While installing some new recessed lighting in my attic today, I noticed this split in one of the ceiling joists. The split is about 4 ft 6 in long starting from the end of the joist. The joist is a 2x12 and is about 19 ft long. The joist is only supported at the ends; there are no other walls or supports underneath it.
How urgent is it to address this? What options do I have for a repair that would be acceptable? The only way I could get another 19 ft joist there to sister the entire joist would be to open up the ceiling.
EDIT: Additional context
The house was built in 1979. It is a single story house in a sort of U-shape:
The joist in question runs along the top of the living room, which has a raised 10 ft ceiling (rest of house has 8 ft ceilings). Here is a view of the living room. The split in the joist in question is above the sliding glass door and runs from the left side of the picture to the right side. The width of the living room along the joist is about 18 ft:
Here is a view of the ceiling looking the opposite direction. The joist still runs left to right across the picture:
This is a view in the attic looking towards where the joist is split. This picture shows how the ceiling is raised. The ceiling joists run from the front of the picture to the back and are perpendicular to the rafters for the roof. The split is on the far side of the attic:
This view shows a close up of how the joists are supported in the foreground of the previous picture:
This shows how the joists are attached to the frame on the far side (where the split is):
EDIT: Nearly final result
Here's what the nearly final result looks like (I still need to replace the block that was between the joists):
I was able to get a 10 ft board in place to sister with the joist with the split. I used pairs of 3/8in bolts spaced about 24 in apart to bolt it together. Near the split, I put the bolts below the split and put one extra one in. I used construction adhesive between the joist and the sistered joist.
In the area that was split, I also drove a few 8in lag screws vertically through the joist. It wasn't able to close the split, but hopefully it helps: