Tracing an orange streak on the chase back to the metal on top

Where the orange streak on a chase starts

A chase is the framed box, finished in stucco, siding or trim, built around the metal chimney pipe that serves a factory-built fireplace. A sheet-metal cover closes the top, and its skirt folds down over the sides. When that cover rusts, every rain carries a little of the rust over the edge and down the wall, leaving the orange or brown streak that most owners notice first.

Rockford Chimney Supply, which fabricates replacement covers, describes the pattern plainly. Most original galvanized tops have a low spot that holds water, the metal starts to rust after a while, and runoff carries the rust down the siding as a stain. Chase cover rust is therefore rarely a mystery. The question is how far it has gone and what else is rusting with it.

Why a flat chase top holds water

A sheet of metal laid flat over a wide box sags between its supports, and a sag holds a puddle. Replacement covers can be made with diagonal creases, sometimes called cross-breaks, bent into the top from corner to corner. Rockford explains that these creases keep water from accumulating, and that a drip edge along the bottom of the skirt keeps runoff from falling straight onto the siding or brick.

An original cover without those details spends much of the wet season holding water in its low spot. Leaves and grit collect in the puddle and keep it damp even between storms. The metal under that puddle is the part that rusts through first, and the runoff from it follows the same path down the chase every time.

How the zinc on a galvanized cover wears through

Galvanized sheet is steel with a thin zinc coating. The American Galvanizers Association points out that the coatings on continuously galvanized sheet, named by ASTM A653 designations such as G60 and G90, are much thinner than what a galvanizing kettle leaves on finished steel, and that the thicker the coating, the longer it serves. A chase cover starts with a limited supply of zinc.

How fast that supply is used depends on the conditions. The same association explains that alternating wet and dry spells are what build the protective film on zinc, and a pan with standing water spends long stretches wet instead of drying out. Near salt water, chlorides from sea spray dissolve that film and expose fresh zinc. Once the zinc is gone from a patch, the steel beneath rusts, and a rusty chase cover then streaks with every storm. Choosing a metal for a home close to the coast is covered in our guide to salt air chimney damage.

Cover, cap, storm collar or flashing?

Several metal parts sit at the top of a chase, and each leaves its own kind of mark. DuraVent's typical installation drawing for its DuraChimney II system labels the chimney cap on top of the pipe, a storm collar around the pipe just above the chase top and the chase top flashing itself. With binoculars from the yard, the starting point of the streak usually tells you which part is responsible.

• A streak that begins right under the skirt edge, often at a corner or along the lowest side, points to the cover.
• Rust running from the top of the pipe down onto the cover suggests chimney cap rusting. Florida's residential code has a factory-built chimney terminated to its maker's instructions (R1005.1), so a replacement cap has to suit that system.
• An orange ring around the pipe where it passes through the cover points to the storm collar or its fasteners.
• A streak that starts at the roofline beside the chase, rather than at the top, points to flashing or other roof metal.
• On a brick chimney, rust stains can have another source entirely. The Brick Industry Association notes that corroding wall ties or joint reinforcement in or next to the brickwork can cause rust-colored stains.
• A green or blue-green stain under a copper cover is copper runoff, which the same association lists among stains from materials above brickwork, not rust.

What rust on top means for the chase below

A streak on the outside is the visible part. The less visible part is what happens when the pan thins to pinholes. Rain then passes through the cover into the chase, runs down the framing and the chimney pipe and can reach the top of the fireplace. DuraVent describes its storm collar as a part used to help keep rain out of the chase, and a perforated cover lets that rain straight in.

Check the inside of the house for the signs. Rust flakes or water on the floor of a metal firebox, damp or stained refractory panels and stains on the ceiling or wall around the chase all suggest the top is letting water in. If the framing or siding of the chase has been wet for a while, chimney chase repair may be needed alongside a new cover.

Why painting over the streak changes nothing

The streak is the last step in a chain that starts on the roof. Painting the siding or stucco hides it until the next heavy rain, when fresh rust runs down over the new paint. Paint on a cover that is still sound can slow corrosion, and the American Galvanizers Association counts a durable paint system among its protections for joints between metals. Paint does not change a top that ponds, though, and it does not bring back steel that has rusted through.

Cleaning the stain has its own risk. The Brick Industry Association's guidance on stains warns that the wrong cleaning agent can leave a mark that is harder to remove than the original. Clean the wall once the source is fixed, test any cleaner on a small area and follow the maker's instructions for the siding or stucco.

What a replacement chase cover should have

A new cover is the chance to remove the causes along with the rust. Look for these details in the written estimate.

• Stainless steel or copper instead of galvanized sheet, which are the replacements Rockford offers for rusted tops.
• Diagonal creases so the top sheds water instead of holding it.
• A drip edge on the skirt so runoff clears the siding.
• A skirt long enough to cover exposed wood, fastened through into the structure behind it.
• Fasteners of a matching metal. The American Galvanizers Association explains that zinc is the metal attacked when it touches stainless steel or copper in wet conditions.
• A sealed storm collar, and a cap that suits the listed chimney system on a factory-built fireplace.

Replacing a rusted chase top in Miami

On a free inspection we trace the streak to its starting point and check the cover, the storm collar, the cap and the chase below it. The written estimate says whether a new chimney chase cover is needed, what it will be made of, how it will be attached and what it costs, all before any work is scheduled. When the work is done, the old metal and debris leave with us, and the written record names the new cover's metal and fastening. Attachment also matters in storm season, which our guide to hurricane chimney prep explains.

Sources

• Rockford Chimney Supply, chimney chase covers: Galvanized chase tops with a low spot rusting and staining the siding, and stainless or copper replacements.
• Rockford Chimney Supply, custom chase covers: Diagonal creases, drip edges and skirt fastening on chase covers.
• American Galvanizers Association, continuous sheet galvanizing: ASTM A653 sheet coating designations and the link between coating thickness and service life.
• American Galvanizers Association, hot-dip galvanized steel in the atmosphere: Wet and dry cycles in forming zinc's protective film, and chlorides from sea spray.
• American Galvanizers Association, dissimilar metals in contact: Zinc corroding against stainless steel or copper, and paint as a protective measure.
• DuraVent DuraChimney II component literature: The cap, storm collar and chase top of a factory-built chimney system, as one manufacturer's example.
• Brick Industry Association Technical Note 23, Stains: Rust stains from embedded steel, copper stains, and cleaning agents that make stains worse.
• Florida Building Code, Residential, 8th edition (2023), chapter 10: Section R1005.1, which has factory-built chimneys listed, labeled and terminated as their maker's instructions direct.