Looking at several modern airliners, the root of the vertical tail doesn't extend right to the aft-most point of the fuselage. This seems counter-intuitive given extending the tail further aft would increase the moment arm and its contribution to the aircraft's directional stability.
In this photo of a 787, you can see what I mean.
There is still a significant length of fuselage behind the tail.
The same can be said for the A320:
This is far less prevalent on the 737 Classics and NG
And the BAE-146 follows (to my ignorance) what seems like the most logical tail design, with the root reaching the very aft of the fuselage.
The potential reasons for the tail not extending fully aft, that spring to mind are:
- CG limits
- APU placement
- Torsional loads on the fuselage where it becomes very thin at the back, not being able to support the loads imposed by the tail.
Is this why it is or is there more to it?