Hi, i have a roof in this shape 1.PNG.

I want the top side offset with "-3.00m" from the left side. and the image should be like this one Roof+Slope+Arrow.jpg.
that one in the image is a simple form only with 4 points and 4 line. when i go with my roof in shape editor mode i get something like this 2.PNG and if i will pick line and offset it i dont get the image i want.
so i want to obtain the same image but i have a more complex boundary.
can someone help me?