The screws should be long enough to hit the metal on the outlet attached to the ductwork.

If you have a system using the stud bay or joist bay as a duct, you may have to use Molly bolts to hold the grates on.

Thanks! It sounds like I need to drill pilot holes through the drywall and into the metal.

I like my frame idea better, though.

The frame should be behind the drywall.

They are often refereed to as a 'register boot.'

The reason the screws holding the grill are often very long is to make sure they can reach all the way to the register boot that comes off the duct work

try changing the angle of the screw.
usually works for me.

and you should use some mastic tape
to secure the supply box to the
sheetrock before reinstalling grill.

best of luck.

What is the supply box? Is that another term for air register?

inside the house you see the supply grill or register
inside the attic is the supply box
the supply grill is attached with screws through
the sheetrock & into the supply box.(usually 2 1/2 to 3" screws)
the duct attaches to the supply box at attic floor
the other end of the duct attaches to supply plenum.

and the knee bone is connected to the leg bone
and the leg bone is connected to the hip bone...

best of luck

If I understand this correctly, the supply box is the same as the register boot.

{{!gwi}}

Hi, Make a frame to fit. I once bought a house where the previous owner paneled one wall. Instead of removing the register, he cut a hole around it. Looked terrible. I built the frame to fit the hole and mounted the register on top the frame. Problem Solved.
Good Luck Woodbucher