How do I seal the duct drop inside a roof curb so water can't get into the building

Plain-English answer

The internal supply and return sheet metal drops must be fastened and sealed to the inside flange of the roof curb using heavy structural polyurethane sealant or flashing tape before the RTU is set.

This ensures that even if condensation forms inside the curb walls, it cannot leak down past the outer duct perimeter into the ceiling plenum of the building below.

Field checklist

• The internal supply and return sheet metal drops must be fastened and sealed to the inside flange of the roof curb using heavy structural polyurethane sealant or flashing tape before...
• This ensures that even if condensation forms inside the curb walls, it cannot leak down past the outer duct perimeter into the ceiling plenum of the building below.

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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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