A beam conflict at one floor can force a chimney offset that changes supports, penetrations and the roof location several levels away. Repeated local field fixes can exhaust the available geometry before anyone has checked the complete route. Review the chimney from appliance connection to termination as one coordinated assembly.
The Geometry Controls
Under IMC 805.4, no part of a factory-built chimney incorporating offsets may exceed 30 degrees from vertical, and the assembly may contain no more than four elbows. Measure the angle from vertical, not horizontal. Count elbows in the entire assembly, not separately per floor or per subcontractor's detail. These are model-code limits; the selected system's instructions may impose additional restrictions.
A Quick Layout Check
For an illustrative straight sloping segment at 30 degrees from vertical, horizontal offset equals vertical rise multiplied by tan(30 degrees). A 24-inch rise therefore produces approximately 13.9 inches of horizontal offset for that idealized segment. This is only a geometry check. Actual elbow geometry, insertion depth, available listed lengths, joints and manufacturer offset tables control a buildable component arrangement. Do not use the simple calculation as an installation schedule.
Support is a Separate Decision
Section 805.5 requires joists, rafters or other structural members that support a factory-built chimney to be designed for the additional load. A visible strap does not establish the capacity of the framing or attachment. Identify the load path from chimney through its listed supports and anchors into the structure. Coordinate chimney weight, support locations, offsets, permitted supported height and project-specific structural/environmental loads with the responsible professionals and product instructions.
What Belongs on the Drawings
Show a continuous route with elevations, offset angles, elbow count, outside dimensions and required clearances. Mark each support and penetration, then reconcile the mechanical sections with framing, shafts and roof geometry. Reference the selected system's support and offset details. Keep attachment design, structural reactions and responsibility clear. Do not cut or notch a truss or structural member simply to make a nominally compliant chimney route fit.
Submittal and Field Review
Request a component layout tied to the plan and section, the manufacturer's offset table, permitted support arrangement and any project-specific attachment design. Before concealment, trace every direction change and support. Record identifiable components, visible attachment conditions and actual clearances; identify concealed or inaccessible areas separately. New field offsets should trigger a coordinated review of elbow count, support, clearances and termination, rather than approval of the elbow alone.
Manufacturer Example
Duravent's DuraChimney II guidance illustrates why routing and support are linked: its planning information discusses permitted offsets and elbow straps. That example is product-specific and does not establish strap spacing, load capacity or accessories for another system. DuraChimney II manufacturer guidance.
Project Boundary and Reference
This article explains IMC 2024 model-code requirements plus an expressly labeled geometry example. Confirm adopted edition, amendments, manufacturer instructions, structural design and AHJ requirements. 2024 IMC Chapter 8, International Code Council.
Continue Learning
Use the Daily Code Talk archive to connect routing review with listing, temperature classification and insulation protection.