Originally Posted by wrldruler
Structurally speaking, which is my better option? Cut the same rafter another 12 inches, or cut a new rafter?
Just to pick a nit, those aren't rafters, they're ceiling joists. Rafters run uphill on the underside of the roof.
By having one joist cut apart in the center, you've weakened the overall ceiling structure by X amount. The weight which originally was carried by the cut joist is now being transferred by those "headers" to the joists on each side. Now, if you cut one of those
joists, you're essentially doubling the load being transferred via headers.
Unless you have a gazillion pounds of stuff stored up there, either approach would be fine. But ultimately, the better structural approach would be to shift the opening along the length of the already-cut joist.