What Do the Chimney Stains Mean? 

Black Stains

If your chimney has black stains, it’s most likely the result of excess creosote and soot build up inside your flue. 

When your chimney isn’t cleaned regularly, creosote continuously builds up, creating a thicker and thicker layer on the walls of your chimney flue. 

The build-up of creosote will keep your chimney from drafting properly. Basically that means you could have smoke filling your chimney instead of exhausting up and out of your chimney. 

Eventually, your dirty flue will show itself by creating soot stains on the exterior. 

Most soot stains can just be the result of age or lack of regular cleaning and maintenance. But it can also indicate a hidden chimney problem. 

When the creosote build-up is bad enough to cause exterior staining, you may need to get your chimney relined. 

NOTE: Gas fireplaces should never have exterior staining from soot. If you have soot stains and your fireplace is gas, this can indicate a carbon monoxide leak or another dangerous malfunction.

Dark Green or Blueish Stains

Dark green or blueish stains are pretty hard to differentiate between black stains caused by soot unless you’re up close and personal with your chimney.

Dark green or blueish stains are caused by algae growth. It’s often seen in areas where the water has been allowed to pool or consistently flows.

For example, a commonplace for this type of stain is around the chimney crown, when the chimney crown doesn’t have an appropriate overhang.

In a perfect world, all components are performing their functions to keep moisture out.

But the biggest design flaw in exterior masonry chimneys is the fact that bricks are porous.

That means they absorb water.

In the wintertime, this could result in freeze/thaw cycles which can cause your chimney bricks to crack and ultimately will lead to your chimney leaking anytime it rains.

It’s important to have these troubled areas corrected so that it doesn’t cause any additional problems later on down the road.

Red or Brown Stains

Red, brown, or “rust-colored” stains are most often seen on the sides of metal or prefabricated chimneys.

It’s rust-colored because it’s….you guessed it….rust!

When the rust stains begin to appear on the sides of your chimney, that’s a sign that your chase cover is probably leaking and you should have a chimney company take a look.

Rusted chase covers can allow for pinholes on top. Those pinholes in the rusty chase cover will allow water to get inside your chimney.

You should have rusted chase covers replaced as soon as possible to prevent water damage to the fireplace and chimney.

Luckily, these days, you can get stainless steel chase covers that will not rust.

If you don’t’ have a metal or prefabricated chimney, but you’re still seeing rust-colored stains, it’s likely that you still have some component rusting.

That could be your cap or the flashing around your chimney.

Rust stains on any chimney are indicative of a potential leak and should be taken care of as soon as possible.

White Stains

In the chimney industry, we call this efflorescence.

That means that water-soluble salts within your chimney’s masonry have mixed with water.

Those salts have traveled to the exterior part of the chimney, leaving deposits that will cause chimney discoloration.

The powdery white staining is easily washed off and does no harm itself.

But when efflorescence appears on the outside of your chimney, it’s pretty clear that you have some type of water damage to your masonry.

The problem with water in the masonry, as I’ve already mentioned, is that during freeze/thaw cycles, your brick will expand and contract.

Over time, this will cause your chimney to deteriorate.

You might notice a pile of debris beneath your chimney. This is evidence of the brick face popping or breaking off from deterioration.

When you see efflorescence the best thing to do is replace the affected bricks. Otherwise, I’m afraid, your chimney will eventually weaken to the point of leaning and potentially collapsing.