, and its page Learn About Asbestos says exposure “may occur only when the asbestos-containing material is disturbed or damaged.” OSHA’s asbestos standard for construction (29 CFR 1926.1101) includes construction mastics in its Class II category of asbestos work. On an older roof, ask whether old roofing cement or mastic at the chimney will be tested before it is scraped or torn out; how to recognize asbestos in roofing covers testing.

If the roof is being replaced, reflash the chimney as part of the job. Step flashing is woven into the old shingle courses and comes off bent and nail-holed, and counterflashing set into tired mortar is the next leak waiting. Write the scope into the contract: new step flashing, new or reset counterflashing, and who repoints any mortar joints the counterflashing goes into.

Do not accept a sealant smear as the whole repair for either system. Photograph each chimney face from the ground and check the attic after the next rain, then send both to the trade those clues point to.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

Sources

• Four Low-Cost Recommendations to Prevent Major Water Damage, Chimney Safety Institute of America
• 2021 Seattle Residential Code, Chapter 9: Roof Assemblies, City of Seattle, Department of Construction and Inspections
• InterNACHI Home Inspection Standards of Practice, International Association of Certified Home Inspectors
• Learn About Asbestos, U.S. Environmental Protection Agency
• 1926.1101: Asbestos, Occupational Safety and Health Administration