My house has a gabled roof and I want to add guttering and wooden edging.
I see I can do comp to select but I don't see an option to select between two faces?
I have shown below in the picture the red lines are where I want to attach the models.
I read this part: Help -
But using comp with ridge doesn't seem to work.
Yes, that is correct. You have to use comp(e) with the selectors eave, hip, valley, and ridge because these selectors are only designed to work for edges.
(See the section on Roof Edges.)