/ Field knowledge · IMC 805.4–805.5
The fourth elbow is already in the field.
A beam conflict on one floor can force a chimney offset that changes supports, penetrations, and the roof opening several levels away. Factory-built chimneys with offsets: no part more than 30 degrees from vertical, and no more than four elbows in the whole assembly.
Educational 2024 International Mechanical Code (IMC) 805.4 and 805.5 from Daily Code Talk. Confirm the adopted edition. Distinct from factory-built listing (805) and decorative shrouds (805.7). The manufacturer’s offset table may be tighter. This is not a chimney layout.
- 805.4: measure the angle from vertical, not from horizontal. Count elbows in the entire assembly — not per floor, not per subcontractor’s detail.
- A local field elbow that “just misses the beam” still counts. Repeated local fixes can exhaust the four-elbow budget before anyone has walked the complete route.
- 805.5: joists, rafters, or other members that support the chimney have to be designed for the additional load. A visible strap does not establish the capacity of the framing or the attachment.
On the set: a continuous route with elevations, offset angles, elbow count, outside dimensions, and clearances. Mark each support and penetration. Do not cut or notch a truss to make a nominally compliant route fit.
The fourth elbow is already in the field
New field offsets should trigger a coordinated review of elbow count, support, clearances, and termination — not approval of the elbow alone. Keep attachment design and structural reactions assigned. The appliance collar is not the support.
Source: Daily Code Talk — IMC 805.4–805.5 · tool: Offset screen · listing: 805 · support/shield: 802.7–802.8.
What to send
The chimney route in section (or photos of every elbow), the listed system name, and whether framing is already carrying the chimney.
If the chimney has to miss a beam
Send the section. Masterbuild will say whether 805.4 still has elbow budget — or 805.5 is an unassigned structural load.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. The 30-degree and four-elbow limits are 2024 IMC 805.4 education from Daily Code Talk — not an offset schedule. Manufacturer instructions may be tighter. This page is not legal advice.