I applied it to the bottom of my floor after a lot of internal deliberation. The previous builder that had started constructing my floor had already applied a coat of roofing tar so it was probably overkill. After applying it, I let it bake in the sun for a couple of days.
I inspected the underside of the floor just the other day now that I've been using it for about a year and it looked to be in as good of shape as the day I applied it and had no signs of sagging anywhere.
If I were to build another one, I would fiberglass the bottom. The glass cloth will prevent the wood from checking over time and the epoxy will protect it from the elements. The bottom of the floor doesn't receive near as much abuse as I had originally thought it might and we've dragged it through mud holes that came up to our doors.