InspectAPedia offers impartial, unbiased advice without conflicts of interest.
We will block advertisements which we discover or readers inform us are associated with bad business practices,
false-advertising, or junk science. Our contact info is at
InspectAPedia.com/Contact.htm.
This article describes modified bitumen roofing materials, choices, installations, inspection, defects, roofing repairs, and product sources.
Page top photo courtesy of W. David Schwaderer.
Because modified bituminous roofing, sold in rolls and applied to low-slope roof areas can be mistaken for mineral-granule-coated roll roofing, also see ROLL ROOFING, ASPHALT.
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.
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.
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.
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.
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.
As Carson Dunlop's sketch (below) illustrates a simple single-ply modified bitumen roof installation and a two-ply modified bitumen roof installation procedure.
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.
Wear and Maintenance on Modified Bitumen Roofs
An 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.
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 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™
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. [See
Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
Additional technical contributors & reference sources for this article are listed below.
Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
ARMA Asphalt Roofing Manufacturer's Association, 750 National Press Building, 529 14th Street, NW, Washington, DC 20045 Tel: 202 / 207-0917. Quoting from the association's website The Asphalt Roofing Manufacturers Association (ARMA) is the North American trade association that represents the majority of the asphalt roofing industry’s manufacturing companies and their raw material suppliers. Together these companies produce a variety of bituminous-based residential and commercial roofing systems, including asphalt shingles, roll roofing, built-up roofing systems and modified bitumen-roofing systems.
- Web Search 07/12/2010 original source http://www.asphaltroofing.org/resources_faq.html#ls2.
Example of SBS heat weldable modified bitumen installation encoding and instructions was found at
http://www.specjm.com/products/system.asp?system=sbsbituminous&type
=heat&main=application&tab=specs Quoting further details about the composition of modified bitumen roofing material
SBS (Styrene-Butadiene-Styrene) modifies the asphalt by forming a polymer network within the bitumen. SBS gives the bitumen rubber-like characteristics and improved resistance to aging and weathering. Most SBS-modified bitumen sheets are either set in hot mopping asphalt, torch-applied or adhered with cold-process adhesives. SBS-modified bitumen sheets that do not have factory applied granule or foil surfacing need some form of field-applied ultraviolet protective coating.
APP (Atactic Polypropylene) is a thermoplastic polymer which forms a uniform matrix within the asphalt. This enhances the bitumen’s performance by increasing its UV resistance, increasing its flexibility at low temperatures and improving its flow resistance at high temperatures. APP-modified bitumen sheets are generally applied using a propane-fueled torch. Applicators use the heat to soften the modified bitumen on the underside of the sheet. The sheet's bottom surface becomes a molten adhesive which flows upon the substrate and then cools to form a waterproof adhesive bond. Some APP sheets can also be applied with cold process adhesives.
"Choosing Roofing," Jefferson Kolle, January 1995, No. 92, Fine Homebuilding, Taunton Press, 63 S. Main St., PO Box 5506, Newton CT 06470 - 800-888-8286 - see http://www.taunton.com/FineHomebuilding/ for the magazine's website and for subscription information.
Johns Manville Corporation, JM Corporate Headquarters, 717 17th Street, Denver, Colorado 80202
Phone: 303.978.2000, (800) 654-3103 - Web Search 05/22/2010 original source: http://www.specjm.com/commercial/roofing/sbsbituminous.asp
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
Katepal Oy, Katepal Oy, P.O. Box 33, FI-37501 LEMPÄÄLÄ
Customer service +358 (0)3 375 9111
Fax +358 (0)3 375 0974
E-mail: katepal@katepal.fi
www.katepal.fi
Quoting from the company's brochure: Founded in 1949, Katepal Oy is the leading manufacturer of bitumen-based roofing materials
and bitumen products in Finland. Katepal roofing shingles comply with the requirements of the
European Standard EN 544.
Katepal roofing shingles are type approved by the Finnish Ministry of the Environment and
several other countries where our products are exported. Katepal complies in all its operations
with the requirements of a certified ISO 9001 quality system.
"On file as /roof/Asbestos-to-Zinc_Metal_Roofing_NPS_2.pdf - From Asbestos to Zinc, Roofing for Historic buildings, Metals- Roofing Today - ", Technical Preservation Services, National Park Service, U.S. Department of the Interior, web search 9./29.10, original source:
http://www.nps.gov/history/hps/tps/roofingexhibit/roofingtoday.htm
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
Roof failure causes in depth (and specific methods for avoiding them)
Roof design fundamentals and flourishes, based on voluminous industry research and experience
New technologies and materials -- using them safely and correctly
Comprehensive coverage of all major roofing systems
pecifications, inspection, and maintenance tools for roofing work
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
Mobile View
