BURs are the toughest of the roofing systems.  Where pedestrian traffic is high, a BUR roof system has no equal.  A BUR roofing membrane is constructed on the roof by combining layers of materials (roll goods) together and in their total they act as the roofing membrane.  In the matrix of a traditional BUR membrane, tar or asphalt act as not only the glue which binds or adheres together the three to four layers of sheet-fabric roll goods, but that very tar or asphalt also acts as the main waterproofing component/ingredient which is being held/contained in place by those three to four layers of sheet-fabric.  Hot tar (also known as 'coal-tar-pitch') has been virtually replaced by hot asphalt, but hot tar remains an outstanding product because of its ability – after it has been installed – to easily melt and flow into voids giving it the title “self-mending.”  

Modified BUR - In the last thirty years, traditional BURs have been somewhat replaced by new high-bred membranes called “Modified Roofing Sheets.”  These modified sheets (roll goods) don’t rely solely on tar or asphalt to be the heart of the membrane; the new sheets are themselves a manufactured complete preassembled waterproofing membrane.   Modified sheets are made by modern formulations that include rubber as well as asphalt materials held together with fabric now called scrim.  Two layers of these modified sheets can provide an excellent finished roofing membrane.  The layers of modified sheets can be adhered together by using hot asphalt, fully torching them together with open flames, with cold adhesives, with cold adhesives that have their side laps torched or heat welded, or some modified sheets come complete as self-adhering sheets.  Modified roofing systems are commonly put down in two or three layers; the bottom layer is commonly known as the base-sheet, the central layer is the interply, with the top layer or the finishing layer known as the cap-sheet.  Capsheets are manufactured with a finished granule surface, or they come with a smooth surface with a final surfacing to be installed above it.  Modified roofing systems can be designed in several different layer configurations with many different schools of thought as to why.  Modified and Built-Up-Roofing systems are excellent for their durability and are considered a more costly roofing membrane/system. 

The Final Surfacing of the built-up-roof is important, specific, and evolving.  Gravel set in a pouring of tar or asphalt has been the traditional finished surface.  Gravel gives the BUR membrane a heavy, durable, attractive final surfacing that does offer some solar heat resistance, and it is extremely durable holding up to foot traffic, hail and other weather conditions.  A final, good quality aluminum coating applied to a smooth or granule surface locks in the BUR’s oils by sealing the roofing system and reflecting the solar rays - potentially adding years to your BUR if regularly maintained.  Then there are the new highly formulated coatings emerging as final surfacings of BURs & single ply roofing systems; time will tell as to which one of them performs best.  Coatings are generally considered a maintenance item, meaning coatings can wear and will need to be redone.  The final surfacing should act as a seal to keep the oils in the roofing system so the system can remain resilient.  A well designed BUR roofing system is the best roofing system tough enough to stand up to the pressure of constant pedestrian traffic.