Summary: To apply line labels and sub-line labels to your RoofSnap sketch, tap the "Edges" tab on the bottom of your screen, select your desired label from the drawer, and tap the line(s) you wish to label.
For sub-line labels, double tap the edge-type in the drawer to expand the sub-line label options. Tap the desired subline labels and tap the line(s) you wish to label.
Watch the video below to see the process: