Tar roofing is a common roofing material for commercial and government buildings. It has to be used on a flat roof (or else it could melt on a hot day and run off the slope.
You will not see a lot of tar roofs here in Virginia because it can freeze in the winter and crack, then when it start raining in the spring you will find out.
One exception that you will see often is outdoor living space. Namely, decks on the second floor of buildings. People often opt for tar under the decks and above the roofs because it's easier to do a patch job than replacing something like metal.
Tar does provide a watertight seal but it also has the negative aspect that it can melt on hot summer days and smell very bad. In our opinion, there are better options for roofs than Tar, at least, for our geographic location.
