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Apprentice answer

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

3rd YearMeasuring, Layout & Field MathCheck 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.

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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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