Modified (Polymer-modified bitumen) Modified bituminous roof membranes continue to be popular option for low-sloped roofs due to their proven performance. Historically these products have been torch-applied, mop-applied using hot asphalt (SBS only), or applied with solvent-based adhesives.

Polymer-modified bitumen sheets are made with polyester and/or glass-fiber reinforcements. Most manufactures of Mod/Bit produce material with reinforcements that are filled, saturated or coated with very fine minerals, such as sand, mica or talc. All polymer-modified bitumen coated sheets are factory-coated on one or both sides with modified bitumen. The sheets are surfaced on one or both sides with very fine minerals, such as sand, mica or talc. The polymer-modified bitumen sheets are produced in a variety of weights and with a variety of reinforcements. Many smooth-surfaced sheets are used as base sheets, interply sheets and top ply sheets in multiple-ply modified bitumen membranes. Granule-surface and metal-foil laminate sheets generally serve as cap and flashing sheets.

5 Basic Components of a Modified Bitumen Roofing System

  • 1

    The "R" Value

    Insulation-provides the "R" value and stable substrate for the roof system

  • 2

    Modified Base Sheet/Ply

    Either a modified or unmodified base or ply sheet used under a modified bitumen top ply

  • 3

    Modified Bitumen Membranes

    A factory fabricated sheet consisting of a copolymer modified bitumen typically reinforced with polyester fiber glass of a composite carrier

  • 4


    Actual waterproofing material and bonding agent that is either applied hot or cold

  • 5


    Provide UV protection and a weathering surface that can be either factory or field applied

Once all steps have been completed, flashings are then applied. This provides waterproofing around roof perimeter, equipment and projections.

Advantages of Modified Bitumen Roofing Systems

  • Modified bitumen membranes are the thickest of the single ply class of roofing materials making them more puncture resistant, more resistant to weathering and to the rate of aging
  • Suitable for a broad range of service temperatures

Limitations of Modified Bitumen Roofing Systems

  • Special safety precautions are required to prevent fire during the installation process
  • Modified bitumens are susceptible to certain chemicals such as compressor oil, mortar sealer and solvents
  • Modified roofing is not effective for flat roofs without slopes; they must have a 1/4 inch per ft. slope to maintain proper drainage
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