Chimney Chase Cover Replacement: How to Measure, Which Material to Choose and How to Order

If you have noticed rust streaks on the siding of your chimney, water in the firebox after a storm, or a stain on the ceiling near the fireplace, there is a good chance the metal lid on top of your chimney chase has rusted through. Replacing a chimney chase cover is one of the most cost-effective repairs a homeowner can make, and with the right measurements it is a straightforward job. This guide walks you through how to tell whether yours needs replacing, how to measure for a new one, which material to choose, and what to expect when you order a custom cover.

What a chase cover is (and why the original one failed)

A chimney chase is the framed, siding- or stucco-wrapped box that houses a factory-built fireplace’s metal chimney pipe. Because it is not masonry, it cannot have a concrete crown; instead a flat metal pan called a chase cover (also called a chase top or chase cap) seals the top, with one or more collars where the chimney pipes pass through.

Most chases built in the 1980s–2000s were topped with a galvanized steel cover. Galvanized steel is inexpensive, but once the zinc coating weathers the steel underneath rusts, usually starting at the low points where water pools. Within a decade or two you see the classic signs:

• Orange-brown rust streaks running down the siding below the top of the chase.
• Water or rust-coloured drips inside the firebox or on the fireplace’s metal face.
• A sagging, dished or visibly rusted top when you look at it from the ground or a ladder.
• Soft framing, mould or a stained ceiling near the chimney.

Patching a rusted cover with sealant buys a season at best. A new cover in stainless steel or copper solves the problem for the life of the house.

How to measure for a replacement chase cover

A custom chase cover is built from four numbers plus a sketch. Take them with a tape measure from the top of the chase (or have your chimney sweep or roofer take them during a service visit):

1. Length and width of the chase top. Measure the outside dimensions of the chase at the very top, where the cover sits, in inches. Measure each dimension at two points and use the larger number. If the siding sits slightly proud of the framing, measure to the outside of the siding — the cover’s skirt must drop over it.
2. Skirt (drop) height. The skirt is the vertical edge that hangs down over the siding; 3 to 4 inches is typical. Our standard cover uses 3.5-inch sides.
3. Flue pipe diameter and position. Measure the outside diameter of each chimney pipe that comes through the top and note where it sits relative to the edges. If there is only one pipe and it is centred, say so; for anything else, a sketch or a photo with the tape in it is the surest way to get the collar in the right place.
4. Collar height. The collar is the raised ring around each pipe that keeps water from running down the pipe into the chase. Note how tall the existing collar is, or how much pipe is exposed above the cover.

Tip: if your siding is tight or slightly out of square, order the cover a quarter inch larger in each direction. Our configurator has an “add 1/4 inch to length and width” option for exactly this reason.

Which material should you choose?

Four materials cover almost every replacement:

• Stainless steel (24 gauge) — the most popular choice. It will not rust, it is less costly than copper, and from the ground it is nearly invisible. If you are not sure, choose stainless.
• Copper (16 or 20 oz.) — the premium option for homes with copper flashing, gutters or accents. Bright at first, it weathers to brown and then to a soft green patina. Lead-coated copper and Freedom Gray are pre-weathered alternatives.
• Kynar-coated aluminum or steel — a factory colour finish (black, dark bronze, medium bronze, charcoal, almond, bone white, copper penny) when you want the cover to blend with a roof or trim colour.
• Galvanized or Galvalume steel — the budget option. It costs the least and lasts the shortest; if you are already replacing a rusted galvanized cover, it is worth stepping up to stainless.

A cross-break — a shallow X-shaped bend across the top — adds rigidity and helps water run to the edges instead of pooling. Covers that need more than one sheet of metal are joined with a locked seam, and copper and stainless covers can be soldered at the corners for a fully sealed top.

Chase cover or chimney cap? Usually both

The chase cover seals the top of the box; the chimney cap sits on top of the pipe that passes through the cover and keeps rain, animals and sparks out of the flue itself. If your cap is a rusted “mushroom” cap, replace it at the same time — the cover and cap are often ordered together, and the collar height on the new cover should be sized so the cap still fits over the exposed pipe. See our chimney caps, dampers and chase covers collection for matching caps.

Installing a chase cover

First Response Fireplace Products fabricates and ships the cover direct to you; we do not install. The good news is that a chase cover is one of the simplest chimney parts to replace. A chimney sweep, roofer or capable homeowner working safely on the roof will typically:

1. Remove the old cap, cut any sealant and lift off the old cover.
2. Check the framing underneath and replace any rotten wood.
3. Set the new cover in place, run a bead of high-temperature, exterior-grade sealant around the collar and the top edge of the siding, and fasten the skirt with exterior screws.
4. Refit the chimney cap and storm collar.

The whole job usually takes under an hour once the cover is on site.

How to order a custom chase cover

Enter your measurements and material in the configurator on our custom chimney chase cover page, upload a sketch or photo if you have more than one flue, and click Request My Quote. No payment is taken at that point — a fireplace specialist checks the dimensions, emails you a firm price and lead time, and the cover is laser-cut to order and delivered to your door once you approve it. Not sure about a measurement? Call (540) 212-9374, Monday to Friday 9–5 Eastern, and we will walk through it with you.

Frequently asked questions

How long does a stainless steel chase cover last?
Stainless does not rust, so a properly installed stainless cover should outlast the chase itself. Copper lasts just as long and simply changes colour.

Can I reuse my old chimney cap?
If it is stainless and in good condition, yes. If it is galvanized and showing rust, replace it with the cover so you are not back on the roof next year.

What if my chase top is not square?
Give us the longest measurement in each direction and add the quarter-inch allowance. For unusual shapes, send a sketch and we will quote it.

How do I know the flue hole is in the right place?
For a single centred flue, no sketch is needed. For an off-centre or multiple flues, a quick top-down drawing with the distances from two edges to the centre of each pipe is all the fabricator needs.