Photograph of  really worn out asphalt roof shingles Modified Bitumen Roofing Types, Product Sources, Installation, Defects, Repairs

Modified bitumen roofs:

This article describes modified bitumen roofing materials, choices, installations, inspection, defects, roofing repairs, and product sources. Modified bitumen roofs or "mod bit" roofs are a single ply roofing systems applied using heat to seal seams between runs of roofing material.

Page top photo courtesy of W. David Schwaderer.

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Modified Bitumen Roofing Materials, Choices, Costs, Life Expectancy, Characteristics

SBS Modified Bitumen - Johns ManvilleArticle Contents

Conventional Modified Bitumen

Modified bitumen (modified asphalt) or "mod bit" roofing material is sold in rolls and applied usually on low-slope or flat roofs. Its seams are sealed using a torch to heat the under-side of the bituminous material that coats both sides of a polyester or fiberglass reinforced mat.

Modified bitumen roofs are installed using one of several methods: hot applied, cold applied, heat weldable (torched seams), and self-adhered.

While we consider this material quite durable, the manufacturer's label typically warrants its life for just ten years.

SBS-Modified Bitumen installation sketch (left) courtesy of Johns Manville Roofing.

What is the meaning of "modified" in modified bitumen roofing? Quoting from ARMA,

Modified bitumens generally use a traditional waterproofing medium -- asphalt -- modified with atactic polypropylene (APP), styrene butadiene styrene (SBS), synthetic rubber or other agents that create a uniform matrix that enhances the physical properties of the asphalt. SBS and APP are the most common bitumen modifiers.

As Carson Dunlop's sketch (below) illustrates a simple single-ply modified bitumen roof installation and a two-ply modified bitumen roof installation procedure. [Click to enlarge any image]

Modified bitumen roof installation sketch one-ply (C) Carson Dunlop AssociatesModified bitumen roof installation sketch 2-ply (C) Carson Dunlop Associates

The Asphalt Roofing Manufacturers' Association (ARMA) describes Modified Bitumen Roofing as follows:

Modified bitumen membranes -- MBS -- combine the features of a built-up roof with the added tensile strength from its polymer modification. Using a reinforced sheet that is prefabricated in the plant, modified bitumen systems require a less labor-intensive application and can be applied cross-platform in both commercial and certain residential applications.

A modified bitumen roofing system is composed primarily of polymer-modified bitumen reinforced with one or more plies of fabric such as polyester, fiberglass or a combination of both. Factory surfacing, if applied, includes mineral granules, slag, aluminum or copper. The bitumen determines the membrane's physical characteristics and provides primary waterproofing protection, while the reinforcement adds strength, puncture resistance and overall system integrity.

Factory-assembled, modified bitumen membranes undergo strict quality control standards to ensure uniform thickness and consistent physical properties throughout the membrane. The finished roofing system is usually a two- to four-ply system consisting of a modified bitumen membrane and a base sheet, with additional plies for added strength if needed. The substrate often determines which ply system is best specified.

The finished roofing membrane may consist of one or more modified bitumen sheets, or it may be comprised of a combination of built-up roofing (BUR) felts and one or more modified bitumen sheets. The type of substrate and the performance objectives influence the specification of the modified bitumen membrane system.

According to the US NPS, in discussing use of modified bitumen on historic buildings in roofing today:

Modified bitumen roofs involve some traditional materials, but use modern fabrication methods, and traditional or more contemporary installation techniques. Modified bitumen roofs are made from prefabricated rolls of modified asphalt (or coal tar) reinforced with a fiberglass or polyester reinforced mat. Rubber-modified asphalts, such as styrene-butadiene-styrene (SBS) materials, are granular surfaced and are normally installed in two or more plies using mopping asphalt, cold adhesives, or torch welding. Plastic-modified asphalts such as atactic polypropylene (APP) systems are smooth or granular surfaced and can be heat welded or laid in cold adhesive.

How to Identify Modified Bitumen versus Roll Roofing on buildings

First, how can an inspector quickly examine roll roofing material on a building surface to determine if it is asphalt roll roofing or modified bitumen roofing?

Try looking at the material edges: modified bitumen roofing is thicker, and its edges are sealed by heating with a torch - you should see a little runout of melted bitumen at the material seams. If there is no runout the roof may have been adhered using some other method, but if it was "torched" it was not heated sufficiently and may be less durable.

Second measure the width of material between seams parallel to the roof eaves. Since modified bitumen is typically 39" wide and overlapped just an inch or two, the measurement will be wider than lapped 36" asphalt roll roofing whose interstitial spaces will be less than 36".

Third, not only is the modified bitumen usually thicker and more pliable than asphalt roll roofing, it also is better at resisting tearing and breaking. If you find that it is easy to (very very slightly) tear into the roof material edge (don't try this where a leak or cosmetic damage will appear) it's probably roll roofing not mod-bit.

Modified Bitumen Roofing Properties & Installation Methods

As stated in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

Most modified-bitumen roofs are torch-applied, although there are also self-adhesive and cold-process systems. The waterproofing membrane, sometimes called “single-ply modified,” consists of asphalt bitumen reinforced with a polyester or fiberglass fabric and modified with polymers to give it greater strength, flexibility, resistance to UV degradation, and resistance to heat and cold.

A variety of different chemical formulations have been tried over the years. It is best to stick to a product with an established track record. In general, modified-bitumen roofs can be applied to slopes as shallow as 1/4 inch per foot.

Modified Bitumen Roofing Product Properties

Modified Bitumen
Application Methods Compatible Decks Warranty Codes, Comments
Firestone UltraWhite[5] Hot asphalt
Heat welding
MB Cold Adhesive
Concrete (structural)
Wood (structural)
20-Year Red Shield® available Granule coated, bright white surface, high reflectivity reduces cooling costs. Does not require liquid roof coating treatment. Available in SBS or APP.
APP-Modified Bitumen - Heat welded
- Cold-applied-adhesives

- Extra wear-resistant granule-coated surface,
- Non-woven polyester mat
- Sold in both smooth & granule-coated forms.
- Sold in various thicknesses
- Used: re-roofing & new roofing

SBS Modified Bitumen - Heat welded
- Cold-applied adhesives
- Hot asphalt
- Mechanical-attachment
    - Extra strength compared with traditional asphalt-based roofing products, high resistance to elongation & fatigue
- Glass-reinforced polyester mat
- Sold in black, white, other colors, granule-surfaced products;
BUR - Hot asphalt
- + mechanical attachment ?

- 4-ply or 6-ply
- Asphalt-impregnated fiberglass mat reinforced felt installed in multiple layers + floodcoat + aggregate
- Can be used in combination with modified bitumen

Self-Adhered Base - Cap sheet torch applied over base sheet     - 2-ply self-adhered base using Firestone MB Base SA + either an APP or SPP cap sheet
Two-Ply SBS or APP, Cold-Adhesive SBS or APP base sheet cold-adhesive bonded to cover board;
SBS or APP cap sheet cold-adhesive bonded to base sheet

Typical application: installed over cover board mechanically glued atop solid insulation that has been mechanically attached to the roof deck.

Two-Ply SBS glass fiber reinforced base+cap sheet SBS glass fiber reinforced cap sheet torch applied over SBS base sheet.     Typical application: installed over two layers of foam board insulation both mechanically bonded to the roof deck & together

1. APP = atactic polypropylene products.

2. SBS = styrene-butadiene-styrene products. SBS polymers are blended with asphalt to obtain properties of flexibility & resistance to thermal movement at low temperatures.

3. BUR = built-up roofing systems

Both APP & SBS modified bitumen roofing from Firestone include a fiberglass / polyester reinforcing non-woven mat. SBS flashing is used on traditional built-up roof (BUR) roofing systems;

SBS cap sheets & flashing are used on hybrid BUR roofing systems
SBS base sheets, cap sheets & flashings are used in 2-ply modified bitumen roofing systems. Firestone's literature describes the product's reinforcing fiber mat as including both non-woven polyester fibers (flexibility & puncture resistance) and continuous glass fiber materials (strength & stability) integrated into a single reinforcing fabric.

Source: Firestone Roofing Products [5]

Modified Bitumen Roofing Product Heat Reflection & Cooling Properties

Product Type Roof Reflectivity1 Roof Emissivity2 Roof Initial SRI3 Comments
SBS Products 0.72 0.84 87  
SBS FR Products 0.72 0.92 89  
APP Products 0.72 0.89 89  
APP FR Products 0.72 0.90 89  

1. Reflectivity =

2. Emissivity =

3. SRI = solar reflectivity index.

Source: Firestone Roofing Products [5b]

Installation Methods for Modified Bitumen Roofs

A torch-applied, or torchdown, roof starts with a nonflammable base sheet made of asphalt-saturated felt or fiberglass that is mechanically attached to the roofing deck. In residential construction, the base sheet is usually attached with roofing nails driven through metal caps.

The second layer is the waterproofing membrane, or cap sheet. This is heated with a torch as it unrolls, fusing it to the base sheet, to itself at seams, and to penetrations such as skylights. Installers must learn to heat the membrane so it is hot enough to fuse but not so hot as to burn through.

Membranes may be either smooth or have a granular surface like roll roofing. Smooth-faced membranes need a third coating, which has colored or reflective pigments to protect against UV radiation. The smooth type is preferable where foot traffic is expected or where decking is going over the roofing.

Torchdown roofing is self-flashing and uses no adhesives or solvents to seal around openings. The material can be run up parapets and abutting wall, and patches are used to seal around metal skylight curbs and similar openings. A special patching compound is used to seal to PVC stacks. If applied correctly, the torchdown membrane is essentially seamless.

Pros and Cons of Modified Bitumen Roofs

Modified bitumen is easily repaired without solvents or adhesives. It is compatible with asphalt shingles and asphalt compounds, although patching with roofing cement is not recommended. The reinforced fabric layer isolates the membrane above from building movement and gives the material enough strength to support occasional foot traffic.

The main drawback of modified bitumen roofing is the risk of fire during installation. While the risk of fire is low in the hands of trained installers, care must be taken when using torchdown on a wood-frame structure. A number of fires have started with sawdust that has accumulated in empty cavities, such as crickets and parapets. Inspection of the roof for sawdust pockets while it is being framed is advised.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Typical Slopes for Modified Bitumen Roof Systems

Modified bitumen roofing is normally installed on low-slope roofing, up to 3" in slope, or depending on the application method, up to 6" of slope per foot may be permitted. Because there are quite a few approved installation methods, manufacturers such as Johns Manville and others typically use a alphameric roof application method name that encodes the basics of how the roof covering should be installed.

For example a JM 3FID-HW modified bitumen roof installation specification would require 3 plies, of Fiberglass cap sheet, installed on an Insulated substrate, D=capped with an SBS granule-surfaced cap sheet, HW=Heat Welded.

Wear and Maintenance on Modified Bitumen Roofs

Flat roof alligatoring (C) Carson Dunlop AssociatesAn an older mod-bit roof, particularly an older bitumen roof whose material lacked the now-common protective coating of mineral granules, you may see fine alligatoring or cracking as its surface dried.

Modified bitumen roofing manufacturers such as Johns Manville advise that at the time of unrolling and installation, "...a small amount of light cracking of the coating can be normal. These minor visual imperfections do not affect the waterproofing performance or integrity of the ... materials. For aesthetic purposes only, a recoating may be applied using JM ... Coating."

But deeper and more extensive cracks (see Carson Dunlop's illustration at left) in any roofing material are an indication of wear, age, and shortened remaining life. In a freezing climate, the action of water and ice in such cracks can lead to an accelerated wear in the latter portion of the roof's life. We have successfully extended the life of modified bitumen roofs with modest cracking by coating with a roof paint formulated for use on this material. Don't use a roof coating not recommended for modified bitumen roofs or its solvents may damage the roof.

Guide to Selection & Use of Coatings, Paints, Sealants for Low-Slope Roof Surfaces

Roof coatings used on low slope roof coverings improve the roof life by increasing the solar reflectivity and may also reduce building cooling costs if the roof surface color is changed from a darker (say black EPDM) to lighter (tan or gray) surface.

Watch out: some roof coatings are chemically incompatible with some roof covering materials and can dissolve or otherwise damage the roof surface. Be sure to check the compatibility of the specific roof paint or coating you are considering using against the properties of the roof covering material itself. And as you'll see in notes to the table below, even when an acrylic roof coating is to be applied, a base coat of special sealant is recommended for EPDM roofing and is required when coating Asphalt based roofing products.

Firestone's AcryliTop™ PC-100 roof coating is an white, tan, or gray acrylic sealant that, according to the company, can be applied to all APP, granulated SBS, smooth BUR, and RubberGuard™ EPDM roof membrane systems. The tan or gray systems are typically applied to EPDM roofs.

Roof Coating Products, Properties, Applications

Product Compatibility Colors Reflectance Emittance Comments
Initial 3-year Initial 3-Year
Acrylic Top Coat1 all APP, granulated SBS, smooth BUR, EPDM White 0.82 0.73 0.87 0.84  
Tan 0.53 0.48 0.68 0.83  
Gray 0.32 0.34 0.88 0.82  
Acrylic Base Coat2 EPDM        
Acrylic Base Coat3 Asphalt        


1. Firestone AcryliTop™ PC-100 Roof Coating

2. Firestone AcryliTop PC-100 Base Coat for EPDM - applied before the top coat

3. Firestone AcryliTop PC-100 Base Coat for for Asphalt roofing must be used when applying the Acrylic Top Coat

Source: Firestone Roofing Products [5b]

Examples of Defects in Modified Bitumen Roofs

Our photographs of modified bitumen roofing shown below indicate three conditions found on a small walk-out balcony on a 1935 Poughkeepsie New York home that had been re-roofed in the prior year.

Modified bitumen roof seam issues (C) Daniel Friedman Modified bitumen roof seam issues (C) Daniel Friedman

Modified Bitumen Roof Wear From Walking-on Traffic

At left we can see that in less than a year there is some granule loss and wear from foot traffic, especially where people step on the modified bitumen roof membrane as exiting and entering the door into the building. While light, careful walking on a modified bitumen roof may be acceptable for the purpose of inspection or maintenance, on a balcony where regular walking is anticipated, the roof will not wear well.

Inadequately Heated Modified Bitumen Roof Seams

Second, in the same photograph we don't see any melted runout at the heated, lapped roofing seam. It is possible that this seam was inadequately torched or heated during installation. Elsewhere on the roof we noticed granule loss at wrinkles in the roofing membrane, perhaps caused by foot traffic over the raised edges of the wrinkles.

Modified Bitumen Roof Edge Bending & Wear

Our modified bitumen roof photo at right shows that the installer allowed the edge of the roof membrane to run wild about 2 inches past the roof edge. You may notice a bit of granule loss on this component as well. While it's more of a problem with asphalt roll roofing than modified bitumen (which is thicker, more flexible, and in some products, reinforced) running an inch or two of unsupported roofing out past the roof edge invites bending and early wear or cracking in that location.

Modified Bitumen Roof Shingles - SuperKatepal™

Super Katepal modified bitumen roof shingles SuperKatepal™ a Finnish SBS modified bitumen [elastomer bitumen] roof shingles look like a more conventional asphalt shingle roof.

But unlike a conventional asphalt roof shingle that is built on a fiberglass or organic (bitumen impregnated felt) base, modified bitumen shingles are constructed from what appears to be essentially the same material as modified bitumen roll roofing products applied on low-slopes.

Modified bitumen is a durable roofing product that tolerates bending and temperature extremes. Mineral-granule-coated modified bitumen roofs [perhaps a precursor to the hybrid product described here], by adding sun protection, are more durable, and a common industry claim is that the material life is double that of ordinary [not-modified bitumen- based roofing products].

Here is how the manufacturer's sales brochure and other product literature from katepal describe the product

SuperKatepal shingles

SuperKatepal shingles are manufactured from SBS elastomer bitumen. The product has glass felt reinforcement, which guarantees excellent dimensional stability. Thanks to the SBS elastomer bitumen the shingles are watertight and pliable.

The shingles have a surfacing of slate and/or mineral granules. The undersurface consists of self-adhesive bitumen under a protective film and a sand coated area. The granular surfacing gives the product its colour, a matt finish that blends in well with the natural and built environment, a rough friction surface, UV protection and the required fire safety properties (fire rating: BROOF (t2) and BROOF (t1)).


Katepal roofing shingles combine the durability of stone and the waterproofing qualities of bitumen with excellent elasticity. Thanks to the staggered assembly all joints are covered, and the roof is waterproof at nailing points and joints. Durability also means that the surface of the shingle does not peel or rust. Snow and ice accumulating on the rough mineral granule surface stay on the roof in the winter and do not fall on pavements or garden plants. In rain and hard wind bitumen roofing shingles are comfortably quiet.

SuperKatepal™ SBS modified bitumen is nonetheless installed over an underlay membrane.


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