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136 Chapter 4 Quantities of Water and Wastewater Flows
describes three methods for calculating the fire flow requirements that were developed by
these organizations:
1. Insurance Services Office Inc. (ISO)
2. Illinois Institute of Technology Research Institute (IITRI)
3. Iowa State University (ISU)
The following general information is taken from the 2006 edition of the ISO’s Guide
for Determination of Needed Fire Flow:
1. For one- and two-family dwellings not exceeding two stories in height, the follow-
ing needed fire flows should be used:

Distance between buildings, ft (m) Fire flow, gpm (L/min)
Over 100 (over 30) 500 (1,893)
31 to 100 (9.1 to 30) 750 (2,839)
11 to 30 (3.1 to 9) 1,000 (3,785)
10 or less (3 or less) 1,500 (5,678)

2. For wood-shingle roof coverings on the building or on exposed buildings, add 500
gpm (1893 Lpm) to the above flows.
3. High-risk areas. Multifamily, commercial, and industrial areas are considered
high-risk areas. The fire flows available for these areas require special considera-
tion. The distribution and arterial mains in the high-risk areas are to accommodate
the required fire flows in those areas.
4. Typically the water main system must be able to meet the flow requirements of
(a) peak day demands plus fire flow demands as a minimum for any water main
design and (b) instantaneous peak demands for special water mains from water
source, water treatment plant, and/or water storage facilities.
5. Water storage facilities should have sufficient capacity, as required by the
Recommended Standards for Water Works (2007 edition), to meet domestic average
daily water demands and, where fire protection is provided, fire flow demands.
Fire demand can also be estimated according to the community size and realty subdi-
vision development by using the following empirical equation from the National Board of
Fire Underwriters:
0.5
0.5
Q fire 1,020 (P ) [1 0.01 (P ) ] (U.S. Customary Units) (4.22)
k
k
0.5
0.5
Q fire 3,860.7 (P ) [1 0.01 (P ) ] (SI Units) (4.22a)
k
k
where Q fire is the fire demand, in gpm for U.S. customary units and L/min for SI units; and
P is the population, in thousands.
k
The National Board of Fire Underwriters requires provision for a 5-hour fire flow in
places with populations of less than 2500 and provision for a 10-hour flow in larger places.
The ISU method is relatively simple and quick to use but yields low fire flow
requirements, whereas the IITRI method produces excessively high rates. The
ISO methodology calculates values in between the other two. The most recent
International Fire Code (International Code Council [ICC], 2006) recommends a mini-
mum fire flow of 1,000 gpm (3,785 L/min) for one- and two-family dwellings having an
2
2
area that does not exceed 3,600 ft (344 m ). Fire flow and flow durations for larger
2
2
buildings having areas in excess of 3,600 ft (344 m ) are not to be less than what is
shown in Table 4.13. Type I-A structures are typically concrete frame buildings made of