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.

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

Article Series Contents

• MODIFIED BITUMEN ROOFING - home
• DEFINITION OF MODIFIED BITUMEN
• IDENTIFY MODIFIED BITUMEN vs ROLL ROOFING
• MODIFIED BITUMEN PROPERTIES & INSTALLATION
• MODIFIED BITUMEN ROOF COATINGS & PAINTS
• MODIFIED BITUMEN SHINGLES - Modified bitumen roof shingles - SuperKatepal
• MODIFIED BITUMEN ROOF DEFECTS & ROOF LIFE
• MODIFIED BITUMEN ROOF DEFECTS & ROOF LIFE FAQs
• MODIFIED BITUMEN ROOF WEAR & MAINTENANCE
• MODIFIED BITUMEN ROOF BUCKLING PROBLEMS
• MODIFIED BITUMEN ROOF LIFE RESEARCH

SBS-Modified Bitumen installation sketch above courtesy of Johns Manville Roofing.

[Click to enlarge any image]

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.

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.

Carson Dunlop Associates' sketches illustrate a simple single-ply modified bitumen roof installation and a two-ply modified bitumen roof installation procedure. [Click to enlarge any image]

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 (Steve Bliss, J Wiley & Sons) , 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

Notes to the table above

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]

FLINTASTIC SA SBS MODIFIED BITUMEN PRODUCT GUIDE [PDF] CertainTeeed, retrieved 2023/01/30, original source: https://www.certainteed.com/resources/RPG_Flintlastic-SA-Brochure.pdf

Notes to the table above: Definitions of Roof Reflectivity, Emissivity, and SRI or Solar Reflectivity Index

1. Reflectivity of a roof surface is defined as the percentage of solar energy that strikes and is then reflected by a surface.

Example: a roof with 50% reflectivity is one that reflects back 50% of the solar energy striking its surface.

The other 50% of solar energy striking the roof surface is absorbed as heat.

2. Emissivity of a roof surface is defined as the efficiency with which a surface releases heat.

Example: a roof with a 50% emissivity is one that subsequently emits or releases 50% of the heat that it has previously absorbed.

The actual total amount of energy or heat released by the surface of any material or surface is, of course affected, perhaps dominated by additional factors including the temperature difference between the roof and the surrounding air and effects of wind.

3. SRI or solar reflectivity index is defined as a measure of the ability of a roof surface to reflect solar heat or heat from sunlight falling on the surface.

To make sense of an SRI or solar reflectivity index of a particular roof surface, color, or material, compare its SRI with these two standards:

A standard black roof surface is defined as one that has a solar reflectivity of 0.05 (5%) and a solar emissivity of 0.90 (90%).

A standard white roof surface is defined as one that has a solar reflectivity of 0.80 (80%) and a solar emissivity of 0.90 (90%).

Source: Firestone Roofing Products [5b] and various definitions from other roofing sources.

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 (Steve Bliss, J Wiley & Sons) .

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.

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.

Notes to the table above

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]

Modified Bitumen Roof Shingles - SuperKatepal™

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 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.

Defects in Modified Bitumen Roofs

This article content has moved. Please see MODIFIED BITUMEN ROOF DEFECTS & ROOF LIFE

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.

Maintenance on Modified Bitumen Roofs

Moved for space and faster loading: please see MODIFIED BITUMEN ROOF WEAR & MAINTENANCE.

Modified Bitumen Roof Buckling

This discussion has moved to MODIFIED BITUMEN ROOF BUCKLING PROBLEMS

Modified Bitumen Roof Life

Please see this new article: MODIFIED BITUMEN ROOF LIFE RESEARCH

...

Continue reading at ROLL ROOFING, ASPHALT & SBS - modified bituminous roofing, sold in rolls and applied to low-slope roof areas can be mistaken for mineral-granule-coated roll roofing, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see MODIFIED BITUMEN ROOFING FAQs - questions and answers about mod-bit roofs that were posted originally on this page.

Or see these

Recommended Articles

• MODIFIED BITUMEN ROOFING - home
• DEFINITION OF MODIFIED BITUMEN
• IDENTIFY MODIFIED BITUMEN vs ROLL ROOFING
• MODIFIED BITUMEN PROPERTIES & INSTALLATION
• MODIFIED BITUMEN ROOF COATINGS & PAINTS
• MODIFIED BITUMEN SHINGLES
• MODIFIED BITUMEN ROOF DEFECTS & ROOF LIFE
• ROOF LEAK SOURCE DIAGNOSIS

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