Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
ROOFING INSPECTION & REPAIR
AMERICAN CEMWOOD ROOFING
BEST ROOFING PRACTICES
BUILT UP ROOFS
CATHEDRAL CEILING VENTILATION
CERTIFICATIONS for ROOFING CONTRACTORS
CHIMNEY FLASHING Mistakes & Leaks
COLD WEATHER ROOF TROUBLE
DECKS, ROOFTOP CONSTRUCTION
EPDM, RUBBER, PVC ROOFING
EXTRACTIVE BLEEDING on SHINGLES
FIRE RETARDANT PLYWOOD
FLASHING on BUILDINGS
FLAT ROOF MOISTURE & CONDENSATION
Green House or Solarium Roof Leaks
HEAT TAPES & CABLES on Roofs for Ice Dams
ROOF ICE DAM LEAKS
MASONITE WOODRUF FIBERBOARD ROOFING
NOISE CONTROL for ROOFS
PLASTIC ROOFING TYPES
PVC, EPDM, RUBBER ROOFING
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF CLEANING RECOMMENDATIONS
ROOF COLOR RECOMMENDATIONS
ROOF INSPECTION SAFETY & LIMITS
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
SADDLE CONSTRUCTION at CHIMNEYS
SNOW GUARDS & SNOW BRAKES
STANDARDS for ROOFING
STRESS SKIN INSULATED PANELS
TEST LABS - ROOF SHINGLE
TREES & SHRUBS, TRIM OFF BUILDING
TRUSSES, Floor & Roof
UNDERLAYMENT REQUIREMENTS on ROOFS
VENTILATION in BUILDINGS
WALK-ON ROOF SURFACES
WARRANTIES for ROOF SHINGLES
WORKMANSHIP & ROOF DAMAGE
Modified bitumen roofs: 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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
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,
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]
The Asphalt Roofing Manufacturers' Association (ARMA) describes Modified Bitumen Roofing as follows:
According to the US NPS, in discussing use of modified bitumen on historic buildings in roofing today:
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 Product Properties
Installation Methods for Modified Bitumen Roofs
Pros and Cons of Modified Bitumen Roofs
-- 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.
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.
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.
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.
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™ SBS modified bitumen is nonetheless installed over an underlay membrane.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs) about modified bitumen roofs: installation, specifications, life, repairs
Questions & answers or comments about mod-bit modified bitumen roofs.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References