What Is a Chimney Chase, and Do You Have One?
A chimney chase is a framed box, built of wood or metal studs and finished in stucco, siding or cement board, that encloses a metal flue running up from a factory-built fireplace. A fireplace chase is the same thing named from the other end. It is not a masonry chimney. It only looks like one from the street, and nearly everything that goes wrong with it goes wrong for different reasons. Telling the two apart is the first diagnostic step, and you can usually do it from the ground.
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What a chimney chase actually is
A chase is a hollow enclosure built to surround a flue, not to carry one. The load-bearing part is a frame of wood or light-gauge metal studs, sheathed and then finished in stucco, lap siding, cement board, or occasionally a thin veneer of brick or stone. Inside that frame, a double-wall or triple-wall metal flue runs from the fireplace up through the roof and out the top. The frame carries only itself. The flue carries the heat and the exhaust, and the two are held apart by a required air clearance.
A masonry chimney is the opposite arrangement. There the structure and the flue are one assembly, brick or block built around clay tile, and it sits on a footing of its own because of what it weighs. A chase is light by comparison and usually rests on the house framing itself. That single difference explains most of what follows. Masonry fails slowly, by absorbing water and shedding material at the surface. A chase fails from the inside out, quietly, because the frame holding it up is wood inside a closed box that nobody looks into.
How to tell a chase from a masonry chimney
Start at the top. A masonry chimney is finished with a crown, a sloped cap of concrete or mortar, with the clay tile standing proud of it. A chase is finished with a flat or slightly sloped metal lid, called a chase cover, with a round collar where the flue passes through. If you can see metal up there, or a decorative metal hood sitting over the whole opening, you are looking at a chase. Binoculars from the yard settle it, and so does a photograph taken with a phone at full zoom.
The face gives it away too. Stucco over a frame sounds hollow when you tap it and has crisp, perfectly square corners, because the corners are formed by a metal bead rather than by stacked brick. Stucco over block or brick sounds solid and dull. Look at the firebox as well. A factory-built firebox has a metal outer shell, refractory panels rather than laid brick, and usually a metal rating plate inside the opening or on the frame above it. A firebox of laid brick with mortar joints belongs to a masonry chimney.
The parts of a chase and what each one does
Working down from the top, the chase cover is the lid that closes the open box, and its job is to shed water rather than hold it. The storm collar is the band that seals the joint where the flue passes up through that lid. A shroud, if there is one, is the decorative hood that shelters the whole termination, though the flue's own rain cap is what actually closes the pipe. Below the cover sits the flue, held by supports, with clearance kept on every side so no part of it touches framing.
Below that is the box itself, made of studs, sheathing, a weather barrier and the finish coat, with flashing where the chase passes through the roof plane and a base where it meets the wall or the deck. Inside, horizontal firestops close off the cavity at the ceiling and roof levels. Every one of those parts has a seam, and a seam is a place water can enter. Treating the chase as a stack of joints rather than as a solid object is the most useful way to think about it while reading symptoms.
How a chase fails, in the order it happens
The usual sequence begins at the cover. A flat cover holds a shallow pool after every storm, and standing water on painted steel finds the seams and the fastener holes first. Rust starts underneath, where nothing shows. Once the cover is perforated or the storm collar has lost its seal, rain runs down the outside of the flue into the cavity. Because the box is closed and the air here is humid year round, that water leaves slowly. The cavity stays damp rather than drying out between storms.
From there it moves in two directions at once. The flue corrodes from the outside, which is the side no one looks at, and the framing and sheathing stay wet long enough to soften. Salt-bearing air near the coast accelerates corrosion, and the cover and the storm collar are the thinnest, most exposed steel in the assembly, so that is where it surfaces. The outward signs arrive late. Rust streaking down a face, a finish that stays dark after rain, or a soft spot in the sheathing all mean the process is already well along.
Interior symptoms, and telling them from a roof leak
Because the cavity is closed, a chase reports its problems indirectly. A ring or tide line on the ceiling near the fireplace wall, paint lifting on the drywall above the mantel, or a musty smell that appears a day after heavy rain and fades in dry weather are all consistent with water in the cavity rather than a roof leak somewhere else. Wind noise is another. A hollow drumming or a rattle in gusts often means the cover has lifted at a corner or the shroud fasteners have worked loose.
Separating the two is mostly a matter of timing and direction. A roof leak tends to follow the pitch and show up away from the chase, while chase water tracks straight down and appears tight against the chimney wall. Note whether the stain grows during the rain or hours afterward, since a slow cavity drains long after the sky clears. Whatever the interior sign, the source sits at the top of the chase far more often than in the middle of a face, so that is where the looking should start.
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Frequently Asked Questions
Is a chimney chase the same as a fireplace chase?
Yes. Both names describe the framed, hollow enclosure that surrounds a metal flue running up from a factory-built fireplace. Some people name it from the fireplace at the bottom and some from the chimney at the top. The structure is identical either way, and so is everything that tends to go wrong with it.
How can I tell a chase from a masonry chimney without going on the roof?
Look at the top with binoculars or a zoomed phone photo. A metal lid and a round collar mean a chase. A sloped crown with clay tile standing above it means masonry. Then tap the wall at chest height. A frame sounds hollow, block or brick sounds dull and solid. The firebox settles it too, since a metal shell with refractory panels means a chase.
Is a chase weaker than a masonry chimney?
Not weaker for its purpose, since it only has to hold itself up and keep weather out. It is harder to read, though. Masonry shows its neglect on the outside surface where you can see it, while a chase can hold the same water inside a closed cavity, out of sight, where the part getting wet is the wood frame holding the box up.
Why does a chase need a chase cover at all?
The box is open at the top when it is framed, and the cover is what closes it. Without a sound cover and a sealed storm collar, every rain puts water directly into the cavity around the flue. That is why the cover, rather than the wall finish, is the first thing worth looking at on any leaking chase.