Walter R. wrote:
The facia boards on my (single story) house are rough sawn 2 x 12.

Due to constant sun exposure (Southern California) the boards have developed
cracks and fissures, some 1/4" wide, 1/2" deep and 5' long.

I have patched the cracks with exterior caulk and repainted them. However,
due to the different rate of expansion between the caulk and the wood, the
caulk keeps popping out.

Due to the deep cracks, the boards have developed some dry-rot, which I have
repaired successfully.

How can I get the caulk to stay in the cracks? Replacing the boards would be
a major undertaking.

I doubt you'll be able to stop the checking. A big piece of wood like
that has all sorts of internal stresses from changes in temperature and
humidity. Those stresses have to be resolved the only way they can -
by gaps opening up.

You didn't mention how long those fascia boards have been in place, but
your periodic caulking and painting is the only "solution".

R