Modified bitumen ("mod bit") looks a lot like roll roofing but seams are typically heat-sealed with a torch. You won't see nails.


Poor or improper installation: Unless you have been able to find manufacturer's recommendation, this will be limited to obvious problems:

  1. Membrane problems: blisters, ridges, deterioration, damage, or poorly-bonded or failed seams;
  2. Membrane slippage: typically due to poorly fastened base sheet or wrong asphalt adhesive type;
  3. Corners: improperly installed;
  4. Roof-to-wall junctions: improperly installed;
  5. Flashings: deterioration, failure, improper installation methods or materials. Pitch pans are NOT recommended for mod bit;
  6. Terminations: deterioration, failure, improper installation methods or materials;
  7.  Asphalt flood coat overly thick: pattern cracking;
  8.  Asphalt flood coat too thin: exposed felt:

Drainage Problems

  1. Inadequate slope (look for accumulated sediment);
  2. Roof features that interfere with the flow of runoff;
  3. Conductors or pipes lying on the roof that will impede runoff flow;
  4. Drains: improperly installed (too high or improper method), blockages, primary and secondary shared downspout, or strainer missing.


  • Natural aging (loss of volatile compounds) resulting in shrinkage, craze-cracking, splitting, or loss of granules;
  • Splitting may be caused by embrittlement over time or membrane shrinkage, especially if expansion joints are missing or over-spaced. Pay attention to reentrant corners;
  • Separation/splitting caused by differential expansion and contraction rates between the membrane and metal flashing.
  • Blisters (in the membrane or in a coating) may be caused by excessive moisture in the system, produced either by air leakage from the building interior (blisters may appear as ridges that mirror joints in the insulation), or moisture trapped during application;
  • Seams: weak, failed, lack of run-out. Seams should be tested with a pick.
  • Improper torching methods;
  • Loss of granules: age, foot traffic;
  • Pipe support methods that do not allow for expansion and contraction.


  • Impact: dropped tools, hail;
  • Wind: peel-back and missiles;
  • Coating spalling, fracture, or displacement;
  • Wear and tear from foot traffic.


  • Roof debris



The proper name for Modified bitumen is "polymer-modified bitumen". "Modified" means that certain characteristics of the asphalt (bitumen) have been enhanced by the addition of polymers to the asphalt blend. These polymers typically improve softening temperatures, durability, and waterproofing, to name just a few.

Modified bitumen often looks similar to mineral-coated roll roofing, except that you should not see fasteners. It may also have a surfacing installed. Surfacing types include flood coats that may or may not have aggregates embedded, or may have another type of liquid-applied coating installed.

Two Types of Mod Bit

You'll often hear modified bitumen called "mod-bit" for short. There are two basic type of mod-bit: atactic polypropylene (APP) and styrene butadiene styrene (SBS).

APP asphalt is modified with a plastic polymer. It is a plastomer

  • APP is more resistant to UV damage than SBS.
  • APP thermal service range: 15°F to 300°F
  • APP is less flexible and will not accommodate substrate movement  as well as SBS.

SBS asphalt is modified with a rubber polymer. It is an elastomer.

  • SBS is more flexible that APP and will accommodate substrate movement better.
  • SBS thermal service range: -20°F to 270°F gives better low-temperature performance.
  • SBS is less resistant to UV damage than APP.

Mat Reinforcement Types

Like with asphalt shingles and rolled roofing, modified bitumen roofing consist of a mat embedded in bitumen. Threee types of mats mauy be used:

  • Fiberglass;
  • Non-woven polyester;  and
  • Composite, which uses both fiberglass and polyester.

Installation Methods

Modified bitumen installation requirements vary both by manufacturer, and by various systems from individual manufacturers. To complicate matters further, manufacturer's system requirements may change over time. As an inspector, you will not know what system is installed unless you have access to documentation, in which case you may want to do some research before walking the roof.

Determining the system used will allow you to identify key installation details for cant strips, base, penetration, cap, roof edge, and termination flashing, and proper installation methods related to seams and corners. One of a variety of surfacings may also be installed like a flood coat (with or without embedded aggregates), or a liquid-applied coating of some sort.

Generally speaking, installation may be as follows:

  1. A base coat is mechanically fastened to the roof deck;
  2. A first layer of rigid foam insulation of some type is mechanically fastened through the base coat to the roof deck;
  3. A second layer of rigid foam insulation is adhered to the first, with joints staggered;
  4. Another layer of base coat is adhered to the insulation; and
  5. A cap sheet is torched down- or adhered with asphalt- to the second base coat. Cap sheets may also be peel and stick.

Note: Instead of a single base sheet, multiple interply sheets may be used in a manner similar to built-up roofs.


Manufacturers typically require bitumen run-out (bleed-out) at the side- and endlap- seams of 1/4" to 1/2". This is true for both torch-down and adhered membranes.


Flashings may be manufacturer/system specific and may consist of base and cap sheets similar to the membrane, may be pre-formed metal or plastic flashings, or liquid-applied flashing.

Areas at which metal and membrane meet (pay attention to roof edges) should be reinforced with stripping plies; strips of membrane reinforcement installed over the areas at which materials with differential expansion and contraction rates intersect.

Roof Cement

Flashings may be required to be embedded in roof cement, which is available in different grades typically related to viscosity and designed for use on either horizontal or vertical surfaces. You may see flashing failures that are related to failure of the applicators to use the specified roof cement. APP is not compatible with asphalt!


The 2011 NRCA Membrane Roof Systems manual list includes seven different recommended coatings for use with smooth-surfaced mod bit and nine for granule-surfaced mod bit. Without current documentation, you will probably not know what you are looking at.