I live in a 100+ year old victorian with a beautiful front porch. A bathroom above the porch leaked and damaged the plaster ceiling. The leak has been repaired.

I would like to cover the existing ceiling with bead board but need some advice:

1. I have purchased 1x3s to run along the rafters (joists?) from which the plaster ceiling hangs in order to have a reliable surface into which I can nail the beadboard. The rafters are approximately 16 to 18 inches apart and will run perpendicular to the length of the beadboard. Would I be better off covering the existing ceailing with lightweight plywood instead of using the 1x3s? Only a small part of the ceiling is damaged.

2. My porch is deep enough that the 96" Aztek PVC beadboard will not cover the entire span in one peice. Can I butt two pieces together and not risk them buckling in the heat/cold if I leave enough expansion room where the ceiling meets the wall?

Thanks!