In short you can join the two wall together. Below is a link to a video. Watch it from the 5:55 mark.https://www.youtube.com/watch?v=1lBO7Xj_hnQ
The main reason most people create a loop is to make the roof easier to draw, yet you can draw a simple face over your walls or any area to create a roof from face. Therefore in this situation there is no need to create a loop.