I’m having a little difficulty describing this in words so here are some pics. I want to keep the angle of the line constant at 27 degrees, but rotate it around y-axis such that it intersects the bottom edge of the orange face.

exteriorform.skp (221.4 KB)Why 27 degrees? Where did that number come from? And why are you drawing the roof based on the hip angle rather than the roof pitch?

This is more of an academic exercise in practicing SketchUp than a practical application.

Do you have any helpful advice?

Here is one idea. Imagine a right-triangle similar to the light gray one in the drawing, but larger such that the length of the side opposite the sharp point of rotation equals the height of the orange edge to be intersected. Use trigonometry to determine the length of the adjacent side (which runs on the “floor”), given the 27 degree angle and the known opposite height.

Adjacent = Opposite / tan(27 degrees)

Draw a new gray triangle with adjacent (horizontal) side as just calculated (running in an arbitrary compass direction), with opposite (vertical) side equal to the height of the intersected line, and with hypotenuse filling in the diagonal. Rotate that new larger triangle until the upper tip (joint between hypotenuse and opposite side) touches the edge to be intersected. Hopefully SketchUp’s inferencing will latch on that point to the edge.

Ah thanks for making a gif, looks good.

I think @TDahl was on the point of saying the same. It’s a version of a common problem that comes up, often in making diagonal bracing, how to inference the center of a line for intersection.