Despite the limitations that have promoted the many alternatives to the traditional bituminous membranes as a technique for waterproofing low-sloped roofs, today’s built-up membrane is itself continually evolving, if more slowly than its single-ply competitors. As demonstrated by its retention of roughly one-quarter of the flat roof market, it remains a formidable competitor of the new systems. As its most notable continuing advantage, few, if any, competitive systems can match the toughness, impact resistance, and general durability of a well-constructed aggregate-surfaced built-up roof systems.         

   Asphalt Built Up Roofing (BUR) and Polymer-Modified Bitumen (MB) systems have excellent credentials. Bitumens won the initial quality contest in the 19th century when asphalt and coal tar pitch were adapted for use on the roofs of commercial buildings. It is estimated that the asphalt BUR originated 170 years ago, during the beginnings of the industrial revolution. Many factories, warehouses, transportation terminals, and tenement buildings that endure today were constructed in the years that followed, and during that time, asphalt in general and BUR systems in particular were established as benchmarks for roofing. The greatest attributes of asphalt are proven durability and excellent water-leak resistance, both resulting from a redundancy of multiple layers of material.

Built Up Roofing - BUR

   These roof systems are sometimes referred to as "tar and gravel roofs" Asphalt BUR is the most common type of flat roof at this time.

BUR systems generally are composed of alternating layers of bitumen and reinforcing fabrics that create a finished membrane. The number of plies in a cross section is the number of plies on a roof: The term "four plies" denotes a four ply roof membrane

construction. Sometimes, a base sheet, used as the bottom most ply, is mechanically fastened. Built up roofs generally are considered to be fully adhered if applied

directly to roof decks or insulation.

   The bitumen typically used in BUR roof systems is asphalt, coal tar or cold-applied adhesive. The asphalt or coal tar is heated in a kettle or tanker and then applied by mop or mechanical spreader. Asphalt is a petroleum product refined from crude oil; coal tar is derived from the distillation of coal. Cold-applied adhesives typically are solvent-based asphalts that don't have to be heated in a kettle or tanker.

Modified Bitumen

   Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. MB roof systems typically are installed as a two-ply system and almost always are fully adhered.

                                                                                     There are two types of MB roofing membranes:

• SBS polymer-modified bitumen membranes commonly are in stalled in hot moppings of asphalt (similar to BUR systems) or cold adhesive. Some SBS modified membranes are self adhering; that is, they contain an adhesive backing.
• APP polymer-modified bitumen membranes typically are heat-welded or torch-applied. Generally, APP modifiers impart a "plasticized" quality to asphalt, and SBS modifiers impart a "rubberized" quality to asphalt.

   Surfacing for MB membranes include aggregate surfacing, mineral surfacing, metal foil-laminate surfacing and smooth liquid-applied surfacing.

   Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. MB roof systems typically are installed as a two-ply system and almost always are fully adhered.


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