Modified Bitumen Roofing • Introduction to Mod Bit Roofs

By Jack Gray, Roof Online Editor • Last updated March 24, 2023

torch-applying a roll of modified bitumen roofing
Torch-applying a roll of modified bitumen roofing

Table of Contents

Newly installed modified bitumen roofing.
Newly installed 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. Modified bitumen is “modified” when a polymer (a synthetic rubber or plastic) is mixed into the asphalt (the “bitumen”) during the manufacturing process.

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 low-slope roofing materials when it comes to maintenance and repair, because of the variety of easy-to-use asphalt roofing products available.

tan modified bitumen roof
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 rubber or plastic) which increase the durability, strength, and flexibility of the asphalt. The final product will contain anywhere from 3% to 15% of the polymer additive, depending on the manufacturer and the specific type of product.

Mod bit 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 applied to each side of a reinforcement fabric, much the same way as asphalt shingles are made. 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 mod bit 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.

Additionally, layers of insulation and cover board below the roof membrane itself may lead to a total depth of the roof system of up to several inches. The actual depth of the roof system will depend on the type of insulation used and the total roof system R-value requirements of the local energy code.

rolls of modified bitumen standing on a roof
Rolls of modified bitumen roofing (cap sheet) placed for easy 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.

APP-Modified Bitumen

APP (atactic polypropylene) is a plastic mainly used to modify roofing asphalt. APP mod bit is less elastic but stronger than SBS-modified and has a higher resistance to UV radiation and high temperatures. APP mod bit 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 (styrene butadiene styrene) is a synthetic rubber with many uses, modifying roofing asphalt being just one. SBS mod bit is more elastic than APP-modified and does not become as brittle in cold temperatures. SBS mod bit 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.

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 or the other of these two types of polymer additive.

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.

Surfacing for Mod Bit 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, the asphalt has 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 on them.

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 mod bit 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 modified bitumen roof flashings
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 mod bit roofing sheets:

Torch-Applied (or “Heat Welded”)

This method uses propane torches (hand-held torches for flashing materials, and rolling “torch wagons” for the field of the roof) to melt the asphalt on the underside of the mod bit rolls. The melted asphalt bonds the roofing to the substrate.

Although this was a very common application method in the past, it has fallen out of favor with the development of alternatives such as cold-process adhesives and peel-and-stick (self adhering) mod bit membranes.

Many municipalities now ban torch-application of modified bitumen due to the risk of fire. Anyone performing a torch-application should also ensure the presence of a fire monitor on the roof for an appropriate time after the day’s work is complete to ensure that no hidden, smoldering insulation, wood blocking, or other flammable substrate results in a fire on the roof.

One hour is typically the minimum amount of time the fire monitor should stay on the roof, but the roofing manufacturer or the local code may have other recommendations or requirements. Always check with the local fire marshal’s office before proceeding with a torch-down roofing project.

Mopped in with Hot Asphalt

This method uses hot asphalt melted in special asphalt roofing kettles to bond the modified bitumen rolls to the substrate. Hot liquid asphalt is transferred from the kettle to the area of application and mopped on to the roof. The mod bit is rolled out into position and set into the hot liquid asphalt.

This method is effective with SBS-modified bitumen but not with APP, which has a more unforgiving application temperature range. Contractor error regarding the use of APP vs SBS modified material is something to watch out for.

The hot asphalt method has also fallen out of favor recently, mainly due to the serious decline of built-up roofing in general. Most roofing contractors no longer maintain the asphalt kettles and other hot asphalt application equipment needed for this method.

Cold-Process Adhesive

Instead of hot asphalt from a kettle, asphaltic adhesives can be brought to the roof in buckets and poured onto the substrate at the ambient temperature. The liquid is then squeegeed out to the proper application rate, and the roofing is rolled into it as with hot asphalt.

The first generation of cold-applied adhesives had a serious odor issue due to the high level of VOC’s in the available products. These fumes could make their way into the building and cause problems for the occupants.

According to the manufacturers, this is no longer an issue, as current products are “low VOC” and have almost no odor.

This video from Siplast gives you a remarkably detailed look at how modified bitumen roofing is installed in cold-process adhesive:

Siplast - Cold Adhesive Application

Self-Adhering (Peel-and-Stick) Membrane

The latest development in mod bit application technology, peel-and-stick rolls of mod bit come from the factory with adhesive already applied to the back of the sheets, protected by a plastic film.

A compatible underlayment, cover board, or both specified by the manufacturer is installed on the roof. The rolls of mod bit are unrolled out into position on the roof. The protective film is then pulled off the bottom of the material and the roof membrane is pressed down for full adhesion, usually with a specialized heavy roller.

This video from the Garland Company walks you through the installation of a peel-and-stick modified bitumen system:

Installing Self-Adhering Modified Bitumen System; Garland Roofing

About the Author

Jack Gray is a principal roof consultant and vice president at the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. He is also the editor of the Roof Online website.

Mr. Gray has worked in the roofing industry for over 25 years, with training and practical experience in roof installation, roof inspection, roof safety, roof condition assessment, construction estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management.

He was awarded the Registered Roof Observer (RRO) professional credential in 2009.

He also served as an infantry paratrooper in the 82nd Airborne Division and has a B.A. from Cornell University. Read full bio.

1. Book Recommendation: Manual of Low-Slope Roof Systems: Fourth Edition by C.W. Griffin & Richard Fricklas

2. 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.

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

4. 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.

5.  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.

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

7. 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.

8. 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.

9. 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.

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

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

12. 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.

13. 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.

14. 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.