Where a rust streak on a chimney starts

Follow the streak up

A rust stain is a map. Rainwater picks up rust from a corroding steel part and carries it down the surface below, so the streak starts just under the source and fans out as it runs. The first job is to find the top of the stain and look at exactly what sits there.

Where the streak begins tells you a lot. Stains that start at the top edge of a framed chimney and run down the siding point to the chase cover. Streaks that start just under the cap's skirt, on the crown or the upper brick, point to the cap or its fasteners. A stain that begins at the roof line points to the flashing. Rust inside the firebox points to the damper, not to anything outside.

Why galvanized steel goes first

Galvanized steel, which is ordinary steel coated with zinc, is widely used for chase covers, caps and flashing. The zinc protects the steel in two ways: it covers it, and it corrodes in the steel's place, slowly and sacrificially. As long as zinc remains, the steel underneath stays sound. When the zinc is used up in a spot, the steel there begins to rust, and a stain appears.

How fast the zinc goes depends on its surroundings. The American Galvanizers Association notes that chloride deposition, from sea spray and marine air or from salt spray, significantly speeds up zinc corrosion. Standing water does too, because it keeps the coating wet for long stretches. Scratches and cut edges from installation expose bare steel from the start.

The chase cover that holds water

Chase cover rust begins where water sits. On a framed chimney, the metal cover across the top is the first place to look when rust streaks run down the siding. A flat cover, or one that has sagged, holds rainwater in a shallow pool around the flue collar. That pool keeps the galvanized coating wet, the zinc wears through, and the steel rusts until it is perforated. At that point the stain on the siding is the smaller problem, because water is now running into the chase and rotting the framing.

A cover that sheds water has cross-breaks or a slope that carries rain to the edges, and a drip edge that throws it clear of the siding. When a rusted cover is replaced, those details and the material are what matter.

Caps, screens and fasteners

A chimney cap rusting follows the same path, and the screen can go first, since its thin wires have little coating to lose. Rust from a cap runs down onto the crown and the upper brick. A cap that is rusting through also loses its grip on the flue and its ability to keep animals out.

Fasteners are a quieter source. Mixing metals where they touch in the presence of water sets up galvanic corrosion, in which one metal is eaten away faster than it would be alone. Plain steel screws in a copper cap, or bare steel fasteners in stainless parts, can bleed rust long before the part itself fails.

Flashing and steel inside the masonry

Steel flashing at the roof line rusts where its coating wears or where it was cut, and it stains the brick just above the roof. Rust can also come from steel set into the masonry itself, such as reinforcing in a crown that has cracked and let water reach it, or a steel frame at the top of the flue. When the source is inside the masonry, the rust is a sign that water has been getting in, and the crack or joint that let it in is part of the repair.

Rust inside the fireplace

Rust can show up inside the house as well. A throat damper that is rusting, flaking or seized is a sign that water is coming down the flue, through a missing cap, a cracked crown or open joints at the top. The steel lintel over a masonry fireplace opening can also rust and stain the face below it when moisture reaches it.

Inside rust is traced the same way as outside rust, upward, but the answer tends to be at the top of the chimney. Replacing a rusted damper without closing the water path above it only starts the clock again.

What replaces a rusting part

The replacement should match the exposure, and the part's maker is the authority on which material suits which location. The broad choices are these.

• Stainless steel: does not rely on a coating. Grade 316 adds molybdenum, improving its resistance to chloride pitting compared with grade 304, so the grade matters where salt spray reaches.
• Copper: does not rust. It weathers to a brown and then green patina, can leave green staining, and should not be paired with galvanized steel or plain steel fasteners.
• Galvanized with a paint or powder coat: protected only as long as the coating is unbroken, so it needs watching where it sits in water or salt spray.

Why location changes the material

The same cover or cap can last very differently on two houses. Salt carried in the air from the ocean or from road spray settles on metal and keeps drawing moisture to it, and the American Galvanizers Association lists that chloride deposition among the conditions that speed up zinc corrosion. A house close to salt water is therefore a case where stainless steel of a suitable grade, or copper, earns its place over galvanized steel.

Away from salt, standing water is the bigger enemy, and the shape of the part counts for as much as its metal. A galvanized cover with a good slope can outlast a better metal installed flat.

Cleaning the stain

Clean last. Once the rusting part has been replaced and the water path fixed, the stain can be treated with a cleaner made for rust on masonry or siding, following its label and testing it first on a small hidden spot. Avoid high-pressure washing, which can damage soft brick and stucco and force water into the wall. On porous brick, a faint shadow of the stain can remain, and on painted siding, repainting may be the cleaner answer. Our chimney chase cover page covers replacing a rusted cover.