What to do you do when drywall is broken on a ceiling heat vent mounting area and there is no backing? Find a scrap of corner bead and using a hammer, tap the corner bead gently around a piece of drywall or osb until it forms a “ u” shaped piece that can slip around the edge of the broken drywall. Then it allows the vent screw to have metal to bite into when mounting the vent cover. This technique can be used for a lot of vent repairs.