How to cut walls along the pitches of the roof I have a problem I have been tied up all the day with and I haven't solved yet.
I am trying to cut the walls of this house along the inclined pitches of this house.
Any ideas? I have tried almost all, add-ons, boolean operations etc. 
I think that the best way in this case would be to cut the vertical walls along the intersection between the roof and the walls themselves (pic. 1). Another option would be to extend the walls up underneath the pitches, but as you can see there are different heights and there are no snapping points to use and this is so annoying ;)
Please be quite clear because, in spite of my age, I am a newbee.
Thanks.
In this case, there should be no difficulty using Knife Project.


Select the roof, then the building
Put the (active) building into Edit Mode.
In an Orthographic view, looking straight down the roof ridge, hit 'Knife Project' in the Tool Region, Tools tab, Add section.
In the Knife Project tool's own panel, check 'Cut through'.
V Rip the newly created edges
Hovering over the unwanted faces, L select all linked, and delete them.