We had originally thought to use beadboard or a plastic like look alike to cover or replace the lanai ceiling, but that is a little more casual than I want to go and it would require putting it over the ceiling or taking the ceiling down and probably would still contract and expand at the seams.
Then we thought that maybe coffering would look attractive and cover the seams and allow them to expand and contract underneath where it won't show.
I will try to find some pictures of coffering on porches.
Here are some good ones;
coffering on porches - Bing images
If you have any other good answer to fixing this problem, please share.