I've figured out how to make the roofs match the house, thank you for your help.
I built each "box" of the house on its own, then moved them together into the final house shape.
This was my process:
1. Create the four walls of the tallest shed roof section. Set the sides to full gable walls, then set the tallest wall to a shed wall. I measured the rise and run of the real house walls to get the roof pitch.
2. Create the four walls of the shorter shed roof section. As above, set sides to full gable walls, then set tallest to a shed wall.
3. Move the shorter shed roof section up to abut the taller one. The taller roof now rises above the shorter one exactly as the real house does.
4. Create the four walls of the gable roof section of the house (the "main" part of the house). Set sides to full gable walls, then move it to connect to the ones I already made.
5. Create the three new exterior walls of the garage, then change the side walls to full gable roof. The gable roof now extends over the garage in a smooth slope, exactly like the real house.