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Modified Bitumen

A modified bitumen membrane (MBS) roof is a system developed by taking the qualities of a BUR system along with the qualities of a single-ply roof. A standard MBS roof system should have a base sheet or ply, like the BUR system, then a waterproofing sheet similar to single-ply.

To break it down, a modified bitumen membrane is composed primarily of polymer-modified bitumen reinforced with one or more plies of fabric such as polyester, fiberglass, or a combination of both. Factory surfacing, if applied, includes mineral granules, slag, aluminum, or copper.

Modified bitumen’s generally use a traditional waterproofing medium — asphalt — modified with atactic polypropylene (APP), styrene butadiene styrene (SBS), synthetic rubber, or other agents that will enhance the asphalt properties. APP and SBS are the most common bitumen modifiers.

Common modified bitumen’s are:

  • SBS (Styrene-Butadiene-Styrene) modifies the asphalt by forming a polymer network within the bitumen. SBS gives the bitumen rubber-like characteristics and improved resistance to aging and weathering. Most SBS modified bitumen sheets are either set in hot mopping asphalt, torch-applied, or adhered with cold-process adhesives. SBS modified bitumen sheets that do not have factory applied granule or foil surfacing need some form of field-applied ultraviolet protective coating.
  • APP (Atactic Polypropylene) is a thermoplastic polymer, which forms a uniform matrix within the asphalt. This enhances the bitumen’s performance by increasing its UV resistance, increasing its flexibility at low temperatures and improving its flow resistance at high temperatures. APP modified bitumen sheets are generally applied using a propane-fueled torch. Applicators use the heat to soften the modified bitumen on the underside of the sheet. The sheet’s bottom surface becomes a molten adhesive, which flows upon the substrate and then cools to form a waterproof adhesive bond. Some APP sheets can also be applied with cold process adhesives.

Advantages of Modified Bitumen Roofing Systems:

  • Since gaining acceptance in the roofing industry during the 1970s, the use of modified bitumen membrane roofing systems has increased steadily. Today modified bitumen membranes are the fastest growing roofing materials in the industry.
  • Considered a quality “hybrid” system, MBS provides the features of a built-up roof with the added tensile strength and elongation of a modified bitumen cap sheet, as well as the quality assurance of in-plant membrane fabrication uniformity and control, and reduced labor requirements for installation
  • In addition, MBS products undergo the same fire and uplift resistance testing standards as BUR, providing the commercial customer full assurance of appropriate fire and uplift resistance.

Disadvantages of Modified Bitumen Roofing Systems

  • Is usually applied using propane torch and assumes the risks associated with open flame.
  • If application is not installed properly possible future problems such as, defective lap seams, shrinkage, blistering, delamination, splitting, ridging/wrinkling and slippage can occur.
  • Is not as effective on flat roofs, mainly because it is only one layer and water can find even the smallest opening when the area is totally submerged.