Flat Roof Dormer vertical dormer cheeks

Hi,

I am trying to model a pitched roof which has a number of flat roof dormers built into the pitch of the roof.

When I go to model the vertical parts of the dormer, the vertical parts of the roof form a rectangle shape which appears to form a wall within the room below the roof.

Is there a way of modelling the vertical parts of the dormer roof so that they form a triangle shape which follows the pitch of the roof?

Thanks

Andrew