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Apprentice Q&A · #134

What is "take-back" or "allowance" when measuring for a slip/drive vs. TDC fitting?

3rd YearField Math & MeasuringYELLOW · Check First

Slip & Drive (S&D): The raw metal edge slides roughly 2 1 inch inside the companion duct pocket. You must add 2 1 inch of material length to your raw layout cuts for each S&D joint end.

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Plain-English answer

Slip & Drive (S&D): The raw metal edge slides roughly 2 1 inch inside the companion duct pocket. You must add 2 1 inch of material length to your raw layout cuts for each S&D joint end.

TDC / TDF Flange: The duct metal is rolled outward to form a 1 2 1 -inch structural matching flange border. The face-to-face measurement is exact, but you must factor in about 8 1 inch of thickness allowance space for the compressed butyl gasket layer sandwiched in the middle.

Field checklist

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Hey boss, I’m checking field math & measuring: What is "take-back" or "allowance" when measuring for a slip/drive vs. TDC fitting? I found the likely issue and want to verify the next step before I lock it in. Do you want me to adjust it now or check the drawing/detail first?

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Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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Related Field Rescue routes

What do I do when the S-cleat side is not fully seated?Field Rescue · Rectangular Duct Assembly: S-and-DriveWhat do I do when not allowing for connector length in run measurement?Field Rescue · Measuring, Field Verification & Layout MarksWhat do I do if the drive cleat is cut too short?Field Rescue · Rectangular Duct Assembly: S-and-Drive

Related mistakes & recovery checks

#025 · Forgetting to add allowance for the slip and drive connection when cutting duct1st Year · Sheet Metal Work#066 · Not properly forming a female end on duct so the joint won't pull apart2nd Year · Sheet Metal Work#007 · Forgetting to account for material thickness when laying out bends1st Year · Measuring & Layout