Measure the horizontal distance between the centers of your two elbow positions, then use an offset chart to find the rise and connector pipe length. For Class A chimney pipe (UL 103 HT listed), you can choose between 15-degree and 30-degree elbow kits. Per NFPA 211, the connector between elbows must not exceed 72 inches, and you are limited to two offsets (four elbows) per chimney system.
A straight, vertical chimney always delivers the best draft and the highest safety margin. But in many homes, obstructions like rafters, ridgepoles, or ceiling joists block a straight path from the stove to the roof.
In those cases, you need a chimney offset - a combination of two elbows and a connecting pipe - to shift the chimney horizontally. This guide from Chimney & Stove Co. LLC explains exactly how to measure and configure an offset that fits your layout and meets NFPA 211 building code requirements.
When Do You Need a Chimney Pipe Offset?
You need an elbow offset any time a structural element blocks a straight vertical chimney path. Common obstructions include:
- Rafters or ridgepoles in the attic
- Wall framing studs
- Roof overhangs or eaves
- Structural floor joists in a multi-story installation
If your chimney runs through the attic, an offset around a rafter is one of the most common scenarios. For cathedral ceilings without attic space, see our cathedral ceiling installation guide.
What Are the Parts of a Chimney Offset?
Every chimney offset consists of three components:
- Offset elbow - the lower elbow that starts the horizontal shift
- Return elbow - the upper elbow that brings the chimney back to vertical
- Connector pipe - the Class A chimney pipe section between the two elbows
How to Measure Your Chimney Offset - Step by Step
You only need one measurement to plan your offset. Here is how to take it:
- Stand at the point where your chimney will exit the stove or adapter.
- Look straight up and identify the obstruction (rafter, joist, stud).
- Measure the horizontal distance from the center of the flue exit to the center of the clear space past the obstruction.
- Keep your tape measure level - do not measure along a slope.
- Write down this number. This is your offset distance.
Once you have the offset distance, use the elbow offset chart below to find the matching rise and connector pipe length.
Elbow Offset Chart - 15-Degree vs 30-Degree
This chart shows the relationship between offset distance, connector pipe length, and total rise for both 15-degree and 30-degree Class A chimney pipe elbow kits. All measurements are in inches.
| Offset (inches) | Elbow Angle | Connector Length (inches) | Total Rise (inches) |
|---|---|---|---|
| 6 | 15-degree | 18 | 31 |
| 6 | 30-degree | 6 | 19 |
| 11 | 15-degree | 36 | 50 |
| 12 | 30-degree | 18 | 30 |
| 16 | 15-degree | 54 | 68 |
| 18 | 30-degree | 30 | 41 |
| 22 | 15-degree | 72 | 86 |
| 24 | 30-degree | 42 | 52 |
How to read this chart: Find your offset measurement in the left column. Then compare the 15-degree and 30-degree options. A 30-degree elbow gives you a shorter rise and shorter connector, but shifts the chimney more per inch of pipe. A 15-degree elbow is gentler and better for draft, but needs more vertical space.
15-Degree vs 30-Degree Elbows - Which Should You Choose?
Both angles work with UL 103 HT listed Class A chimney pipe systems, but each has trade-offs:
| Factor | 15-Degree Elbow | 30-Degree Elbow |
|---|---|---|
| Draft performance | Better - less restriction | Good - slightly more restriction |
| Vertical space needed | More - higher total rise | Less - lower total rise |
| Connector pipe length | Longer | Shorter |
| Best for | Tall attics, maximum draft | Low ceilings, tight spaces |
If you have plenty of vertical room (like a tall attic), go with 15-degree elbows for the strongest draft. If vertical space is tight, 30-degree elbows keep the total rise lower.
Offset Installation Rules
Always use the smallest offset that provides the required clearance. A straight, vertical chimney provides the best draft and is preferred whenever possible.
- Use only compatible 15-degree or 30-degree Class A chimney elbows.
- Do not exceed 30 degrees from vertical.
- Do not combine a 15-degree elbow with a 30-degree elbow to create a 45-degree offset.
- Use no more than two chimney pipe sections between the elbows of a single offset.
- The total chimney pipe run between the elbows must not exceed 72 inches.
- Maintain at least 2 inches of clearance to combustible materials throughout the complete offset.
- Secure all chimney pipe joints with the required locking bands.
- Install the supplied Elbow Strap at the upper elbow. If two chimney pipe sections are installed between the elbows, install a second Elbow Strap at the joint between those sections.
- Do not cut chimney pipe sections or combine components from different chimney systems.
For a complete overview of chimney building codes by state, read our chimney building codes guide. If you need help with chimney support systems, we have a separate guide for that.
Common Offset Mistakes to Avoid
- Measuring at an angle - always measure horizontal distance with a level tape. Measuring along the slope gives you a wrong offset number.
- Forgetting the 2-inch clearance - the chimney pipe plus 2 inches on each side needs to fit through the opening. A 6-inch inner diameter pipe with insulation has roughly an 8-inch outer diameter, so you need at least 12 inches of clear space.
- Using stove pipe in the offset - stove pipe (single-wall or double-wall black pipe) is only rated for use in the same room as the stove. The offset section must use Class A insulated chimney pipe if it passes through any ceiling, floor, or wall.
- Skipping the support straps - an unsupported offset will sag over time, creating stress on joints and potential gaps.
Need Help Measuring Your Offset?
Not sure which elbow angle fits your situation? Use our free chimney design service - send us your measurements and photos, and we will send you a custom chimney plan within 24 hours. Or call us at 888-8340-880 (9 AM - 9 PM, 7 days a week) to talk it through with one of our chimney specialists.
You can also browse our complete chimney kits or use the chimney liner kit configurator to build your system online.
Frequently Asked Questions
Can you use 45-degree elbows on a Class A chimney pipe?
No. NFPA 211 only permits 15-degree and 30-degree elbows for Class A chimney pipe offsets. You must never combine two elbows to create a 45-degree angle. Steeper angles restrict draft and increase creosote buildup risk.
How many offsets can a chimney system have?
A single chimney system is limited to two offsets, which means a maximum of four elbows total. Each offset consists of one offset elbow and one return elbow connected by a straight section of Class A chimney pipe.
What is the maximum connector pipe length between elbows?
Per NFPA 211, the connector pipe between two elbows in a single offset must not exceed 72 inches (6 feet). If your offset measurement requires a longer connector, you need to adjust the elbow angle or reposition the chimney path.
Do you need extra support for a chimney offset?
Yes. Every pipe section in the offset needs an elbow strap to prevent the weight of the chimney from pulling the joints apart. Without proper support, the offset can sag and create dangerous gaps. See our chimney support systems guide for details.
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