Here is a GIF that shows one way of achieving it.
You can place 2 Guide Lines along the two roof plane end edges.
Then modify the Editing Plane, and use the Pick Plane option to pick one of the Roof surfaces.
Then you can place another Guide Line on that plane at the intersection point of the two Guide Lines.
Then you can just select the top edge of the Roof Plane using CTRL-SHIFT-click. and move it onto that Guide Line.
Do the same with the other roof plane.
Then you may have to remove some vertical edges that stays there on the vertical surfaces after the move actions.
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