Thank you all for your help
I'm trying to build a monastery and have done a rough and ready mock-up using available parts from my castles to try to figure out what I needed - basically a museum roof for the top of the tower and then two half-roofs for each of the transepts - plus many, many windows!
I've attached a picture of it (ugly I know, but I'm hoping it will show what I mean!) and if I've understood correctly, I'm thinking I will need 4 x 30 02 858, 8 x 30 02 859 and no connectors to get those parts of the roof done
Thank you all again!!