The link below leads to a NEC-2 study of a short vertical monopole
for two mounting/operating conditions stated in the study.

It shows about a 13 dB advantage in the surface wave launched by
the roof-mounted configuration in this comparison.

Similar affects should apply to verticals used in the ham bands,
so this result might be useful to some readers here.

http://www.keepmyfile.com/download/3ef1691784487

RF