Chimney Chase Cover: Materials, Sizing and Signs It's Failing

A chimney chase cover is the metal pan that caps a chimney chase, the framed, sided box that surrounds a factory-built (prefab) fireplace flue. Where a masonry chimney has a concrete crown, a chase has a chase cover. It sheds water off the top of the chase and has a collar where the metal flue pipe comes through. When a chase cover rusts or sags, water pours into the chase and down onto the fireplace, so it's a part worth checking every year.

Do you have a chase?

If your chimney is boxed in with the same siding as the house (vinyl, wood, fiber cement or stucco) and your fireplace is a metal firebox insert with a metal flue, you have a chase. Brick or stone chimneys usually have a crown instead; see our chimney crown guide.

Materials

Material	Life expectancy	Notes
Galvanized steel (original builder grade)	Often rusts within 5 to 10 years	Thin, flat, holds water; the usual reason people shop for a replacement
Aluminum	Doesn't rust	Soft; can't take high flue temperatures near the collar on some systems
Stainless steel (304 or 316)	Decades	The most common upgrade; 316 for coastal areas
Copper	Decades	Develops patina; keep runoff off steel or aluminum below

Design features that matter

• Cross-break or sloped top. A creased "X" pattern or a pitched top makes water run off instead of pooling. Flat covers rust from standing water.
• Skirt with a drip edge. The sides should come down at least 2 inches over the siding, ending in a hemmed drip edge.
• Raised collar. The hole for the flue should have a raised lip so water doesn't run into the chase around the pipe.
• Storm collar and cap. A storm collar sealed to the flue pipe sheds water over the collar, and a chimney cap tops the flue.

Measuring for a custom cover

Chase covers are almost always custom-made to the chase's size.

1. Measure the length and width of the chase top at the outside of the siding, in two places each. Chases are rarely perfectly square.
2. Measure the flue pipe's outside diameter and its position: distance from the center of the pipe to two adjacent sides.
3. Note how many flues pass through.
4. Choose the skirt length (2 to 4 inches is common).

Order the cover about 1/4 inch larger than your measurements so it slips over the siding.

Signs your chase cover is failing

• Rust streaks running down the siding of the chase
• A sagging, dished top that holds water
• Water in the firebox, or rust on the fireplace's metal parts
• Water stains on the ceiling or walls near the fireplace
• Rotted sheathing or framing at the top of the chase

Replacing one

Replacement means working on the roof at the chase top: removing the chimney cap and storm collar, unscrewing the old cover, setting the new one, fastening it through the skirt into the chase framing with stainless screws and neoprene washers, and refitting the collar and cap. Many chimney sweeps do it in an hour or two. While the old cover is off, check the framing for rot. If the plywood is soft, replace it first or the new cover will sag too.

A wet chase can also make a fireplace smell musty in summer; see chimney smell for other causes.