Roofing System 7


Long and proven history when properly installed.Particularly sensitive to experience and knowledge of the installer
Easily repairedAsphaltic materials do not naturally possess optimal long-term performance characteristics.
RobustSafety issues related to installation
Noxious fumes

Although coal tar is the only roof covering noted in the International Building Code as being suitable for slopes as low as one-eighth unit vertical in 12 units horizontal for new construction and is still available, the vast majority of BURs are constructed with asphalt. Coal tar pitch is suspected to be carcinogenic and most owners and designers avoid it for that reason, in spite of the excellent performance characteristics.


The physical characteristics of asphalt are actually not well suited to roofing; they are not UV stable, get brittle with age and cold, crack, alligator, and otherwise degrade relatively quickly. In order to make the asphalt more suitable for roofing, it is modified with other chemicals. MB membranes exhibit general toughness and resistance to abuse. They are typically composed of pre-fabricated polymer-modified asphalt sheets with a reinforcement layer. Polymers are added to bitumen to enhance various properties of the bitumen. The quality of MB products is highly dependent on the quality and compatibility of the bitumen and polymers, and the recipe used during the blending process. They are also highly dependent on the reinforcement within the sheet. High quality manufacturers carefully monitor the source of their raw asphalt and how it is modified. There are unfortunately many MB manufacturers and not all are as diligent.

There are three primary types of MB sheets, as well as field-applied modified mopping asphalt:

Figure 3. This APP modified bitumen cap sheet is being applied with a multi-head torch. The seam is being rolled with a roller.

Atactic polypropylene (APP): APP polymer is blended with asphalt and fillers. The mixture is then factory-fabricated into rolls that are typically one meter wide. The prefabricated sheet is typically reinforced with fiberglass, polyester, or a combination of both. The sheets are available in base, interply, and cap sheet variety. Sheets are smooth (i.e., un-surfaced); embedded with mineral granules of a variety of colors; or factory-surfaced with metal foil such as aluminum, copper, or stainless steel. The aluminum foil is available in colored finishes. APP MB membranes are generally resistant to high-temperature flow.

To avoid surface cracking from ultra-violet radiation, a field-applied coating (such as aluminum-pigmented asphalt, asphalt emulsion, or acrylic) may be applied. Cap sheets with factory-applied surfacing of granules or metal foil should be specified.

APP MB roofing systems are typically composed of a base sheet, an interply sheet, and a cap sheet. The cap sheet is either heat-welded (i.e., torched) to the base sheet, or it is adhered in cold adhesive. Mechanically attached systems are also available.

Note: APP MB sheets are also available with a factory-applied adhesive on the underside of the sheet, which permit them to be self-adhering. Several manufacturers introduced these products in the early 2000s.

Sometimes one or more fiberglass ply sheets (as used in BUR) are mopped to the base sheet or additional layers of APP sheet (sometimes referred to as “interply sheets”) and then the cap sheet is installed. The interply sheet(s) provide redundancy. Note that these systems diminish some of the benefits of MB by introducing unmodified asphalt.

APP MB membranes can also be used in a protected membrane roof (PMR) configuration, also called inverted roof membrane assembly (IRMA). In a PMR, XPS insulation is placed over the membrane. The insulation is protected from ultraviolet (UV) radiation and wind blow-off by concrete pavers or large aggregate. When aggregate is selected, a filter fabric should be specified between the aggregate and insulation in order to keep the aggregate from getting into the board joints and underneath the boards.

Styrene-Butadiene-Styrene (SBS): SBS polymer is blended with asphalt and fillers. The mixture is then factory-fabricated into rolls with reinforcement and surfacing similar to APP MB sheets. SBS sheets generally have good low-temperature flexibility and weatherability compared to APP.

SBS MB is susceptible to premature deterioration when exposed to UV radiation and is typically specified with a factory-fabricated mineral surfaced modified bitumen cap sheet.

SBS MB membranes are typically specified as two or three ply systems. Specify 3 ply consisting of nailed modified bitumen base sheet, modified bitumen interply sheet, and modified bitumen cap sheet over nailable deck. The base sheet should be adhered when decking is non-nailable.

Note: SBS MB sheets are also available with a factory-applied adhesive on the underside of the sheet, which permit them to be self-adhering after removal of a carrier sheet. Several manufacturers introduced these products in the early 2000s.

SBS MB roofing systems can also be used in a Protected Membrane Roof (PMR) (sometimes referred to as an Inverted Roof Membrane Assembly (IRMA)) configuration. If a PMR system is specified, a slip-sheet recommended by the membrane manufacturer should be placed between the membrane and the XPS to prevent the insulation boards from bonding to the membrane. Otherwise, membrane tearing could occur when the insulation moves or floats during a rainstorm.

Styrene-Isoprene-Styrene (SIS): These seldom-used self-adhering sheets are blended with SIS polymer, asphalt and fillers. The mixture is then factory fabricated into either 3 feet or 1-meter wide rolls. The top of the prefabricated sheet is available with embedded mineral granules or a factory-laminated UV-protective surfacing, such as aluminum foil. The bottom surface has a release paper to keep the sheet from bonding to itself while rolled.

A similar product is commonly used under steep-slope roof coverings to provide ice-dam protection. However, the steep-slope underlayments do not have a UV-protective surfacing. SIS MB roof membranes currently capture a very small share of the low-slope market.

Styrene-Ethylene-Butylene-Styrene (SEBS): SEBS polymer is blended with asphalt in a factory. The SEBS modified asphalt is then reheated at the job site in specially designed tankers or kettles. The hot modified asphalt is applied in a manner that is virtually identical to BUR. The membrane is typically surfaced with aggregate. SEBS modified mopping asphalt is extremely expensive and therefore not commonly used. SEBS can be used to adhere MB sheets and not compromise performance compared to typical asphalt.

Modified mopping coal tar was introduced in the mid-1990s, but it has very limited market share.