Overview
If you’ve ever seen a tall, siding-covered box running up the outside of a house with a chimney cap sticking out of the top, you’re probably looking at a chimney chase.
A chimney chase is essentially a framed enclosure built around a chimney system. It can protect an exterior chimney from the elements, give the installation a more finished appearance, and help shelter the chimney from wind and weather.
What it does not do is change how the chimney itself needs to be installed.
The Class A chimney pipe inside the chase still needs the proper clearance from combustible materials, listed supports and firestops, and the correct termination height above the roof.
For Shasta Vent All-Fuel HT Chimney, that means maintaining at least 2 inches of clearance to combustible materials around the chimney, including inside a chase.
Here’s what homeowners and installers should know before building one.
What Is a Chimney Chase?
A chimney chase is a framed structure built around chimney pipe, most commonly along the exterior wall of a home.
Think of the chimney and the chase as two separate things.
The chimney is the listed pipe system carrying exhaust safely out of the home.
The chase is the structure built around it.
That distinction is important because enclosing Class A chimney pipe inside a chase doesn’t eliminate or reduce any of the chimney manufacturer’s installation requirements.
If you’re still figuring out how your chimney should exit the house, start with our complete guide to wood stove chimney pipe sizes, clearances, and kits.
Why Build a Chimney Chase?
You don’t necessarily need a chase for an exterior Class A chimney. Listed chimney systems can be installed exposed on the outside of a home when properly supported.
But there are several reasons homeowners choose to build one.
Weather Protection
An exterior chimney is constantly exposed to rain, snow, ice, wind, sunlight, and changing temperatures.
A properly built chase gives the chimney another layer of protection from those conditions and can help protect joints, locking bands, and exterior components over the life of the system.
The chase top also creates a defined location for managing water around the chimney penetration.
Protection From Wind
Exterior chimney systems can be exposed to cold winter winds, which can contribute to heat loss through the chimney wall.
A chase helps shelter the chimney from direct wind exposure.
It’s important not to confuse that with insulating the chimney itself, though. The chimney still needs to be a properly insulated, listed system.
Shasta Vent All-Fuel HT Chimney uses a solid-filled, double-wall design with high-temperature ceramic blanket insulation between two walls of .020-inch Type 304 stainless steel.
The insulation inside the chimney is what helps maintain flue temperatures. The chase simply provides additional shelter around the system.
A More Finished Appearance
This is probably the most obvious reason people build chimney chases.
Instead of having stainless chimney pipe running several feet up the side of the house, the chase can be finished with siding, trim, stone, or other materials that complement the home.
For homeowners who don’t love the appearance of an exposed exterior chimney, a chase can make the installation look much more intentional.
Chimney Chase Clearance Requirements
Clearance is one of the most important considerations when building a chimney chase.
Shasta Vent Class A chimney pipe requires 2 inches of air space between the outside of the chimney and combustible materials.
That clearance needs to remain around the chimney for the full height of the chase.
The framing, sheathing, insulation, blocking, wiring, plumbing, or other combustible materials cannot enter that required clearance zone.
Shasta Vent All-Fuel HT Chimney is ETL listed by Intertek to the HT requirements of UL 103 and tested at:
- 1,000°F continuously
- 1,400°F for one hour
- Three 10-minute cycles at 2,100°F
That final test is designed to simulate the extreme temperatures associated with a chimney fire.
The takeaway for a chase installation is simple: design the chase around the chimney’s required clearance, not the other way around.
How Big Should a Chimney Chase Be?
The required interior size of a chimney chase depends on the outside diameter of the chimney.
Shasta Vent chimney pipe has approximately a 1-inch wall thickness, which means the outside diameter is 2 inches larger than the flue diameter.
Once you add the required 2-inch clearance on every side, you get the minimum clear interior dimensions.
6-Inch Chimney
A 6-inch Shasta Vent chimney has an outside diameter of approximately 8 inches.
Add 2 inches of clearance on both sides:
8" chimney OD + 2" + 2" = 12" minimum clear interior
That means you need at least approximately 12 inches of clear interior space in both directions.
8-Inch Chimney
An 8-inch chimney has an outside diameter of approximately 10 inches.
Using the same calculation:
10" chimney OD + 2" + 2" = 14" minimum clear interior
So an 8-inch chimney requires approximately 14 inches of clear interior space in both directions.
Those numbers represent minimum clearance, not necessarily the ideal size to frame.
Giving yourself additional room makes installation easier and provides some margin for framing irregularities, chimney joints, and minor variations in how the chimney runs through the chase.
Always confirm dimensions and clearances against the installation instructions for your specific chimney system before framing.
Do Chimney Chases Need Firestops?
Yes. Firestopping is an important part of a chimney chase installation.
When a chase passes through different floors, ceilings, or framing levels, the chimney system needs the appropriate listed firestop components.
Without firestopping, a tall open chase can create a continuous vertical pathway through the structure. Firestops help separate those spaces at each level.
Where the chimney passes through a ceiling, a listed ceiling support box may also serve as part of the required support and firestopping system.
We break down support boxes, firestop spacers, insulation shields, and other components in our chimney pipe components guide.
What About Insulation?
Insulation should never be packed around Class A chimney pipe or allowed to enter the required clearance space.
If insulation is present in or near the chase, the appropriate listed insulation shield should be used to keep it away from the chimney.
This is particularly important with loose-fill attic insulation, which can shift or migrate over time.
Chimney Chase Tops: Where Leaks Often Start
If you’re building a chase, pay close attention to the top.
The chimney chase top, sometimes called a chase cover or chase pan, is the metal cover that sits over the top of the framed enclosure.
This is one area where poor installation can eventually turn into a water problem.
A good chase top needs to handle three things well.
Proper Slope
Water shouldn’t be allowed to sit on top of the chase.
The pan should be designed with enough slope to direct rain and melting snow away from the chimney and off the chase.
The Pipe Penetration
The chimney passes through an opening in the chase top. That opening needs to be properly designed and sealed according to the chimney system requirements.
Simply cutting a hole and relying on a large bead of sealant is not a good long-term strategy.
Storm Collar and Chimney Cap
Above the chase top, the storm collar helps direct water away from the chimney penetration.
The chimney cap then completes the top of the system.
These components are worth inspecting periodically, especially before winter.
Material matters here too. Stainless steel generally provides better long-term corrosion resistance than lower-grade alternatives, particularly in wet, coastal, or freeze-thaw environments.
Our guide to 304 vs. 430 stainless steel chimney pipe explains the difference in more detail.
Does a Chimney Chase Change the 3-2-10 Rule?
No.
Your chimney still needs to meet the proper termination height even when it runs through a chase.
Under the commonly referenced 3-2-10 chimney rule, the chimney should terminate:
- At least 3 feet above the point where it penetrates the roof
- At least 2 feet higher than anything within 10 feet horizontally
You can read our full homeowner-friendly explanation of the 3-2-10 rule for chimneys.
It’s smart to calculate the final chimney height before deciding how tall to build the chase.
Otherwise, you can end up with a beautifully finished chase only to discover that several additional feet of chimney need to extend above it.
Our chimney height calculator can help you estimate the required height before installation.
Tall sections extending beyond the chase may also require additional support or bracing.
The same applies to offsets inside the chase. Shasta Vent's listed limits for elbows and offsets still apply when the chimney is enclosed. See our guide to how many elbows a chimney can have for more information.
How to Build a Chimney Chase
Every home and chimney installation is different, so this shouldn’t be treated as a substitute for installation instructions or local building requirements.
But the general planning sequence looks something like this:
- Determine the correct chimney size. Your wood stove or appliance determines the required flue diameter. Our guide to what size chimney pipe you need is a good starting point.
- Calculate the required chimney height. Use the 3-2-10 rule and account for the roofline and anything nearby.
- Check local permit requirements. A chase is part of the structure of the home, so local building requirements may apply.
- Plan the chimney penetration and supports. For a through-the-wall installation, this can include the wall thimble, tee, tee cap, and tee support. See our Class A chimney installation guide.
- Frame the chase around the chimney clearance. Maintain the required air space around the chimney throughout the enclosure.
- Install the required firestops. These should be incorporated as the chase passes through each applicable level.
- Use insulation shields where necessary. Keep insulation completely outside the chimney's required clearance zone.
- Sheathe and finish the exterior. Weatherproof the chase and finish it with siding or another suitable exterior material.
- Install the chase top. Make sure it is properly sloped and detailed around the chimney penetration.
- Install the storm collar and chimney cap. Finally, verify that the completed chimney still meets the required termination height.
If you’re building a chase around an existing chimney, inspect the chimney before enclosing it.
Look for corrosion, damaged components, loose joints, or other signs that the system may need service or replacement.
Once the chase is finished, those components become much harder to see and access.
How Much Does a Chimney Chase Cost?
There isn't a particularly useful one-size-fits-all price for building a chimney chase.
A short, one-story chase finished in basic siding is a very different project from a two-story chase requiring custom flashing, scaffolding, structural work, and matching stone veneer.
The biggest cost factors typically include:
Height. Taller chases require more material, labor, and often more complicated access.
Structural support. Depending on the design, the chase may need its own footing, ledger, or another structural solution.
Exterior finish. Standard siding may be relatively straightforward. Matching discontinued siding, brick, stone, or custom trim can add considerably more cost.
Chase top. A properly fabricated stainless steel chase top costs more than a basic galvanized cover, but it can offer better long-term corrosion resistance.
Permits and inspections. Requirements and fees vary considerably by location.
If you're hiring the project out, get multiple local quotes and make sure the chase top material and construction are actually included in the proposal.
Frequently Asked Questions About Chimney Chases
What is a chimney chase?
A chimney chase is a framed enclosure built around chimney pipe, usually on the exterior of a home. It protects and conceals the chimney while allowing the exterior to be finished to match the house.
Does a chimney chase change chimney clearance requirements?
No. The chimney still needs to maintain the clearance required by its listing and manufacturer.
Shasta Vent All-Fuel HT Chimney requires 2 inches of clearance from combustible materials, including when installed inside a chase.
How much clearance does Class A chimney pipe need inside a chase?
For Shasta Vent All-Fuel HT Chimney, maintain at least 2 inches of air space between the outside of the chimney and combustible materials on all sides.
Always follow the installation instructions for your specific chimney system.
Do chimney chases need firestops?
Yes, appropriate listed firestop components are generally required where the chimney chase passes through floors, ceilings, and other framing transitions.
Check your chimney installation instructions and local requirements for your specific installation.
Is a chimney chase required for exterior chimney pipe?
No.
Listed Class A chimney systems can generally be installed exposed on an exterior wall using the appropriate supports and brackets.
A chase is primarily added for weather protection, shelter, and appearance.
How big should a chimney chase be?
The chase needs enough interior space to accommodate the outside diameter of the chimney plus the manufacturer's required clearance on every side.
For Shasta Vent, that works out to approximately 12 inches of minimum clear interior space for a 6-inch chimney and 14 inches for an 8-inch chimney.
Many installations provide additional space beyond the minimum to make installation and maintenance easier.
Can you insulate a chimney chase?
You cannot pack insulation around the chimney or allow it into the required clearance zone.
If insulation is present near the chimney, use the listed components required to prevent it from contacting the chimney.
Why is my chimney chase leaking?
The chase top is one of the first places to inspect.
Look at the slope of the chase cover, the area where the chimney passes through it, and the storm collar above the penetration.
Water intrusion around any of these areas can eventually make its way into the chase.
Can you build a chase around an existing chimney?
Yes, but inspect the chimney first.
Check its condition, joints, locking bands, exterior wall, supports, and other components before enclosing everything behind a finished chase.
The Bottom Line
A chimney chase can be a great addition to an exterior chimney installation. It protects the system from direct exposure, gives the home a cleaner finished appearance, and can help shelter an exterior chimney from wind and weather.
But the chase is still just the structure around the chimney.
The chimney itself needs to be installed exactly as its listing and manufacturer require, including proper clearances, firestops, supports, insulation shields, and termination height.
For Shasta Vent All-Fuel HT Chimney, one of the most important numbers to remember is 2 inches. Maintain at least 2 inches of clearance between the outside of the chimney and combustible materials throughout the chase.
And before framing anything, check your installation instructions and local requirements. A little extra planning before the chase goes up is much easier than opening it back up later.
Disclaimer
This article is based on our experience with the Shasta Vent All-Fuel HT Chimney system and is intended for general educational purposes.
Chimney chase requirements can vary based on the chimney system, home construction, local building codes, snow loads, wind exposure, and other site-specific factors.
Always follow the installation instructions supplied with your chimney system and verify applicable requirements with your local building authority. NFPA 211 may also apply to your installation.
For structural work, roof penetrations, or installations you're not comfortable completing yourself, work with a qualified chimney professional, installer, or contractor.
