Rust and brown stains on a chimney

Start at the top of the stain

Water runs downhill, and stains follow it. The most useful question about a chimney stain is where it begins. Stand back, use binoculars or zoom in with a phone camera, and trace the streak upward.

• Starts at the edge of a metal top on a siding-covered chimney: the chase cover is the prime suspect.
• Starts at the cap or the flue tile on a masonry chimney: the cap, its fasteners or the flue top.
• Starts at the roofline: flashing, counter-flashing or roof nails.
• Bleeds through the brick face in patches, not from an edge: moisture moving through the masonry from inside.
• Appears on an inside wall or ceiling near the chimney: water or condensation is reaching the interior.

This guide treats each one separately because the fixes are different.

Chase covers on siding-clad chimneys

Many homes with factory-built fireplaces have a chimney chase: a wood-framed box covered in siding, with a metal pan on top called a chase cover. A metal flue pipe runs up through it.

Chase covers made from galvanized steel are common and inexpensive, and over time the zinc coating wears off and the steel rusts. A builder writing in the Journal of Light Construction recommends stainless steel over galvanized for this reason, noting galvanized can rust within several years in coastal conditions. Flat covers make it worse because water sits on them instead of running off.

The pattern is distinctive: orange or brown streaks running down the siding from the corners or edges of the chase top. Over time, rusted covers can also develop holes that let water into the chase, where it can rot framing, wet insulation and drip onto the firebox below.

If you see rust streaks on a chase, look for sagging, pooling water and holes. Replacement with a sloped or crowned cover in a corrosion-resistant metal is the usual fix. See chase covers.

Caps, screens and flashing

On a masonry chimney, rust stains often trace back to:

• A galvanized or painted steel cap. The Internet Center for Wildlife Damage Management recommends stainless steel caps for the same reason: galvanized steel rusts. Rust from the cap runs onto the crown and down the brick.
• Old steel fasteners or brackets holding caps, antennas or straps.
• Steel flashing or counter-flashing at the roof, or roofing nails exposed at the chimney.
• A steel top-sealing damper frame that has started to corrode.

Rust from a cap is often cosmetic at first. Once the cap or screen corrodes through, it no longer keeps out rain or animals, and a corroded screen can shed pieces into the flue. Rusted flashing is more urgent because it is the seal against roof leaks. See chimney caps and flashing repair.

Brown staining from flue condensation

Brown, tan or tarry stains that seep through the brick face, often near the top of the chimney, are a different story. They are frequently linked to moisture forming inside the flue.

Gas and oil furnaces, boilers and water heaters produce a lot of water vapor. DuraVent's relining guide explains that modern, efficient appliances send less heat up the chimney, so exhaust can cool to its dew point inside a masonry flue. The resulting condensate is acidic. It can erode clay tiles, brick and mortar from the inside, and it carries soot and dissolved material through the masonry, leaving brown or white stains on the outside.

Wood-burning fireplaces that run cool and smoky can also push creosote-laden moisture into masonry, creating dark staining near the top.

Condensation staining matters because it means the flue may not be containing exhaust as designed. If you see this kind of staining, stop using the connected appliance until the flue has been checked, and make sure your carbon monoxide alarms are working. If a CO alarm sounds, leave the house and call 911.

Stains inside the house

Brown or yellowish stains on walls or ceilings next to a chimney usually mean water is reaching the interior. The source may be flashing, the crown, porous masonry, or condensation in the flue. Stains on a basement chimney wall near a furnace or water heater connection point toward condensation or a deteriorated flue.

A damp, discolored area with a sharp or smoky smell, especially in heating season, deserves prompt attention because it may mean flue gases are moving where they should not. Read Why is my chimney leaking? and Carbon monoxide and your chimney.

Quick comparison

Treat the table as a starting point. More than one source can be active at once.

Can the stains be cleaned?

Cleaning a stain before fixing its source only resets the clock. Once the cause is dealt with, rust on brick or siding can often be lightened with cleaners made for that material. Masonry is easy to damage with strong acids or pressure washing, so test any product in a small area and follow the manufacturer's directions. Some old rust or tar staining does not come out fully, especially on porous brick.

Painting over stains on brick is not recommended, because a film coating can trap moisture in the masonry. See Why is my chimney brick flaking?

Preventing new stains

Most rust and condensation staining can be prevented, or at least slowed, with a few choices made at the right time:

• Choose corrosion-resistant metal when a cap, chase cover or flashing is replaced. Stainless steel and copper last far longer than galvanized steel in wet, salty or acidic conditions.
• Make sure water runs off. A chase cover with a raised center, sloped panels or cross-breaks sheds water. A flat pan holds it, and standing water speeds up corrosion.
• Keep drip edges clear of the siding so water drops off the cover instead of running down the wall.
• Match the flue to the appliance. When a furnace, boiler or water heater is replaced, the chimney it vents into should be checked to confirm it is still suitable. See The orphaned water heater problem.
• Watch older fasteners and straps on the chimney and replace corroded ones before they fail.

A yearly look at the top of the chimney, from the ground with binoculars or during a sweep, catches most of these before they stain.

How we assess chimney staining

We start with an assessment. We trace the stain to its source from the roof when it is safe, check the cap, chase cover, crown and flashing, and run a camera through the flue when the flue allows to look for condensation damage or liner problems. We check the attic side when accessible and use controlled water testing when the source is unclear. You get findings with photos before any repair is scheduled. Depending on the result, the work might involve a chase cover, a cap, flashing, or a liner. Request service to get started.

Questions homeowners ask

Sources

1. Journal of Light Construction: Capping a Chimney Chase
2. Internet Center for Wildlife Damage Management: Chimney Cap
3. DuraVent: Relining Chimneys for Gas Appliances (Ventinox technical guide, PDF)
4. US Consumer Product Safety Commission: Carbon Monoxide Information Center

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