Why chase covers are priced to measure

What a chase cover does on a factory-built system

Listed metal pipe carries the exhaust of a factory-built fireplace, and where that pipe climbs an exterior wall, a framed and sided chase encloses it. The chase cover is the sheet-metal top that closes that box. It sheds water off the chase, seals around the pipe with a collar, and lets the pipe pass through to its termination cap.

FBC Residential R1005.1 requires a factory-built chimney to be installed and terminated per the maker's instructions, and those instructions cover the parts at the top: the termination cap, the storm collar and any listed chase-top parts. A replacement cover has to fit around them without changing how the listed system works.

Why a stock lid is a poor fit

Chases are framed on site, so their outside dimensions vary from house to house, and the pipe can sit off center in the box. A stock-size lid that is close but not exact leaves gaps at the skirt or pushes the collar off center, and either one lets water in.

A made-to-measure chase cover price is higher at the start than a stock lid's. It is also the reason the cover fits the chase it sits on and the pipe that passes through it.

Metal choice and gauge

The metal comes first. A galvanized lid depends on its zinc skin, and when that wears through, rust runs down the siding and the lid starts holding water in low spots. Stainless steel stands up to standing water and weather much longer. Copper is the most costly to form, and it ages to a patina and does not rust.

Thickness is the second part of the same choice. A thin lid flexes, dents under a ladder foot and lets water pond, while a heavier gauge stays flat and sheds. The estimate names both the metal and the gauge.

Chase size and the number of pipes

Covers are fabricated to the outside dimensions of the chase, with a skirt that folds down over the top of the siding. A larger chase means more metal and a bigger piece to carry and set on the roof.

Each pipe that passes through the cover needs its own opening and collar, sized to that pipe. A chase that carries two fireplaces, or a fireplace plus another vent, means more collars to lay out and seal, and more measuring to get the spacing right. A chase that steps in width, or carries a second pipe added later, needs a cover laid out from a careful drawing rather than two outside measurements.

Shedding water: cross-breaks, slope and a drip edge

A flat sheet of metal on top of a chase sags and holds water. Covers are formed with cross-breaks, diagonal creases that lift the center and send water to the edges, or built with a slope or a raised center for positive pitch. The skirt ends in a drip edge that throws water clear of the siding.

A sloped or peaked design takes more fabrication than cross-breaks. It is the stronger answer on a large chase, where a flat panel would sag between its edges.

Fasteners matter here as well. Screws through the skirt should match the cover's metal and carry sealing washers, so the fastener holes do not become the next leak.

The collar, the storm collar and the listed pipe

The collar is the raised ring around the pipe opening, and it keeps water on the lid from running down the pipe into the chase. The storm collar above it is a band clamped to the pipe that sheds water off the pipe and over the collar.

Both are sized to the listed chimney pipe, following the chimney maker's instructions. A collar cut too large leaves a gap that sealant has to fill, and sealant ages. Reading the pipe size from the model plate or the pipe itself is part of the measuring visit.

What is under the old cover

A rusted, ponding cover has been dripping into the chase, sometimes for years. When it comes off, the top plate and framing below can be soft, and the sheathing and siding at the top of the chase can be stained or rotted.

Rotted framing is repaired before a new cover goes on, because the cover fastens to it. That framing and siding work is chimney chase repair, a separate line, with the siding assigned to whichever trade will handle it.

Water in the chase can also reach the fireplace below. Rust on the top of the firebox, a stain on the ceiling near the unit, or a musty smell from the fireplace can each trace back to a failed cover, so the water is followed down to see how far it went.

Chase cover jobs from plain to involved

From the plainest job to the most involved, with the roof held equal:

• A stainless cover with cross-breaks for one pipe, on a sound chase.
• The same cover on a larger chase, or with two collars.
• A heavier-gauge or sloped cover for a large chase.
• A copper cover, formed to measure.
• Any cover plus repair of rotted top framing or sheathing.
• A cover job that also replaces a rusted termination cap or storm collar, using parts listed for that chimney.

Height, staging and the measuring visit

Height and roof access add time to every job on that list. A chase on a two-story wall needs more staging than one rising from a low roof, and a chase beside a steep tile roof takes more care to reach.

Because the cover is made to fit, the job runs in two steps. The first visit measures the chase, the pipe and its position, and the skirt depth. The second visit, after fabrication, removes the old cover and sets the new one.

Comparing two chase cover quotes

Chase cover replacement cost compares fairly only when two quotes cover the same work. Ask for the metal and the gauge, how the cover sheds water, how the collar is sized, whether a new storm collar is included, and what happens if the framing under the old cover is soft. Ask too whether the termination cap is being reused. If it is being replaced, ask whether the new one is listed for that chimney, since a cap from a different system can change how the listed chimney performs.