Rust Stains Running Down Your Chimney? It's Probably the Chase Cover

If you have a chimney that is wood-framed and covered in siding, and you see orange or brown streaks running down from the top, the cause is almost always the chase cover: the flat metal lid that closes off the top of the chase. Most chase covers installed with factory-built fireplaces are galvanized steel. In a climate that gets about 40 inches of rain a year and roughly 150 wet days, the zinc coating wears off, the steel underneath rusts, and rainwater carries that rust down the siding. The streaks are the warning. The hole that follows is the problem.

The fix is a new chase cover in stainless steel or aluminum, built to the right size with a proper slope, drip edge and a collar around the flue pipe. Our chase cover replacement special starts at $299, every job gets a free quote, and same-day service is often available around Seattle, Tacoma and Everett. Here is how to tell what you are looking at and what happens if you wait.

What a chase cover is (and why it is not a chimney cap)

A factory-built (prefab) fireplace does not have a brick chimney. It has a metal flue pipe running up through a framed box called a chase, which is sheathed and sided to match the house. The top of that box has to be closed off against weather, and that is the chase cover: a shaped sheet of metal that covers the whole top, turns down over the edges, and has a hole with a raised collar where the flue pipe comes through.

A chimney cap is the smaller hood with a screen that sits on top of the flue pipe itself. Both are needed on a prefab chimney. The cap keeps rain and animals out of the pipe; the chase cover keeps rain out of the framed box. When people say "my chimney cap is rusting," they are often looking at a rusting chase cover. If the rust is on the big flat lid, it is the cover. If it is on the small hood on the pipe, see our chimney cap replacement page.

Chase covers are everywhere in the Puget Sound housing stock built after the mid-1980s. Renton's median home was built in 1988, Redmond's in 1993, Bothell's in 1992, Gig Harbor's in 1998. Subdivisions like Lakeland Hills in Auburn, Canyon Park in Bothell, or the South Hill plats in Puyallup are mostly prefab fireplaces with framed chases. A lot of those original galvanized covers are now 25 to 35 years into a climate they were never going to survive.

Why galvanized chase covers fail in Seattle

Galvanized steel is plain steel with a thin zinc coating. Zinc holds up for a while, but every scratch, every screw hole, every seam and every spot where water pools is a place the coating gives out. Three things in Western Washington speed that up:

• Constant wetting. The wet season from October through March brings about 29.65 inches of rain. November alone averages 6.31 inches. The cover never gets a long stretch to dry.
• Flat installs that pool water. Many original covers were installed flat or have sagged in the middle over time. Standing water on galvanized steel is the fastest way to rust it through.
• Moss and debris. Regional roofing guidance is consistent that Western Washington moss spreads fastest from October to April on shaded, slow-drying surfaces. A chase on the shaded side of the roof collects moss and fir needles that hold water against the metal.

Rust starts at the collar around the flue pipe, at the corners, and in any low spot. Once it perforates, water drops straight into the chase.

What the rust streaks are telling you

Read the streaks like a timeline:

• Light orange tint near the top edge: the zinc is gone and surface rust has started. The cover is still keeping water out, but it is on the clock. This is the cheapest time to replace it.
• Dark streaks down the siding, heaviest below the corners: rust is active and the cover has likely lost its drip edge function. Water is running over the edge instead of off it, which also soaks the top of the siding.
• Rust plus a visible dip or sag in the cover, or staining inside the house: the cover is probably perforated. Water is getting into the chase. Look for water stains on the ceiling or wall around the fireplace, a musty smell, or rust on the firebox top or the damper of the fireplace itself.

Note that not every stain is rust. White chalky streaks on masonry are efflorescence (salts carried out by water), and black streaks are usually algae or soot. Orange-brown on a sided chase is rust, and the chase cover is the first suspect.

What happens if you wait

A leaking chase cover is the most common way a framed chimney ends up needing a full rebuild. Water that gets past the cover soaks the plywood sheathing and the framing at the top of the chase. In our climate, that wood does not dry out between storms. Rot starts at the top plates and works down. By the time the siding bulges or goes soft, the framing behind it is often gone. A chimney chase rebuild starts at $299 and a localized chase repair is less, but both are a lot more than a $299 cover. Water dripping onto the metal fireplace unit can also rust the firebox top, the damper and the pipe connections, which is a safety issue, not just a cosmetic one.

That is the whole argument for acting on the streaks instead of the hole. The cover is a wear part. Replacing it on time protects everything under it.

What a proper replacement looks like

Not every new chase cover is a good one. When we replace a cover we build it to outlast the house, not to match the one that failed:

• Stainless steel or aluminum. Neither one rusts the way galvanized does. Stainless is the stronger choice for larger chases and for homes near salt air along the Sound.
• Measured, not guessed. We measure the chase, the flue pipe diameter and the pipe's position so the cover fits with no gaps at the skirt and the collar is centered.
• Built-in slope or cross-break. The cover is bent so water runs to the edges instead of pooling in the middle.
• Drip edge. The skirt that turns down over the chase has a hem that throws water clear of the siding instead of wicking it back under.
• A tall collar with storm collar and sealant. The hole around the flue pipe is the number-one leak point. A raised collar plus a storm collar keeps water out of it.
• Wood checked before the cover goes on. If the top plates or sheathing are soft, we tell you before the new cover hides it. A new lid on rotten framing is a wasted lid.

At the same visit we look at the cap on the flue pipe and the condition of the siding and trim. If the cap is shot too, it is far cheaper to do both in one trip.

What it costs

Our chase cover replacement special starts at $299. The final number depends on size, material and the condition of the flue collar, and you get it in writing as a free quote before we start. We price match written quotes from other companies and take another 10 to 25 percent off. If someone else has quoted you a chase rebuild, get a free 2nd-opinion quote first; sometimes the framing is fine and the cover is the whole job. And if you're not happy, we're not done: we re-do the work at no charge.

What to do next

Please do not go up on the roof to check. You can see what you need from the ground:

• Walk around the house and look at the chase from each side. Note where the streaks are heaviest and whether the cover looks flat, dished or lifted at any corner.
• Inside, check the ceiling and walls around the fireplace for stains, and look at the top of the firebox for rust or drips.
• Call 206-453-2045 or request a free quote online and describe what you see. Photos from the ground help.

We cover Seattle, Tacoma, Everett, Bellevue, Renton, Kent, Federal Way, Auburn and the rest of King, Pierce and Snohomish counties, plus Gig Harbor over the Tacoma Narrows Bridge. See our Seattle service area page for details. Certified and insured, satisfaction guaranteed, competitive pricing.

Related

• Chimney Crown Repair vs. Rebuild: How to Decide
• Do I Need a Chimney Cap? (Yes — Here's Why in Seattle)
• Chimney Chase Cover Replacement service