Moving on with two point perspective, let’s imagine that we’re drawing a simple house and we want to figure out where to put the peak of the roof.
Our basic house starts with a rectangular box. The green line is the horizon line/eye level. Starting with the corner nearest to me, I hold my pencil up to catch the angle along the base of the roof to determine the vanishing point on the right side (off the page for this view). For the vanishing point on the left side, I draw a line from that bottom of the front roof to the bottom of the back roof and continue that line to the horizon.
When drawing the roof it’s helpful to know how to find the center of the gable side to find the roof peak. Draw diagonal lines (red in the picture) across that side – usually from bottom of the roof to the opposite base of the building. Then simply draw a vertical line through the mid point and you have the center of the wall to position the peak of your roof. To get the correct roof pitch, lay your pencil along the roof in your photo, or in the air in front of your subject, and transfer that exact roof angle to your paper. There it is!
Here’s a video from an early post showing the procedure with a barn.
As you can see with the video, I did all the preliminary figuring without a ruler. I started with the horizon line, and built it up from there. I don’t use a straight edge when I draw. If I find the roof looks off somehow, then I draw those diagonal lines to clarify the mid point. I let my eyes be my guide.
And, because you might want to see the finished barn drawing: