How to cut the roof?

Hey guys, I created a roof and want to cut it, but it doesnt work. I tried a lot of stuff, but it didnt help…

You see the old roof behind. And I created with to arrays a new roof, and want set it to the old shape :confused:


Apply the array, if you haven’t.
Then top view(keypad-8) ortho(keypad-5)
knife tool(K) cut through(Z)
and cut from vertex to vertex on the old roof.

ok, I will try

Hey, that also doesnt work :confused: It cuts but than I have to delete all faces I dont need?