Replacing a split tread on an old staircase
If the tread is loose enough to close the gap by tapping it with a hammer you may be able to glue it .If it is loose take two 3/4x3/4 inch boards that are as long as the crack.Drill four holes thru both boards big enough for a 10-24 screw.With some dry wall screws attach the boards to the tread on both sides of the crack.Apply some gorilla glue to the gap. Using the 10-24 screws draw the gap closed, allow the glue to dry, remove the 3/4" boards.Sand any glue squeezed out.