IMC 603 starts with the duct construction baseline. Before supports, sealants, and penetrations, the drawings need to show the right sizing method, pressure class, and material standard.
Plain English:
603.1 General
Duct systems must deliver the required air distribution.
Duct installation cannot reduce required IBC fire protection.
Ducts must be constructed and installed for strength and durability.
603.2 Duct Sizing
Single dwelling units use ACCA Manual D, manufacturer instructions, or another approved method.
Other buildings use ASHRAE Fundamentals or equivalent calculations.
Manual D usually looks like residential room-by-room cfm tied to branch sizes and return paths.
ASHRAE usually looks like system/zone cfm tied to main and branch sizing using velocity, friction, and fan-static assumptions.
603.3 Duct Classification
Ducts are classified by maximum operating pressure.
The selected pressure class must meet or exceed the design pressure.
603.4 Metallic Ducts
Metallic ducts must comply with SMACNA 006.
Single dwelling units can use Table 603.4 only within its limits.
Over 20 inches by dimension or over 1-inch w.g. goes back to SMACNA 006.
603.4.1-603.4.2 Fasteners and Duct Lap
Round metallic ducts need at least 3 screws or rivets per joint.
Crimp joints for round and oval ducts must lap at least 1 inch, with male end in direction of airflow.
603.5 Nonmetallic / Gypsum / Phenolic Ducts
Nonmetallic ducts must be Class 0 or Class 1 and comply with UL 181.
Fibrous glass ducts must follow SMACNA or NAIMA standards and stay below 250 degrees F.
Gypsum ducts are limited to return air under specific temperature/dew-point conditions.
Phenolic ducts must follow SMACNA Phenolic standards.
On Plans: Why it matters
A lot of IMC 603 comments start with missing design basis: wrong sizing method, no pressure class, residential table used outside its limits, or nonmetallic ductwork specified without the required standard.
Code Path: Where to show it
M-001: sizing basis, duct pressure class, and material standard
M-101: duct size callouts
M-501: joint details, pressure class and special material conditions
M-601: duct schedule with pressure class and construction standard
Check: Do
Tie duct construction to the correct material standard
Review Risk: Don't
Don't leave duct pressure class implied
Don't use the dwelling-unit table outside its limits
Don't specify gypsum or nonmetallic ducts without the correct limitations
Field Tip:
House set: trace room cfm to branch size. Commercial set: trace system cfm to main size, pressure class, and fan-static basis. If that chain is missing, expect a redline.
Masterbuild QA Lens
Duct construction requirements (603.1-603.4) define material, gauge, and configuration. Confirm the duct schedule includes: material type for each system, minimum gauge per Table 603.4, and any exception basis for non-standard materials. Missing gauge specifications are the most common first-cycle duct-construction comment - the reviewer cannot verify a gauge that is not stated.
Drawing / Submittal Check
For 603.1-603.5.2 (duct construction basics): the duct schedule must show gauge, construction class, seam type, joint type, and any required reinforcement for each duct size and pressure class. Confirm the schedule covers all duct sizes on the drawings - schedules that end at a certain size leave larger ducts without a construction requirement. Risers and large supply ducts are commonly missed.
Common Review Risk
Duct gauge errors are the most common construction non-conformance on large duct systems. Lighter gauge was installed than specified - either because the spec was not communicated to the installer or because it was value-engineered in the field. Require duct shop drawings that show the gauge for each duct size and pressure class, reviewed before fabrication begins.
When To Escalate
Escalate when the duct construction non-conformance has been discovered after installation and the contractor disputes the required remedy. Non-conforming duct that has been concealed requires either a testing protocol to verify the installed performance or a full replacement. Neither option is inexpensive, and the contractor's position on which option is acceptable will determine the delay.
Duct System Coordination
Duct construction sections 603.1-603.5.2 set minimum metal gauge, joint type, and reinforcement by duct size and pressure class. The duct schedule must include pressure class and construction standard (SMACNA or equivalent) for every system.