Modified Bitumen Roofing Guide for Building Owners

Roof Online Staff

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Torch-applying a roll of modified bitumen roofing.
Newly installed modified bitumen roofing.
Newly installed modified bitumen roofing.

Pictures of Modified Bitumen Roofing

What is Modified Bitumen?

Modified bitumen is also known as polymer-modified bitumen or rubberized asphalt. It is usually called “mod bit” in the roofing industry.

While the word “bitumen” can technically refer to either asphalt or coal tar pitch, “bitumen” means “roofing asphalt” in Europe, where modified bitumen was invented and first adopted. Coal tar pitch is not used in the production of mod bit.

Modified bitumen is an asphalt-based roofing product. It is compatible with other asphalt-based roofing products, such as asphalt roofing cement, mastic, asphalt cutback, asphalt coatings, etc.

This gives it an advantage over many other types of roofing when it comes to maintenance and repair, because of the variety of easy-to-use asphalt roofing products available.

View of a two-ply modified bitumen roof surfaced with tan granules.

How Modified Bitumen is Made

Mod bit is produced by blending raw asphalt with polymeric additives (synthetic plastic or rubber) which increase the durability, strength, and flexibility of the asphalt.

Modified bitumen maintains a more uniform strength and flexibility over a wider range of temperatures compared to the unmodified asphalt used in built-up roofs.

Modified bitumen roofing is manufactured in sheets, with layers of modified asphalt over a reinforcement fabric. The reinforcement fabric is made from either fiberglass or polyester, or a combination of the two.

The finished mod bit sheets are typically 39 inches (1 meter) wide and sold in rolls. The rolls are typically between 25 feet and 33 feet (10 meters) long.

Weight and Thickness of Modified Bitumen Roofs

Individual sheets of modified bitumen are typically between 120 mils (3.05 mm or 0.12 inches) and 180 mils (4.6 mm or 0.18 inches) thick.

A typical sheet of modified bitumen roofing weighs about 1.1 lbs. per square foot.

The vast majority of installed mod bit roof membranes have two plies, a base sheet and a cap sheet. So the average modified bitumen roof membrane (not including other roof system components) is about 300 mils (7.62 mm or 0.3 inches) thick and weighs around 2.2 pounds per square foot.

Rolls of modified bitumen roofing (cap sheet) placed for quick access during a torch-down roof installation.

Types of Modified Bitumen

Modified bitumen is referred to by the type of polymer added to the asphalt.

Although roofing asphalt is available with other modifiers, such as SEBS (styrene ethylene butadiene styrene), mod bit roof membranes are almost exclusively manufactured using one of two types of additive: APP (atactic polypropylene) or SBS (styrene butadiene styrene).

Differences in chemical structure and physical properties between the two types of modified bitumen make them slightly more or less appropriate than the other type for use on any given roof.

The chief difference is that APP-modified bitumen is preferred in warmer climates and SBS-modified bitumen is preferred in colder climates.

There aren’t any actual rules requiring the use of one type and not the other simply because of local temperatures. Other considerations may come into play with any particular roof installation. Both types of mod bit can be found wherever mod bit is used for roofing.

Both types of mod bit sheets look pretty much the same, and you will not be able to tell the difference simply by looking at or handling the material.

APP-Modified Bitumen

APP-modified bitumen is less elastic but stronger than SBS-modified and has a higher resistance to UV radiation and high temperatures. APP-modified bitumen should not be installed in hot asphalt (hot-mopping technique); it doesn’t melt and bond properly when applied this way.

SBS-Modified Bitumen

SBS-modified bitumen is more elastic than APP-modified and does not become as brittle in cold temperatures. SBS-modified bitumen can be installed in hot non-modified asphalt. Historically, this increased its popularity when the only other installation option was torch-application, which requires using open flames to melt the asphalt on the underside of the sheet.

Surfacing for Modified Bitumen Roofing

Unprotected asphalt is highly vulnerable to the sun’s UV rays. The UV radiation breaks down the long-chain hydrocarbon molecules that give asphalt its strength and durability. This is an issue common to all asphalt-based roofing materials.

In order for asphalt-based roofing to perform for an acceptable length of time, they have to be protected from the sun. This is why asphalt shingles are covered with ceramic-coated mineral granules, and it’s the main reason why built-up roofs have gravel surfacing.

There are three main types of surfacing for the cap sheet (top ply) of mod bit roof systems:

Granule-Surfaced Mod Bit

Ceramic-coated mineral granules are embedded in the top surface of mod bit cap sheets (the top layer of a multi-layer roof membrane) at the factory, as with asphalt shingles or roll roofing. This is by far the most common surfacing for mod bit roofing.

These granules are most often some shade of gray, but custom colors are available and tan, brown, black, or white granules are common.

Foil-Surfaced Mod Bit

Metallic foil is laminated to the top surface of the cap sheet in the factory. Aluminum foil is used most often, but copper and stainless steel foil are also used.

Foil-surfaced modified bitumen is often used as flashing material on mod bit roofs which use granule-surfaced modified bitumen for the rest of the roof. This is due to the increased sun exposure of the (mostly) vertical flashings compared to the flat roof surface.

While granules tend to wear off as time goes by, gradually exposing the asphalt to more and more sunlight, laminated foil generally provides total UV protection for the life of the roof.

Foil-surfaced mod bit is relatively expensive, and the foil does not handle foot traffic well.

Foil-surfaced mod bit used as flashing material. The foil has gradually delaminated in places due to differential movement between the roof deck and the walls over the years.

Coated Mod Bit

Roof coatings are typically applied after the roof has been installed, although factory-applied reflective roof coatings are becoming more common as manufacturers try to meet the demand for reflective roofing due to changing energy codes and LEED requirements.

When applying a roof coating to an existing, uncoated mod bit roof, extremely thorough preparation of the surface is necessary. Modified bitumen does not hold a coating as well as unmodified asphalt.

When the loss of granules becomes noticeable on a granule-surfaced roof, and the asphalt on parts of the roof is exposed, a common roof maintenance practice is to apply a roof coating to those areas of the roof.

Attachment Methods for Modified Bitumen Roof Membranes

After the specified amount and type of insulation is installed over the roof deck, a cover board compatible with the specified mod bit membrane attachment method is installed over the insulation.

A cover board is always used in situations where the insulation may be damaged by a direct application of the roof membrane. There are a number of reasons that a roof membrane may not be suitable for direct application over the insulation; heat damage to the insulation from hot asphalt, for example. Cover board typically consists of a layer of gypsum board, wood fiberboard, or perlite board an inch or less thick.

The modified bitumen sheets are then attached to the cover board.

There are four acceptable methods for installing modified bitumen roofing.

  1. Torch-applied (or “heat welded”)
  2. Mopped in with hot asphalt
  3. Peel and stick
  4. Cold process adhesive

1. General: See this very informative, in-depth general article about various roof systems on the Whole Building Design Guide site, which is maintained by the National Institute of Building Sciences.

2. General: “Polymer-Modified Bituminous Roofing Systems” is a good, succinct introduction to mod bit. The article is from 2007 and archived at

3. General: “Advantages, Limitations and Selection of Modified Bitumen” was published in 1987, but still has a lot of useful and valid information. Made available by the NRCA on their website’s Technical Services Section.

4.  Building Codes: “R905.11 Modified Bitumen Roofing” from the 2018 International Residential Code and “1507.11 Modified Bitumen Roofing” from the 2018 International Building Code. Both available on the UpCodes website.

5. Chemical Compatibility of Modified Bitumen: “Potential Effects of Contaminants on Modified Bitumen Sheet Materials” contains useful information. Available on the website of the Asphalt Roofing Manufacturers Association.

6. Inspection of Modified Bitumen Roof Systems: Membrane and Flashing Condition Indexes for Modified Bitumen Roofs: Inspection and Distress Manual is an extremely detailed and valuable resource put together by the US Army Corps of Engineers’ Engineer Research and Development Center.

7. Safety: See this material safety data sheet for health and safety information about SBS-modified bitumen roofing at the Siplast website. This is only an example and other products and brands may be different.

8. Safety: See this material safety data sheet for health and safety information about APP-modified bitumen roofing at the Polyglass website. This is only an example and other products and brands may be different.

9. Technical: See this application manual for insight into how a modified bitumen roof system is put together. This guide is available on the GAF website.

10. Technical: “Cold-Applied Adhesives for Modified Bitumen Membrane Roofing” is a basic overview put together by the Roof Coatings Manufacturers Association.

11. Technical: “Self-Adhesive Modified Bituminous Roofing Membranes” – a basic introduction to peel-and-stick mod bit membranes provided at the website of the Asphalt Roofing Manufacturers Association.

12. Technical: See this product data sheet for the technical characteristics of SBS-modified bitumen roofing at the Siplast website. This is only an example and other products and brands may be different.

13. Technical: See this product data sheet for the technical characteristics of APP-modified bitumen roofing at the Polyglass website. This is only an example and other products and brands may be different.

14. Manufacturers: See this list of modified bitumen roofing manufacturers.