There are a number of different roof systems used to protect commercial buildings.  These can include membrane roofs which are good for particularly large, uninterrupted surfaces, and composite or built-up roofs.  Composite or built-up roofs are particularly useful to cover roof surfaces requiring a significant amount of detail work. 
Originally, built-up roofs were simply a layer of tar paper overcoated with a layer of tar and ballast.  More recently, reinforced rubberized asphalt composite roofing has been employed.  This roofing system generally includes primer, then a first layer of molten rubberized asphalt applied onto a roof deck.  A fabric, typically non-woven polyester, is then laid over the initial layer of molten rubberized asphalt.  This is overcoated with a second layer of rubberized asphalt.  Rubberized asphalt itself is a blend of asphalt and thermoplastic polymer.  This remains flexible even at low temperatures, i.e., -20.degree.  F. Further, it exhibits better elongation than asphalt. 
Built up waterproofing systems are relatively labor-intensive.  The application of two coatings of rubberized asphalt and the proper placement of the reinforcing mesh significantly increases labor costs. 
This system is also difficult to apply to surfaces with high air or moisture contents.  For example, lightweight structural concrete surfaces do not accept this asphalt coating particularly well.  Either the first or the second layer can form pockets or bubbles and pinholes can also form.