But the correct specification is for 20 degrees minimum
I have scaled it out on paper and the difference between the pitch of the slates and the pitch of the rafters is only about 1 degree (effectively each slate is 8mm higher at its tail than at its head)
alastairreid asked in an earlier post and onions 1066 confirmed at 12:33am they were modern tiles. This is where I have been thrown as yes you are right if copper rivets have been used then there the fibre cements and the difference is only 3.5 degrees as Eternit have confirmed. So Mr joiner has messed up. It will need sorting.