A Pittsburgh seam was not knocked down and is leaking

A Pittsburgh seam leak usually needs the mechanical seam completed first, then sealed. Do not try to fix an open pocket seam with mastic alone.

Rectangular Duct & Fittings 1st Year GREEN

Watch out

Do not smear mastic over an open Pittsburgh pocket and call it sealed.

Check

Find the exact length of open or loose Pittsburgh seam.
Check whether the duct is already insulated, hung under tension, or connected to equipment that could be damaged by hammering.
Confirm you can close the seam without oil-canning the panel or deforming the fitting.
Finish the mechanical lock before applying approved sealant.
Recheck for whistle/leak after correction.

Steps

Find the exact length of open or loose Pittsburgh seam.
Check whether the duct is already insulated, hung under tension, or connected to equipment that could be damaged by hammering.
Confirm you can close the seam without oil-canning the panel or deforming the fitting.
Finish the mechanical lock before applying approved sealant.
Recheck for whistle/leak after correction.

Say this to your foreman

The Pittsburgh seam at [location] was not knocked down for about [length]. I checked access and panel support. Do you want me to finish the seam and seal it, or replace/correct the section if it is damaged?