Important Safety and Code Notice: This guide provides general installation information only. Always follow the chimney system manufacturer's listed installation instructions, the connector pipe manufacturer's instructions, and local building and fire codes. Where differences exist, manufacturer instructions and local code requirements take precedence over this guide.
Scope: This guide covers a single-story home (one ground floor + one attic space) where the chimney passes through one ceiling into the attic and exits through the roof. For multi-story installations, see our guide on chimney installation in a multi-story house.
System Overview
The through-the-attic system uses two distinct pipe types:
- Connector pipe (black stovepipe) inside the living area, from the stove up to the ceiling transition.
- UL 103HT listed Class A insulated chimney pipe from the ceiling transition through the attic and above the roofline.
The transition and vertical load support are provided by a ceiling support box.
Required Components
A complete "Through-the-Attic Kit" typically includes:
- Stove Pipe Adapter: transitions from connector pipe to Class A chimney.
- Class A Insulated Chimney Pipe (UL 103HT): the chimney system through the attic and above the roof.
- Ceiling Support Box (11" / 24" / 36"): supports chimney weight and acts as a firestop at the ceiling transition.
- Attic Insulation Shield: keeps insulation and debris away from the chimney in the attic.
- Roof Flashing and Storm Collar: weatherproof sealing at the roof penetration.
- Chimney Cap with Spark Screen: termination and basic spark/animal protection.
- Telescoping Connector Pipe: adjustable interior black pipe section for easier fit.
Exact parts and requirements depend on the chimney manufacturer's listing and your local code/inspector (AHJ). View the complete chimney kit for this installation.
Ceiling Support Box Selection (11" / 24" / 36")
Support box length is chosen by the total thickness of the ceiling structure, not by the roof pitch. The ceiling support box must pass completely through the ceiling structure and extend at least 2 inches below the finished ceiling.
| Support Box Size | Use When | Ceiling Type |
|---|---|---|
| 11" Support Box | Total ceiling thickness is 9" or less | Flat ceilings only |
| 24" Support Box | Ceiling thicker than 9", or vaulted/cathedral up to 8/12 pitch | Flat or vaulted |
| 36" Support Box | Steeper than 8/12 pitch, or deep construction | Steep vaulted/cathedral |
Common mistake: Modern ceiling assemblies can exceed 11" total thickness. For example: 2x10 joist (actual ~9.25") + 2x4 nailer (~1.5") + 1/2" drywall (~0.5") = ~11.25" total. An 11" box is too short in that case, and a 24" box is required. When in doubt, choose the longer support box - many models can be trimmed per manufacturer instructions.
How to Measure Total Ceiling Thickness
Measure the total thickness from the finished ceiling surface to the top of the ceiling structure. Include all layers:
- Actual height of ceiling joists or rafters
- Drywall or other ceiling finish
- Furring strips, nailers, or added framing
- Attic floor sheathing (if present)
Do not rely on nominal lumber sizes (e.g., "2x10"). Always measure actual thickness.
Installation Steps
- Plan the layout and mark the centerline. Position the stove per the stove manual's clearance requirements. Mark the flue centerline at the stove collar, then transfer that center point to the ceiling using a plumb bob or laser. Drill a small pilot hole to confirm the path is clear in the attic.
- Cut the ceiling opening and frame it correctly. Cut the ceiling opening to the support box manufacturer's specified dimensions. If structural members must be altered, reframe the opening with headers/trimmers as required by code.
- Install the ceiling support box. Install per the manufacturer instructions. Verify it passes through the entire ceiling structure and extends at least 2 inches below the finished ceiling. Secure it to framing as specified.
- Connect to Class A chimney pipe. At the ceiling transition, use the correct adapter to switch from connector pipe to UL 103HT listed Class A chimney. Attach the first Class A section to the support box and lock it in place per the brand's locking method.
- Install the attic insulation shield. Install the shield around the chimney where it passes through the attic floor area. It must stand upright and keep insulation and debris away from the chimney.
- Maintain required clearances in the attic. Keep a minimum 2-inch air clearance between Class A chimney pipe and any combustible materials (wood framing, stored items), unless your specific chimney listing states otherwise.
- Cut the roof opening and install flashing. Transfer the chimney centerline to the roof and cut the opening per the chimney/flashing requirements. Install roof flashing matched to the roof pitch and roofing type. Install the storm collar above the flashing and seal with high-temp silicone. For help measuring your roof angle, see our roof pitch measurement guide.
- Set termination height (3-2-10 rule). The chimney must terminate at least 3 feet above the point where it exits the roof, and at least 2 feet higher than any part of the building within 10 feet horizontally. Install the chimney cap with spark screen. For more on this rule, see our article on required chimney height above the roof.
- Final checks. Verify all locking bands/fasteners, confirm clearances, confirm support-box extension below ceiling, and confirm weather-tight sealing at the roof. Obtain inspection/approval if required by your local authority (AHJ).
Safety Clearances - Quick Reference
| Component | Typical Clearance to Combustibles |
|---|---|
| Single-wall connector pipe | 18 inches (verify the connector listing) |
| Double-wall connector pipe | Reduced clearance per manufacturer listing |
| Class A chimney (UL 103HT) | 2 inches (verify the chimney listing) |
Never reduce clearances unless the product listing and manufacturer instructions explicitly allow it. For more about how support and clearance components work together, see our chimney support systems guide.
Common Mistakes to Avoid
- Choosing an 11" box for thick ceilings: always measure total thickness and confirm 2" extension below the finished ceiling.
- Missing the attic insulation shield: insulation or debris must not contact the chimney.
- Clearance violations: do not allow framing or storage within the chimney clearance zone.
- Ignoring termination height: follow the 3-2-10 rule for proper draft and safety.
- Mixing non-listed parts: do not mix chimney parts across brands unless explicitly allowed by the manufacturer.
Ready to Order Your Through-the-Attic Kit?
A complete Class A chimney kit for this installation type includes all required components for a safe and code-compliant setup. Use our chimney kit configurator to match the right parts to your layout, or take advantage of our free chimney design service for a custom recommendation within 24 hours. You can also call us at 888-8340-880 (9 AM - 9 PM, 7 days a week).
FAQ: Through-the-Attic Chimney Installation
What size ceiling support box do I need?
Measure your total ceiling thickness including joists, drywall, nailers, and any sheathing. Use an 11" box for ceilings 9" or less, a 24" box for thicker flat ceilings or vaulted ceilings up to 8/12 pitch, and a 36" box for steeper pitches or deep construction.
Can I use stove pipe through the attic?
No. Stove pipe (connector pipe) is only for use inside the room. Through any ceiling, attic, or roof penetration, you must use UL 103HT listed Class A insulated chimney pipe.
Do I need an attic insulation shield?
Yes. Whenever the chimney passes through or near insulation in an attic, an insulation shield is required to keep insulation and debris at a safe distance from the chimney pipe.
How high does the chimney need to extend above the roof?
Follow the 3-2-10 rule: at least 3 feet above the roof penetration point, and at least 2 feet higher than any part of the building within 10 feet horizontally. See our full guide on chimney height requirements.
What if my ceiling is thicker than 11 inches?
Use a 24" or 36" ceiling support box. Many modern ceiling assemblies with deeper joists and additional framing layers exceed 11" total. Choosing a box that is too short is one of the most common reasons for returns.
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