I.E. Remodeling Solutions a Division of David Fabbo

Built-up roofs are the traditional approach to flat commercial or industrial roofing in North America. Multiple layers of felt with interply moppings of asphalt or coal tar are assembled in the field to "build up" a finished roof assembly. These type of roofs are classed by the number of plies of felt that is used in their construction. Ex. "Three ply roof" or a "four ply roof".
Coal tar built up roofs are best used when roof areas have a very low slope
 and ponding water is probable. Coal tar is completely impervious to ponding water, one of the few roofing materials that has that attribute. Coal tar also
 has a low melting point, which means in the summer months, the coal tar becomes fluid and will "self-heal".
Asphalt built up roofs are best used when roof areas have significant slope where the low melting point of coal tar might cause "slippage" of the membrane. Normally any roof system which has in excess of 1/4" per
 running foot of slope should utilize asphalt if the building owner wishes to utilize a built-up system.
These type roof assemblies have a good life expectancy, normally in the
range of 15 to 25 years. To get a good life span out of these systems, a protective surface must be applied to the top of the roof. The typical surfaces to protect the roof from solar radiation are embedding gravel in the asphalt/coal tar, applying a surface coating, or applying a granular-surfaced "cap" sheet.