Repairing Ceiling Cracks
Ceilings sustian damage from many things. Sometimes an old ceiling will sag and pull away from the rafters or ceiling joist. Other times a water leak causes the ceiling to sag.
It might simply be a settling crack or as you say the ceiling could be separating from the ceiling framing. Here are some ceiling repair tips.
Try pushing on the area, does it move? If yes, and if the sagging is only slight, or covering a small area, it can be reattached to the lath by using long drywall screws fitted with plaster washers.
Plaster washers, also known as ceiling buttons, are metal washers with a series of holes to hold drywall compound and are designed to increase the surface area of a screw head to avoid punching the screw through the plaster and also to provide holding power. Most hardware stores carry these washers, I get mine at Concord Lumber Corp.
To avoid cracking the plaster and creating more damage, install a few washers around the crack or across the sagged area and drive them snug against the plaster face. Then tighten each of them slowly, moving from one to another, so that the plaster gradually pulls tight against the lath. You want to gently work the sag back into place.
Ceiling Repair Tips Article
Once the ceiling is reattached you can apply drywall tape and drywall compound or plaster to patch over the washers and crack. Read how to repair an old plaster wall and ceiling.