Cleats/clips pull the long TDC/TDF flange spans together so the gasket compresses across the whole joint, not just at the corners. Use the shop/project detail for spacing. If the approved cleats are not available, ask the foreman before using 5/16 self-tappers through the flange as the field fallback.
Do not rely on corners alone for long flat flange spans. Do not substitute screws for cleats without foreman/shop approval. Do not use fasteners to drag a twisted joint into alignment.
Check
Check that the duct is level and aligned before adding cleats or screws.
Confirm gasket is on one flange face and is not rolled or stretched.
Seat the corners first, then work the long flange spans evenly.
Use the required cleats/clips per shop/project detail.
If cleats are unavailable, verify whether 5/16 self-tappers in the flange are approved for this job before shooting them.
Steps
Look for daylight, whistle, or gasket gap along the center span.
Confirm corner bolts are installed and gasket is continuous.
Check approved TDC/TDF clip/intermediate fastener detail for the duct size/pressure class.
Install clips evenly and recheck seal compression.
Retest during fan or leakage check.
Say this to your foreman
The wide TDC/TDF joint at [location] is sealed at corners but bowing open in the center. I checked gasket and corners. Do you want flange clips/intermediate fastening added per detail?