40 degrees is a practical minimum for slate or peg tiles. I wouldn't
consider pantiles at anything less either.

Below that you really need to be careful, overlaps may need increasing,
or maybe even a roof of plywood covered with waterproof membrane and the
roof material more decoration than weatherproofing.

Consult a good architect.


My architect told me today that 25 degrees is "about as low as he would go" for slate tiles, we settled on 30, as "you have to be an idiot to get it wrong at
30 degrees".


Rick