TinnerFlow™3rd YearForeman Skills & Jobsite ProductivityCheck first
If only one panel is sloped and the duct depth shrinks, the fitting becomes a restriction. Keep the internal area consistent or get an approved revised fitting detail.
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A 45° offset should move the duct without stealing throat depth. If only one side is sloped and the opposite side stays flat, the internal depth can shrink through the fitting and speed the air up through a choke point.
The recovery is usually a true parallel-plane offset, longer transition, or approved revised fitting that maintains the required internal area. Do not hammer the fitting into place and hope the air balance crew fixes it later.
Use this as training guidance. Foreman direction, approved drawings, project specs, manufacturer instructions, employer safety policy, and AHJ/code requirements always control the final answer.