When I said look, I ment look in the tables, not just take my word that it will work. Usenet is a great place to get ideas, but because there are some real nutters out there you have to check them yourself using independant sources.The only person that you should be able to
100% trust on this is the BCO, and he may require you have advice form a strutrial engineer, which he will check.You may find that you need to lay taller joists next to the existing ones. Simply nailing an extra few inches on does not work.
To get them in, you gate a few tiles out if the roof, this is not too hard with modern tiles, but is somewhat a pain with a thatch.
Rick