Why Is My Chimney Leaking?

On this page

1. The parts that keep water out
2. Six ways water gets in
3. Water in the fireplace vs. on the ceiling
4. Reading the clues
5. When it isn't rain at all
6. What to do now
7. Common misconceptions
8. What an inspection settles
9. Sources

The parts that keep water out

A masonry chimney sheds water in layers. Knowing what each layer does makes the causes below much easier to follow.

• Cap. A metal cover above the flue opening. It keeps rain, animals and debris out of the flue. A cap is not the same thing as a crown, although the words get mixed up.
• Crown. The concrete or mortar slab that covers the top of the brickwork around the flue. It should slope away from the flue and overhang the brick so water drips clear of the chimney face.
• Flue liner. The clay tile, metal or cast lining inside the chimney that carries smoke and exhaust. The flue tile usually sticks up a few inches above the crown.
• Brick and mortar. The walls of the chimney. Both absorb some water. Mortar joints are the weaker of the two and wear first.
• Flashing. Metal at the joint between the chimney and the roof. Step flashing is woven into the shingles. Counterflashing is set into the mortar joints and laps over it, so water runs onto the roof instead of behind it.
• Damper. The plate that closes the flue when the fireplace isn't in use. Throat dampers sit just above the firebox. Top-sealing dampers sit at the top of the flue and also act as a seal against rain.

Six ways water gets in

1. An open flue

Without a cap, rain falls straight down the flue. In a light shower that may only dampen the smoke chamber. In a heavy, windy storm it can run onto the damper and into the firebox. This is often the easiest cause to spot: look up the flue opening from outside, or check for rust on the damper.

2. The crown

Crowns crack as they age, and many were never built well. A common shortcut is a thin layer of mortar troweled flat across the top course of brick. Mortar shrinks as it cures and doesn't flex with the flue tile as it heats and cools, so these thin caps tend to crack early and let water into the brickwork below. Brick industry guidance calls for a crown that overhangs the chimney with a drip edge, and for the crown not to be bonded rigidly to the flue tile so it can move without cracking.

3. Brick and mortar

Masonry is porous. When it soaks up water and that water freezes, it expands and widens small gaps. Over many winters this is what causes brick faces to flake off (spalling) and mortar to crumble. Mortar joints are the most common path for water through a brick wall. White powdery deposits on the brick, called efflorescence, are salts left behind as water moves through the masonry and evaporates.

4. Flashing at the roof line

Flashing failures tend to show up quickly, because water that gets past the flashing runs directly into the house. Common problems are counterflashing pulling out of the mortar joints, rusted or cracked metal, flashing that relies on roofing caulk alone, and flashing disturbed during a roof replacement.

5. Condensation inside the flue

This one isn't rain. It's covered in its own section below because it's commonly missed.

6. A roof leak next to the chimney

Valleys, vents and worn shingles near the chimney can leak too. From the room below, a roof leak a few feet away can look exactly like a chimney leak. Attic access often settles it.

Water in the fireplace vs. on the ceiling

Where the water appears is one of the most useful clues you have.

Reading the clues

Timing tells you a lot. Keep notes for a few storms if you can.

• Drips during or right after rain usually mean a direct path, such as flashing or an open flue.
• Damp that shows up a day or more later and lingers suggests water absorbed into the masonry. A saturated chimney can keep releasing water after the rain stops.
• Only in storms from one direction points to the side of the chimney facing that wind.
• Worse in winter, even in dry weather points to condensation or to snowmelt around the base of the chimney.
• A musty fireplace smell in humid summer weather is often creosote odor drawn down the flue rather than a leak.

When it isn't rain at all

Burning natural gas or oil produces water vapor. In a properly sized, warm flue, that vapor leaves the chimney before it can condense. When a modern, efficient furnace or water heater vents into an old masonry flue that is much larger than it needs, the exhaust moves slowly and cools, and the moisture condenses inside the chimney. The condensate is acidic, and over time it can deteriorate mortar and clay flue tile.

Signs include staining on the outside of the chimney, water at the base of the appliance flue, and problems that are worse in cold weather. The usual fix is a liner sized for the appliance. Sealing the outside of the chimney won't help.

What to do now

• Photograph the stain, and note the date and the weather. Mark the edge of the stain with pencil so you can see whether it grows.
• If water is dripping, protect the floor and move anything valuable. Don't use the fireplace until the system has been checked.
• If the leak is near a furnace or water heater flue, or you notice exhaust odors, soot or a carbon monoxide alarm, have the appliance and flue checked before further use.
• Look from the ground with binoculars at the cap, crown and flashing. Leave climbing to someone with the right equipment.

Common misconceptions

• "A coat of sealer will fix it." Water repellents can help sound masonry shed water. They don't fix cracked crowns, missing caps or failed flashing. Paint and non-breathable coatings can trap moisture inside the brick and speed up damage.
• "It's only a little water." Repeated wetting is what drives freeze-thaw damage, rusted dampers and deteriorated flue liners. Small leaks are cheaper to fix early.
• "The chimney was inspected last year, so it can't be the chimney." Crowns crack and flashing loosens between inspections, especially after a hard winter or roof work.

What an inspection settles

The national fire safety standard for chimneys, NFPA 211, calls for chimneys, fireplaces and vents to be inspected at least once a year. A routine inspection covers the readily accessible parts of the chimney. When a leak's source isn't obvious, the inspector may need roof access, a camera inspection of the flue or a water test to pin it down.

This guide explains what's common. It can't diagnose a particular chimney, and two leaks that look alike can have different causes. If you'd like us to look at yours, see chimney leak repair.

Sources

1. Homeowner Resources and FAQs
2. Water Causes More Chimney Damage Than Fires
3. Brick Brief: Flashing Chimneys
4. Mason Contractors Association of America
5. Replacing a Leaking Chimney Cap
6. publication 326023

This guide is general information. It can't diagnose a specific chimney; an inspection can.