Spontaneous Combustion of Roofers Mops: A Review

May 15, 2003

August 2003 Interface • 9
Conventional built-up roof
(BUR) assemblies have
lost market share in
recent decades to other roofing
options. While BUR can provide
years of dependable service at a
competitive price, concerns
regarding the use of hot asphalt
have been expressed by building
owners and insurance companies
with primary concerns
regarding fumes and fire. One
particular cause of fire is the
spontaneous combustion of
roofers’ mops. A recent case
involving this phenomenon
revealed the absence of readily
available information. This article
is intended to provide a
summary of the basics that are
important to understanding this
phenomenon, the information
available, and recommended
steps to avoid such an incident.
Summary of the
Spontaneous combustion of roofing mops can occur when oxygen
from the air slowly unites with the flammable asphalt. As oxidation
takes place, heat accumulates in the mop. The solidified
asphalt on the exterior of the mop head retains the heat inside of
the mop, and smoking of the mop begins. Under the right circumstances
(mop head material, the mass of asphalt available, ambient
air temperature, etc.), the mop will eventually catch on fire. If
the mop remains on the roof or is adjacent to other flammable
building components, this process can be disastrous.
This fire at a large mall in the Southeast started as the result of spontaneous combustion of
a roofing mop used earlier in the day.
10 • Interface August 2003
Available Information
There is a fair amount of information available on the Internet
related to this type of spontaneous combustion. However, much of
this information is in the form of incident reports, newspaper articles,
and technical bulletins that are not common to the library of
the typical roofing contractor and/or consultant. A little more
research revealed the information summarized below.
• In the January 1971 issue of Professional Roofing, NRCA
printed an October 15, 1970, Eastman Kodak Company
memorandum that discussed “Self-Heating of Asphalt
Saturated Roofers’ Mops.” This memo indicated that spontaneous
combustion of cotton mops had occurred when
mops that were saturated with asphalt were set aside.
Testing of both cotton mops and fiberglass mops was performed
by Kodak and summarized by this memo. The tests
were able to produce spontaneous combustion of the cotton
mops and were not able to produce spontaneous combustion
of the fiberglass mops.
The memo also indicated that cotton is degraded when
exposed to elevated temperatures. This degradation
increases the tendency of self-heating. Additionally, the
tests revealed that the cotton mop exposed to a breeze
ignited in a shorter amount of time than the cotton mop
that was in a protected environment. This condition suggests
that the breeze increased the rate at which the
asphalt was oxidized and thereby increased the rate of
temperature gain.
The memo concludes by recommending that “fiberglass
mops should be substituted for the cotton mops in any
roofing operations in the future.”
• Page 23 of NRCA’s student manual, “Introduction to Builtup
and Modified Bitumen Roof Membranes,” (developed in
July 2000) states, “When you are done with a mop, do not
leave it sitting on a roof. Hot mop heads stay hot for a long
time. When they are left bunched up, they can burst into
flames. Allow mop heads to sit out in an open area before
you throw them away. At the end of a day, spin mop heads
out and set them on something that is not combustible,
such as a piece of plywood. Do not leave them on the roof.”
It should be noted that there may be better choices of
“non-combustible” material than plywood.
While no information was found specifically concerning spontaneous
combustion, the following relevant excerpts were found in
the 2001 Code of Federal Regulations, Title 29, Part 1926, Labor
• 1926.24–Fire Protection and Prevention: “The employer
shall be responsible for the development and maintenance
of an effective fire protection and prevention program…”
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August 2003 Interface • 11
• 1926.25–Housekeeping:
“Containers shall be provided
for the collection
and separation of waste,
trash, oily and used rags,
and other refuse.
Containers used for
garbage and other oily,
flammable, or hazardous
wastes, such as caustics,
acids, harmful dusts,
etc., shall be equipped
with covers. Garbage and
other waste shall be disposed
of at frequent and
regular intervals.”
• 1926.150–Fire Protection:
“Access to all available
equipment shall be
maintained at all times.”
• 1926.252–Disposal of
Waste Materials: “All solvent
waste, oily rags, and
flammable liquids shall be kept in fire resistant covered
containers until removed from worksite.”
Based on the information presented above, the following steps
are recommended for avoiding spontaneous combustion of roofers’
1. Consider the mop head material (fiberglass vs. cotton);
2. Remove excess asphalt from roofing mops at the conclusion
of use;
3. Consider quenching the mop head in a bucket of water;
4. Store used mops in sealed metal containers;
5. Keep used mops off of the roof and away from combustible
materials; and,
6. Comply with all applicable standards and/or ordinances
regarding worksite safety and fire protection. ■
NRCA, “Spontaneous Heating of Roofers’ Mops,” Professional
Roofing, January 1971.
NRCA, “Introduction to Built-up and Modified Bitumen Roof
Membranes” Student Manual, July 2000.
Code of Federal Regulations, Title 29, Part 1926, Labor, July
1, 2001.
HVAC equipment on the roof of the mall destroyed by the fire.
Derek A. Hodgin, PE, RRO, RRC,
CDT, is a forensic engineer employed
by Campbell, Schneider and
Associates, LLC (CSA), an A/E consulting
firm based in Charleston, SC.
Hodgin is licensed as a Professional
Engineer in 14 states, registered as a
Roof Observer and Roof Consultant
with the Roof Consultants Institute,
is certified as a Third Party EIFS
Inspector and Moisture Analyst with
the Exterior Design Institute (EDI),
and as a Construction Document Technologist with the
Construction Specification Institute (CSI). Derek currently manages
a branch office for CSA in Westminster, SC (near
Clemson). Hodgin specializes in failure investigations of all
types of building envelopes and roof systems. He has investigated
numerous types of residential and commercial building
failures related to hurricanes, tornadoes, hail, fire, ice, and
deficient construction. He has also designed high wind resistant
roof assemblies for projects in the southeastern U.S. and
Caribbean. His technical articles have appeared in numerous
trade publications and symposia proceedings.
Construction workers with drug problems have twice the rate of hospitalization for injury
care as non-drug users, according to a pair of studies by a Duke University researcher. The
study was recently published in the American Journal of Industrial Medicine and may be
obtained by calling Health Matrix at 703-918-4930.
— Construction Executive &
DDrruugg AAbbuussee