Chase cover vs chimney cap: which one your roof has

Which kind of chimney is on your roof

The two parts belong to two different kinds of chimney, so the first question is which kind is on your house. A masonry chimney is built of brick, block or stone, sometimes finished in stucco. At its top you will see a concrete or mortar slab with one or more clay flue tiles standing up through it. A factory-built chimney is a metal pipe listed with the fireplace below it, usually made by the same company.

Where that pipe climbs an outside wall or rises through the roof, builders often box it in with framing and siding so it looks like part of the house, and that box is the chase. Those metal chimneys are tested and listed as systems, under UL 103 for the chimney and UL 127 for the fireplace, and their installation instructions govern every part up to the cap.

From the yard the difference is easy to see. If the chimney looks like a wall of the house that carries on above the roof, clad in siding, shingles or stucco, and a metal cap sits on a pipe at the top, it is a chase. If it is bare brick or stone with clay tiles at the top, it is masonry. Indoors, a factory-built fireplace has a metal firebox lined with panels and a model plate, which may sit behind a grille below the opening.

What the chase cover does

A chase is a small building of its own: studs, sheathing and siding, open at the top where the chimney pipe comes out. The chase cover is its roof. It is one sheet of metal laid across the box and fastened down, pierced for each pipe, with a short upright ring formed around each hole.

Rain crossing the sheet meets that ring and runs around it rather than into the opening. The edges bend down over the siding and finish in a small outward kick, the drip, so water falls clear of the wall instead of running back under the metal.

Fireplace makers describe the chase top in their installation manuals. One manual from a large maker states that every chase installation needs one, allows it to be made on site, and asks for a turned-down edge with a drip, a 2-inch collar at the pipe opening, a small gap around the pipe, and a slope falling away from the opening. The same page warns against caulking the pipe to the collar, because the pipe needs its air space. So the chase cover is made to measure for each chase, following the fireplace maker's drawing.

What the chimney cap does

A chimney cap sits directly over a flue opening. On a masonry chimney it is a metal hood on legs or a frame, fitted to the clay flue tile, with a screen around the sides. The hood sheds the rain that would otherwise drop into the flue, and the screen keeps birds and animals out and holds back embers. For masonry chimneys, the residential volume of the Florida Building Code (R1003.9.2) sets the rules that screen has to meet where one is fitted: its open area, its strength, the size of its openings, and that it can be removed for cleaning.

On a factory-built chimney the cap is different in one important way. It is part of the chimney system, tested with the fireplace, and its manual lists it by part number. The manuals are direct about it: the termination has to be the one approved and listed for that fireplace. A masonry-style cap clamped onto a metal chimney pipe is not a substitute for it.

How each one fails

The two parts fail in different ways, and the symptoms point to which one is involved.

• Chase cover, flat or sagging: water stands on it after rain, the metal rusts from the puddle outward, and pinholes let water into the chase.
• Chase cover, rusting: brown or orange streaks run down the siding from under its edge.
• Chase cover, loose: one edge lifts in wind and rain blows under it onto the framing.
• Storm collar, slipped or unsealed: water runs down the pipe and past the collar opening.
• Masonry cap, missing or bent: rain reaches the smoke shelf and damper, and leaves or nests collect above the throat.
• Masonry cap, torn screen: birds or animals get into the flue.
• Metal chimney cap, damaged: rain or debris enters the pipe, and the system no longer matches its listing.

Why they are replaced differently

A chase cover is fabricated. Its size comes from measuring the outside of the chase over the finished siding, its pipe holes from measuring each pipe's position, and its collar, gap and slope from the fireplace manual. Metal and gauge are a choice: manuals give a minimum, and a heavier stainless sheet holds its shape and resists rust better than light galvanized sheet.

A masonry cap is chosen from a maker's range to fit the flue tile, or built to measure for an odd tile or for a cover spanning the whole crown. A factory-built chimney cap is ordered from the chimney's maker for that model. When a chase cover is replaced, a sound listed cap on top of the pipe is taken off and put back on over the new cover. When the cap itself is damaged, it is replaced with the listed part, and the chase cover stays if it is sound.

The names that cause mix-ups

Part of the confusion is vocabulary. People call a chase cover a chase cap, a chase top, a chase pan or simply a chimney cover. Some call a masonry chimney's concrete top a cap, and the residential code does the same: section R1003.9.1 of the Florida Building Code, Residential, calls for a sloped concrete, metal or stone top on every masonry chimney, finished with a drip edge and a caulked bond break around the flue liners. That top is the part the trade calls the crown. Our note on chimney crown vs cap sorts out that pair.

The chase cover vs chimney cap difference is easiest to hold onto by describing what each part sits on. A metal sheet over a framed box is a chase cover. A hood over a clay tile, or the manufacturer's top on a metal pipe, is a cap. A concrete slab across the top of brick is a crown.

What you can check without a ladder

The warning signs can be seen from the ground with a pair of binoculars. Look for streaks on the siding below a chase top, a sag or low spot in its surface, an edge that is lifted, or a cap that is leaning or missing its screen. Inside, look for stains on ceilings close to the fireplace, rusty flakes on the floor of a metal firebox, or water in the fireplace after rain.

Stay off the roof and off the chase top itself. Thin sheet metal dents under a foot or a knee, and each dent holds a little pool after the next shower. Whichever part it is, expect the written estimate to name which part failed, what it will be replaced with, the metal and gauge, and, for a chase, the fireplace model the new top was built to.