Fireplace Drywall Ceiling Damage
This fireplace drywall ceiling damage was caused from
water leaking in between the chimney and the roofing. There's a good
chance that the metal flashing has separated from the brick chimney.
Most of the time water damage to the wood framing, ceiling or walls
around the fireplace can be attributed to the metal flashing around the
fireplace chimney, somehow.
If the roofing isn't installed properly, along with the metal flashing, this could become a problem also.
Home Repair Tip: Drywall ceilings that have suffered from water damage, could have mold, mildew or even a fungus growing on the other side of it. If there's enough moisture, the brick chimney itself could have something growing on it also.
Make sure that you replace any water damaged drywall, to prevent the possibility of mold growth in the future. More important than that, make sure that you locate and repair the roof flashing or roofing that has created the ceiling water damage.
The arrow is pointing to the roof flashing. This is the first place to inspect, when looking for the cause of your ceiling water damage.