I'm installing a microwave/range hood. To preserve usable depth in the cabinet above, I'd like to use 3.25 x 10 duct through the cabinet, and transition to round duct in the attic. However, a ceiling joist is blocking me from running the duct straight up. Is it allowed by code (and not problematic any other way) to build an offset which would move the duct 3 inches to the side, within the cabinet?

If it's allowed, can it be done with simple parallel bending as shown in the sketch, or would it require actual curves?

sketch of duct offset



If I were doing that job, I would contact an HYAC company or if you can find one, a duct fabricator, and have them make what you need. Their offset would be smooth and without the flaws as compared to something you could make in your basement.

Another possibility would be to purchase a piece of 3 1/4" X 14"(this duct comes in 10", 12", and 14" widths) and use it in between the 2-10" ducts to produce the offset. It won't be a clean and pretty transition but it can be done. As far as any "code" there may be would be determined by where you live.

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