What's In This Article
It Started as a Routine Inspection
The homeowner in O'Fallon called for a standard chimney inspection. Nothing was leaking inside. No stains on the ceiling. No smell. No obvious reason to be worried.
From the ground, the only thing that stood out was faint vertical streaking on the siding at the base of the chimney chase — dark green and black, running downward.
That streaking is the most commonly dismissed warning sign in the entire trade. It's frequently called algae, and sometimes that's exactly what it is. Which is precisely why it gets waved off.
When It's Algae, and When It Isn't
Algae grows on siding that stays damp. Fair enough. But ask the next question: why is that particular strip of siding staying damp when the siding either side of it is dry?
Algae on a north-facing wall, spread evenly, in a shaded yard — that's algae.
Algae in a narrow vertical band, starting at the base of a chimney chase, running straight down, on a wall that's otherwise clean — that's not a growth pattern. That's a drainage pattern. Something above it is releasing water at a consistent point, and that water is running down the same line every time it rains.
Algae follows shade. Water follows gravity. If the stain is a straight vertical line under a chimney, it's telling you where water is exiting — and if water is exiting there, it has already been somewhere it shouldn't have been.
The Decision to Open It Up
You cannot diagnose the inside of a wall from outside it. At some point, someone has to remove a panel and look.
Most inspections stop before that point, and there are understandable reasons: it's invasive, it takes time, and if you're wrong you've opened a wall for nothing. But the alternative is writing "possible algae — monitor" on a report and driving away from a house that may be quietly rotting.
We took the panel off.
What Was Behind the Siding
The chain of failure was straightforward once we could see it:
- The chase cover on top had failed — water was getting past it at the collar.
- Water ran down the inside face of the chase wall, behind the siding.
- It saturated the house wrap until the wrap gave out.
- It soaked the OSB sheathing until the sheathing rotted.
- The excess found its way out at the bottom, ran down the outside of the siding, and grew the green streak that we spotted from the driveway.
The streak was the last step in the sequence, not the first. By the time it was visible, the damage inside was years old.
The Full Repair
Note the order of operations, because it matters: we fixed the source first. Replacing rotted sheathing without sealing the chase cover would have been rebuilding a wall and then pouring water on it.
What It Would Have Cost to Wait
Same Leak, Different Year
The homeowner caught this at stage two. Another few winters and the water would have been into the framing, and we'd have been talking about a structural repair rather than a wall repair.
If you have a framed chimney chase, the single highest-value thing you can do is look at the chase cover — because everything in this article started there.
Frequently Asked Questions
What do green or black streaks on siding under a chimney mean?
In a vertical band directly below a chimney chase, streaking usually indicates water is exiting the wall at that point — meaning it has been inside the wall. General algae growth follows shade and spreads evenly; a straight vertical line under a chimney follows gravity from a leak.
Is streaking on siding always a leak?
No. It can be algae, especially on a shaded, north-facing wall where the staining is spread evenly. The distinguishing feature is the pattern: a narrow vertical band starting at a chimney chase is a drainage pattern, not a growth pattern, and warrants investigation.
Do you have to remove siding to find a chase leak?
Sometimes, yes. You cannot see the inside face of a wall from outside it. When the external signs point to water inside the chase, removing a panel is the only way to know the true extent — and it's far cheaper than discovering it years later.
What causes a chimney chase to leak?
Most commonly a failed chase cover — the metal lid on top of the framed chase — leaking at the flue collar or through rust-through. Water then runs down the inside of the chase wall behind the siding, where nobody can see it.
How much does chase leak repair cost?
It depends entirely on how long it ran. Catching it at the cover is $800–$2,000. Once sheathing and siding are involved it's typically $3,000–$6,000. If water reaches the framing, structural repair can exceed $15,000. Our inspection and written estimate are free.
See Streaks Near Your Chimney? Get It Looked At.
Free inspection across Greater St. Louis and St. Charles County. We photograph everything and show you exactly what we find — including when the answer is "it's fine."
