Why is there a gap under the heel of a round saddle tap

Plain-English answer

If the saddle curvature does not match the main duct, a wide heel gap becomes a structural air leak. Mastic alone will crack if the metal is not fitted first.

Remove the tap, reshape/dress the flange to match the barrel, confirm the opening size, reset with approved gasket/sealant, and fasten evenly around the perimeter.

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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.

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