Every Class A chimney installed inside a house needs a ceiling support box where the stove pipe connects to the insulated chimney pipe. Additional components - attic insulation shields, floor/ceiling radiation shields, and roof support brackets - depend on your ceiling type and how many floors the chimney passes through. This guide covers all five installation scenarios so you pick the right parts the first time.
Table of Contents
- Direct Through Roof (No Attic)
- With an Attic
- Multi-Story House
- Support Components Comparison Table
- Quick Summary: When Do You Need Each Component?
- Need Help Choosing?
- FAQ
How Do You Support a Class A Chimney Through the Roof Without an Attic?
A) Flat Ceiling (Room to Roof)
When your room has a flat, horizontal ceiling with nothing above it but the roof, you need the simplest setup:
- Ceiling Support Box 11" - installed at the flat ceiling opening. This box carries the full weight of the chimney and transfers the load to the ceiling joists. It also acts as a firestop, maintaining the required 2-inch clearance to combustibles per NFPA 211.
- Roof Flashing + Storm Collar + Chimney Cap - at the roof penetration. The flashing seals the opening, the storm collar prevents rain from running down the pipe, and the cap keeps out rain, debris, and animals.
- Optional: Exterior Brace - required when the chimney extends more than 5 feet above the roofline. The brace prevents wind-related movement.
You do not need an attic insulation shield in this scenario because there is no attic space.
B) Cathedral Ceiling (Ceiling Follows Roof Pitch)
A cathedral ceiling slopes at the same angle as the roof and has no attic space above it. This changes the support requirements:
- Ceiling Support Box 24" or 36" - the longer box is needed because it must be trimmed to match the roof pitch. The angled cut lets the box sit flush with the sloped ceiling while still supporting the chimney's weight.
- Roof Flashing + Storm Collar + Cap - same as the flat ceiling setup.
- Alternative: Roof Support Bracket + Roof Radiation Shield - in A-frame homes or when you prefer not to use a ceiling support box. The bracket mounts at the roofline and the radiation shield provides the firestop.
- Optional: Exterior Brace - for chimneys extending more than 5 feet above the roof.
What Extra Components Do You Need When the Chimney Passes Through an Attic?
A) Flat Ceiling with Attic Above
When your room has a flat ceiling with an attic above, you add one component compared to the no-attic scenario:
- Ceiling Support Box 11" - at the room ceiling, same as before.
- Attic Insulation Shield - always required when the chimney passes through an attic. This shield maintains the 2-inch clearance required by NFPA 211, keeping insulation material and electrical wiring safely away from the hot chimney pipe.
- Roof Flashing + Storm Collar + Cap - at the roof exit.
- Optional: Exterior Brace - for tall chimney stacks above the roofline.
B) Vaulted Ceiling with Attic Space Above
A vaulted ceiling is pitched (angled), but unlike a cathedral ceiling, it has an attic space between the ceiling and the roof. This setup needs:
- Ceiling Support Box 24" or 36" - installed in the pitched ceiling. The longer model gets trimmed to fit the slope.
- Attic Insulation Shield - in the attic above, separating insulation from the pipe.
- Roof Flashing + Storm Collar + Cap - at the roof.
You do not need a roof support bracket in this case. The ceiling support box handles the load, and the attic insulation shield provides the clearance above.
How Do You Install a Class A Chimney Through a Two-Story House?
A two-story chimney installation is the most complex inside-the-house scenario. The chimney starts at the ground floor stove, passes through the second floor, through the attic, and out the roof. Here is what you need:
- Ceiling Support Box 11" - installed at the first ceiling above the stove (ground floor to second floor transition). This box carries the full weight of the entire chimney stack above it.
- Floor/Ceiling Radiation Shield - at the second floor to attic penetration. This shield maintains safe clearance where the chimney passes between conditioned living spaces. It is not the same as an attic insulation shield - the radiation shield is designed for floor-to-floor transitions.
- Attic Insulation Shield - in the attic, keeping insulation away from the chimney pipe.
- Roof Flashing + Storm Collar + Cap - at the roof exit.
- Wall Straps - used to secure the chimney pipe to the framing as it passes through the second floor for stability.
- Optional: Exterior Brace - if the chimney extends more than 5 feet above the roof.
For a complete walkthrough of the two-story installation process, see our two-story chimney installation guide.
Support Components Comparison Table
| Component | Cathedral Ceiling (No Attic) | Flat Ceiling (No Attic) | Vaulted Ceiling (With Attic) | Flat Ceiling (With Attic) | Multi-Story House |
|---|---|---|---|---|---|
| Ceiling Support Box 11" | - | Yes | - | Yes | Yes |
| Ceiling Support Box 24"/36" | Yes | - | Yes | - | - |
| Attic Insulation Shield | - | - | Yes | Yes | Yes |
| Floor/Ceiling Radiation Shield | - | - | - | - | Yes |
| Roof Support Bracket | Alternative | - | - | - | - |
| Roof Radiation Shield | With bracket only | - | - | - | - |
| Roof Flashing + Storm Collar + Cap | Yes | Yes | Yes | Yes | Yes |
| Exterior Brace (if >5 ft above roof) | Optional | Optional | Optional | Optional | Optional |
Quick Summary: When Do You Need Each Component?
Ceiling Support Box: Almost every inside chimney installation needs one. The 11" model fits flat ceilings. The 24" or 36" model fits pitched (vaulted or cathedral) ceilings. The box carries the chimney's weight and acts as a firestop at the ceiling penetration.
Attic Insulation Shield: Required any time the chimney passes through an attic - whether you have a flat ceiling, a vaulted ceiling, or a multi-story house. The shield keeps insulation and wiring at the required 2-inch clearance from the pipe, per NFPA 211.
Floor/Ceiling Radiation Shield: Only needed in multi-story homes where the chimney passes between conditioned floors (like ground floor to second floor). Not the same as an attic insulation shield.
Roof Support Bracket: An alternative for cathedral and A-frame ceilings when you do not want to use a ceiling support box. Always paired with a roof radiation shield.
Roof Flashing, Storm Collar, and Chimney Cap: Every chimney installation needs these at the roof exit - no exceptions.
All support components must be UL 103 HT listed and from the same manufacturer as your Class A insulated chimney pipe. Mixing brands voids the UL listing and violates NFPA 211.
Want to see how the support box connects to the stove? Read our through-the-attic installation guide for a step-by-step walkthrough. For cathedral ceiling builds, check our cathedral ceiling chimney installation guide.
Not Sure Which Support Components You Need?
Every house is different, and choosing the wrong ceiling support box or skipping an insulation shield can create a fire hazard. Use our free chimney design service - send us your measurements and photos, and we will send you a custom parts list within 24 hours. Or call us at 888-834-0880 (9 AM - 9 PM, 7 days a week) to talk it through with a chimney expert.
Browse our full range of complete chimney kits - every kit includes the right support box, insulation shields, flashing, and cap for your installation type.
Frequently Asked Questions
What is the difference between a flat ceiling, a vaulted ceiling, and a cathedral ceiling?
A flat ceiling is horizontal and may have an attic above. A vaulted ceiling is pitched but still has an attic space between the ceiling and the roof. A cathedral ceiling follows the roof pitch with no attic space at all. Each type requires a different ceiling support box size and may or may not need an attic insulation shield.
Do I always need a ceiling support box for an inside chimney?
Yes, in almost every case. The ceiling support box transfers the chimney's full weight to structural framing and acts as a firestop at the ceiling penetration. The only exception is a cathedral ceiling where you use a roof support bracket instead.
Can I skip the attic insulation shield if my attic has no insulation?
No. Even if your attic is currently uninsulated, building codes require the shield because insulation may be added later. The shield also keeps wiring and stored items at the required 2-inch clearance from the chimney pipe per NFPA 211.
What is the difference between a floor/ceiling radiation shield and an attic insulation shield?
A floor/ceiling radiation shield is used between conditioned living floors (like ground floor to second floor). An attic insulation shield is used where the chimney enters the attic space. They serve different purposes and are not interchangeable.
How tall can a chimney extend above the roof before it needs an exterior brace?
Most manufacturers and local building codes require an exterior brace when the chimney extends more than 5 feet above the roofline. The brace prevents wind loads from causing movement. Check your pipe manufacturer's installation manual for the exact requirement - some specify bracing at shorter heights.
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