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Roof Drainage Design

May 15, 2007

Doorways to the Future
RCI 22nd International
Convention & Trade Show
Orlando, Florida • March 1-6, 2007
© RCI, Inc.
1500 Sunday Drive, Suite 204 • Raleigh, NC 27607
Phone: 919-859-0742 • Fax: 919-859-1328 •
Roof Drainage Design
Stephen L. Patterson, RRC, PE
Roof Technical Services, Inc.
Fort Worth, Texas
Doorways to the Future
Proceedings of the RCI 22nd International Convention Patterson – 123
The purpose of this paper/presentation is to provide the reader with an overview of
roof drainage design requirements. This will include an historical overview of roof
drainage design, a discussion of the evolution of the various national codes, and an
overview of the current International Building and Plumbing Code requirements with
a comparison to the Uniform and National Standard Plumbing Codes. Also included
will be a discussion of the critical life safety issues involved with roof drainage design,
a case history of various roof collapses with a discussion of the causes, and an
overview of the roof design requirements involving roof drainage design requirements
for today’s roofs.
STEPHEN L. PATTERSON is a licensed professional engineer and a Registered Roof
Consultant with over 33 years of experience in the roofing industry. Mr. Patterson’s
experience includes working as a roofing contractor, director of engineering for a roofing
manufacturer, and consulting engineer specializing in roofing for the past 23
Stephen co-authored Roof Design and Practice, a roof design manual published by
Prentice Hall, and Wind Pressures on Low Sloped Roofs and Roof Drainage, published
by the Roof Consultants Institute Foundation. Mr. Patterson has designed numerous
roof drainage systems and investigated more than 30 major roof collapses involving
roof drainage issues.
Patterson – 124 Proceedings of the RCI 22nd International Convention
Every year, roofs collapse primarily
as a result of improper roof
drainage design and other related
issues. Roof collapses are catastrophic
roof failures that cause
the loss of millions of dollars in
property damage. More importantly,
many times roof collapses
result in the loss of life. On May 5,
1995, a series of severe thunderstorms
passed over the Dallas/
Fort Worth area, causing more
than $2 billion in property damage
from large hail. Softball size
hail injured hundreds of people
who were caught outside at an
outdoor festival. However, the
only fatalities were the result of a
roof collapse due to heavy rains.
A correctly designed roof
drainage system provides for a
roof that slopes properly without
resulting in ponding water and
similarly provides for a properly
designed primary and secondary
drainage system to prevent an
excessive build-up of water on the
roof. This build-up of water is
almost always the key factor in
roof collapses. This paper discusses
the evolution of the codes
and industry design standards
and the issues related to roof
drainage design.
The Early Codes
Roofing problems and building
codes have been linked together
since the first building ordinances
in America. The first
building ordinances in America
dated back to the late 17th
Century and involved restrictions
on thatch roofing in Boston and
New Amsterdam (New York). The
ordinances were the result of performance
issues and fire hazards
associated with thatch roofing.
Roofing problems were a prominent
issue in the late 19th
Century. Major fires devastated
many cities in the last half of the
19th and early 20th Centuries.
Fire losses were estimated at $1.2
billion between 1860 and 1915.
The famous Chicago fire of 1871
resulted in the loss of 17,450
buildings alone. These fires led to
research by the forerunner of
Underwriters Laboratories. This
research provided the basis for
the first modern building codes
dealing with roofing issues. Roof
drainage was also included.
Roof Drainage
in the Building Codes
Roof drainage requirements
were included in the first edition
of the Uniform Building Code
(UBC) published in 1927. Issues
of slope, drainage, and overflow
were addressed simply but rather
eloquently in Section 3206 of the
Code, which stated the following.
Roof Drainage, Section 3206.
Roofs of all buildings shall be
sloped so that they will drain
to gutters and downspouts
which shall be connected with
conductors to carry the water
down from the roof underneath
the sidewalk to and
through the curb. Overflows
shall be installed at each low
point of the roof to which the
water drains.
From 1927, the evolution was
gradual. It was not until the 1964
Edition of the UBC that roof
drains are specifically mentioned
in the code. Section 3206 of the
1964 UBC stated:
Proceedings of the RCI 22nd International Convention Patterson – 125
Roof Drainage Design
Roof drains shall be installed
when required at each low
point of the roof to which the
water drains, and shall be
adequate in size to drain the
roof. Overflow drains shall be
installed with the inlet flow
line located two inches (2″)
above the low point of the roof,
or overflow scuppers may be
installed in parapet walls at
each low point of the roof with
the flow line not more than two
inches (2″) above the adjacent
There is no reference to a specific
plumbing code in the Uniform
Building Code until 1967.
However, it should be noted that
the National Bureau of Standards
published the first national
plumbing code in 1928 (Publication
BH 13). This publication was
commonly referred to as the
Hoover Code. The National Association
of Master Plumbers
published its Standard Plumbing
Code in 1933. The American
Standards Association published
a preliminary Plumbing Code A40
in 1942. The Western Plumbing
Code drafted its first Uniform
Plumbing Code in 1938. After
World War II, a joint committee,
the Uniform Plumbing Code Committee,
was formed and extensive
research was performed at the
National Bureau of Standards and
several universities, which ultimately
culminated in the American
Standard National Plumbing
Code, ASA A40.8-1955.
Clearly, roofing and the building
codes have been intertwined
from the beginning, generally as a
result of increased problems in
the industry. Today’s International
Building Code represents
the merging of the three national
building codes – the BOCA Building
Code, the Uniform Building
Code, and the Standard Building
Code – into one unified building
code. The goal was to have a single
national building and plumbing
code. The International Building
Code has been universally accepted
for the most part, but there
are still three national plumbing
codes, which include the Uniform
Plumbing Code, the National
Standard Plumbing Code, and the
International Plumbing Code,
each with slightly different philosophies.
Key Drainage Issues in the Codes
The key design issues addressed
in the codes include roof
slope and the structural issues
related to ponding, the design of
the primary roof drainage system,
and the design of the overflow or
secondary drainage systems.
From the beginning, Building
Codes have stated the obvious –
roofs should drain, there needs to
be a backup drainage system in
case something happens to the
primary drainage system, and the
structures should be designed to
support all the anticipated loads
so they do not collapse. Those are
pretty simple concepts, but it is
alarming how many roof drainage
problems persist and how many
roofs collapse every year.
Roof Slope Defined
The need for sloping roofs
should be obvious. Roofs that
drain well last longer than roofs
that do not drain well, and roofs
that do not accumulate water are
not likely to collapse. Clearly,
however, there are still misunderstandings
regarding the issues
related to roof slope. In 1927, the
UBC stated, “Roofs of all building
shall be sloped so that they will
drain.” How could it be any clearer?
The NRCA published its first
Manual of Roofing Practice and
stated, “enough slope should be
built-in so that water does not collect
in the low areas between the
roof-framing members.” Unfortunately,
the NRCA finished the
sentence with “… so that the roof
is completely dry 48 hours after it
stops raining.” Water standing for
48 hours does not constitute good
drainage, but this 48-hour standard
found its way into the roofing
literature and became part of
the first International Building
Code in 2000.
The 2006 International Building
Code defines positive drainage
The drainage condition in
which consideration has been
made for all loading deflections
of the roof deck, and the
additional slope has been provided
to ensure drainage of
the roof within 48 hours of precipitation.
The Code further requires lowsloped
roofs to slope a minimum
of 1/4:12 except for coal tar pitch
built-up roofs, which can be
sloped 1/8:12. It is important,
however, to understand that there
can be serious problems with
roofs having slopes of less than
1/4:12. These issues are addressed
in the structural requirements
in the codes for ponding
Structural Issues
Roof slope is not only important
for roof performance, but roof
slope is extremely important in
the structural design of a roof.
Roof structures with slopes less
than 1/4:12 must be evaluated
for ponding instability. The concept
of ponding instability is not
new. The requirement to specifically
design roofs for ponding
water first appeared in the
Uniform Building Code in 1967 in
Section 2305.(f).
2305.(f) Water Accumulation.
All roofs shall be designed
with sufficient slope or
camber to assure adequate
drainage after the long-time
deflection from dead load or
shall be designed to support
maximum loads including pos-
Patterson – 126 Proceedings of the RCI 22nd International Convention
sible ponding of water due to
defection. See Section 2307 for
deflection criteria.
Ponding instability is critical
in roof drainage design, because
one of the typical factors in many
roof collapses involves the failure
of the structure due to progressive
deflection under water loads. As
water builds up on a roof, the roof
structure deflects, and if the slope
is inadequate to permit free
drainage, accumulation of water
will result. Water continues to
build up deeper and deeper as the
structure deflects. This progressive
build-up of water is one of the
key factors in many collapses.
Below is the section from the
2006 IBC dealing with ponding
instability, which is a new revision.
1622.2 Ponding instability.
For roofs with a slope less
than 1/4-inch per foot [1.19
degrees (0.0208 rad)], the design
calculations shall include
verification of adequate stiffness
to preclude progressive
deflection in accordance with
Section 8.4 of ASCE 7.
Water weighs 62.4 pounds per
cubic foot (pcf), and therefore
each inch of water weighs approximately
5.2 pounds per square
foot (psf). It is precisely this
weight of water that provides the
live load on most roofs that leads
to roof collapses. The codes have
long provided for live-load reductions,
which commonly allow the
reduction in the live load on bar
joists and other roof framing
members to be as low a 16 psf,
which equals slightly more than 3
inches of water uniformly distributed
across the roof.
A standard deflection criteria
allows for a deflection equal to
1/240th of the length of the span
of the structural element, which is
2 inches of deflection in a 40-foot
span. Potentially, roof slopes of
less than 1/4-inch in 12 inches
can have enough deflection to
restrict the drainage and allow for
water build-up.
Below are illustrations that
show how slope and deflection are
interrelated and can result in the
build-up of water on roofs. The
illustration at the top shows a roof
deck with a slope of 1/8-inch per
foot and a deflection equal to
L/240 (the length of span divided
by 240). The deflection is equal to
2.5 inches at the mid-span, which
results in no slope from the midspan
to the eave (2.5 inches in
deflection and 2.5 inches in fall).
The illustration that shows that
roof with the 1/4-inch slope still
has a slope of 1/8-inch from the
mid-span to the eave, which eliminates
any ponding. This is an
important reason for having a
minimum slope of 1/4-inch per
The International Building
Code that allows roof slopes to be
reduced to 1/8-inch per foot for
coal tar pitch roofs ignores the
importance of positive drainage.
Also, the code requires that anytime
there is less than a 1/4:12
slope, the roof should have been
evaluated for ponding instability.
The illustration on the next
page from Roof Drainage shows
the importance of slope and the
depth of water that can accumulate.
The lower the slope, the
greater the load caused by the
head (depth) of water at the
perimeter. Decreasing the slope
from 1/4:12 to 1/8:12 doubles
the load on the structure.
The more slope on a roof, the
less likely that significant water
accumulations will occur, and the
less likely a roof is to collapse as a
result of rainwater loads. The
manner in which water builds up
on a roof is discussed below, in
the section on “Controlling Water
Depth of Roofs (Overflow Drainage
Systems).” Suffice it to say that
water does build up on roofs, and
the lack of slope can result in catastrophic
consequences unless
the structure is designed for those
Proceedings of the RCI 22nd International Convention Patterson – 127
The building codes have taken
a step backwards in their requirements
relating to slope, particularly
as it relates to reroofing. The
1988 Uniform Building Code
addressed the issues related to
roof slope and reroofing better
than any other code before or
after. Below is an excerpt from
Chapter 32 in the Appendix to the
1988 UBC.
Sec. 3210. New roof coverings
shall not be applied without
first obtaining an inspection
by the building official
and written approval from the
building official. A final inspection
and approval shall be obtained
from the building official
when the re-roofing is
complete. The pre-roofing
inspection shall pay special
attention to evidence of accumulation
of water. Where
extensive ponding of water is
apparent, an analysis of the
roof structure for compliance
with Section 3207 shall be
made and corrective measures,
such as relocation of
roof drains or scuppers,
resloping of the roof or structural
chances, shall be made.
An inspection covering the
above-listed topics prepared
by a special inspector may be
accepted in lieu of the preinspection
by the building official.
Further, the 1988 UBC
required that the new roof meet
the requirements of Chapter 32 in
this building code, which required
the roof to have a slope of 1/4:12.
Essentially, the roof had to be
inspected and if there were drainage
problems, the roof structure
had to be analyzed and the drainage
It is a mistake to assume that
because a building has been
around for a long time that the
drainage system is adequate. One
of the first collapses investigated
by this author involved a 50-yearold
building that never had a
drainage problem – that is, until
someone tossed a Sunday Fort
Worth Star Telegram newspaper
up on the roof. That paper was the
perfect size to block the scupper
drains on this building. And
unfortunately for the owner of the
building, there were no overflow
drains or scuppers.
Slope is one of the three critical
elements of roof drainage
design. It is essential that the roof
have adequate slope to drain the
roof without accumulating water
on the roof. The depth of water
that can accumulate on the roof is
critical, and the next section deals
with the concept of overflow
design in order to prevent excessive
water build up.
The Need for an Overflow
Drainage System
Probably the most important
element of roof drainage design is
controlling the depth of water that
can accumulate on the roof.
Controlling this depth is a function
of the overflow drainage system
or, as it is sometimes referred
to, the secondary drainage system.
Overflow scuppers are prominently
mentioned in the 1929
Standard Practice in Sheet Metal
Work, which is the forerunner to
the Architectural Sheet Metal
Manual published by the Sheet
Metal and Air Conditioning Contractors
National Association, better
known as SMACNA. Also, overflow
was one of the requirements
in the first Uniform Building
Code. Why, then, is it that so
Patterson – 128 Proceedings of the RCI 22nd International Convention
many existing buildings have no
overflow drainage system?
Most of the collapses investigated
by this author have been
caused, in part or completely, by
the lack of an overflow drainage
system. At some point it is likely
that a roof drain is going to
become blocked. It may take 50
years before someone throws a
Sunday newspaper on the roof or
a hailstorm occurs severe enough
to block the drains or blowing
debris in a hurricane blocks the
drains. At some point, the primary
drainage system will likely
become blocked. Water flowing to
the drains will carry debris and
deposit it at the low point of the
roof, which is where the primary
drains are located. Some drains
are more likely to become blocked
than others. Drainage scuppers
are more susceptible to blockage
than roof drains. Below is an
illustration from Roof Drainage.
As can be seen, the roof drain
with a proper strainer can still
function, even with debris accumulated
around the perimeter of
the strainer, whereas the scupper
is virtually blocked by the same
The American Society of Civil
Engineers recognized the importance
of blocked drains and provided
the following recommendations
for structural engineers in
the first ASCE Standard entitled
ANSI/ASCE 7-88, Minimum Design
Loads for Buildings and
Structures, which replaced ANSI
8.2 Blocked Drains. Each
portion of a roof shall be
designed to sustain the load of
all rainwater that could accumulate
on it if the primary
drainage system for that portion
is blocked. Ponding instability
shall be considered in
this situation. If the overflow
drainage provisions contain
drain lines, such lines shall be
independent of any primary
drain lines.
Overflow Drainage
There have been revisions of
ASCE 7, but the issues related to
overflow have remained the standard
in ASCE 7 since its inception
in 1988 and the International
Building Code since its inception
in 2000. The structural engineer
must assume the primary drains
are blocked, calculate the depth of
water that could accumulate on
the roof, and design the structure
to support those loads. Again,
these are pretty simple, commonsense
concepts, which have been
universally accepted in today’s
design standards.
However, there are variations
in overflow drainage design theory.
Historically, most primary and
secondary roof drainage systems
were designed based upon the
1-hour duration, 100-year rainfall
rate. This 1-hour duration, 100-
year rainfall rate is the amount of
rain that is likely to fall in one
hour once every 100 years. However,
in 1991, the Standard Building
and Plumbing Code changed
the requirements for overflow
design. The 1991 Standard
Plumbing Code adopted the 15-
minute duration rainfall rate for
design overflow drainage systems,
which is the amount of rainfall
that can be expected to fall in 15
minutes once every 100 years. To
put it in perspective, the 15-
minute, 100-year rainfall rate is
approximately twice the 1-hour,
100-year rainfall rate.
1507.3 Maximum Rainfall
Rate for Secondary Drains.
Secondary (emergency) roof
drain systems or scuppers
shall be sized based on the
flow rate caused by the 100-
year, 15-minute precipitation
as indicated on Figure 1507.3.
The flow through the primary
system shall not be considered
when sizing the secondary
roof drain system.
The National Standard Plumbing
Code has also adopted this
criterion. The rationale for using
the 15-minute duration, 100-year
rainfall is based on the concept
that roof drains may become
blocked during extreme storms
like hailstorms and hurricanes.
The Standard Plumbing Code was
formerly the Southern Building
Code, which was adopted primarily
throughout the Southeast part
of the United States, including the
hurricane-prone Gulf and Atlantic
Coast states. Blowing debris during
hurricanes commonly block
roof drains, and hurricanes often
produce extraordinary rainfall.
The combination of blowing debris
and extraordinary rainfall can be
catastrophic for roof drainage systems.
The first International
Plumbing Code (1997) also adopted
this 15-minute, 100-year rainfall
rate, but reverted to using the
1-hour, 100-year rainfall rate for
overflow systems in 2000.
Also gone is the old Uniform
Building Code standard of using
Proceedings of the RCI 22nd International Convention Patterson – 129
overflow scuppers three times the
area of the roof drains. The Uniform
Building Code first provided
the overflow scupper option in the
1967 Edition, which follows:
(c) Overflow Drains and
Scuppers. Where roof drains
are required, overflow drains
having the same size as the
roof drains shall be installed
with the inlet flow line located
two inches (2″) above the low
point of the roof, or overflow
scuppers having three times
the size of the roof drains may
be installed in adjacent parapet
walls with the inlet flow
line located two inches (2″)
above the low point of the
adjacent roof and having a
minimum opening height of
four inches (4″).
Overflow drains shall be
connected to drain lines independent
from the roof drains.
There is a major flaw in this
drainage criterion. This criterion
does not take into account the
depth of water that develops at
the scupper based upon the
geometry of the scupper. The fundamental
function of the scupper
is to control the depth of water, so
as to limit the load on the roof
structure and prevent a collapse.
The geometry of a scupper is critical.
For example, a 6-inch
drain has an opening of
approximately 28 inches.
Using the old Uniform Building
Code’s standard of providing
an overflow scupper
three times the area of the
roof drain, the overflow
scupper size would be 84
square inches. The problem
with this standard was that
a designer could select an 8-
inch-wide by 10.5-inch-high
scupper and meet the code
requirements. However, the
head (depth) of water at the
scupper would have to be
8.4 inches to achieve the
flow rate required.
The net result would be a
depth of water of 10.4 inches
at the scupper, as the
overflow scupper is located
2 inches (10.4 + 2) above the
roof. This depth of water
could easily cause a collapse
in many circumstances.
Based upon the old
UBC, it was possible to collapse
a roof using overflow
scuppers three times the
area of the roof drains.
Roof drains are roughly
three times more efficient
than scuppers in terms of
flow. The 8-inch-wide by
10.5-inch-high scupper has
the flow capacity of almost
790 gallons per minute,
which is slightly more than
a 6-inch drain. Unfortunately,
the head of water required
to achieve that flow
through the scupper is 8.4
inches, while the head of
water required to achieve
approximately the same
flow is only about 3.6 inches.
In 1991, the Standard Plumbing
Code introduced a requirement
to determine the depth of
water that can accumulate on the
roof and to notify the structural
engineer to design the structure
based upon this depth of water,
assuming the primary drains are
blocked, which is today’s standard.
The excerpt from the 1991
Standard Plumbing Code (above)
shows the chart used to determine
the depth of water that can
accumulate at a scupper.
The 1991 Standard Plumbing
Code began requiring designers to
assume that the primary drains
are blocked and to calculate the
depth of water that could accumulate
over the secondary or
overflow drainage system. This is
the basis of the design requirements
provided in the initial
ASCE 7 in 1988 and the initial
International Building Code in
2000. This is the most logical
approach and simply reinforces
the concept that the structural
engineer should design the structure
to support the loads that
could be expected to occur on a
ASCE/SEI 7-05 provides a
basis for calculating the head of
Patterson – 130 Proceedings of the RCI 22nd International Convention
water over drains and
scuppers. There are also
methods for calculating
the head of water over
scuppers and drains
provided in Roof Drainage.
There are methods
for determining the
head of water (depth) in
scuppers and over
drains. At the right is
the chart from data
included in ASCE/SEI 7-05.
The overflow drainage system
controls the depth of water that
can accumulate on a roof and is
the most critical part of the
drainage system. The roof structure
should be designed to support
all the loads anticipated on
the roof, and water build-up is
one of the most important sources
of loads on a roof. The requirement
for overflow systems goes
back to the beginning of the modern
national codes, and these
requirements have been refined
over the years. Today, the codes
are pretty simple and straightforward.
Calculate the depth of water
that can accumulate on the roof
assuming the primary drainage
system is blocked, and design the
structure to support those loads.
The structure must be evaluated
for ponding instability any time
the slope is less than 1/4:12.
These are basic concepts that
have been articulated in codes
and standards for years.
Location of Overflow
Drains and Scuppers
Overflow drains and scuppers
should be located above the low
point of the roof to help prevent
the overflow drains and scuppers
from becoming blocked. Debris is
carried by the flow of water on the
roof, which is to the low point. The
primary drains are located at the
low point, which is what makes
them susceptible to becoming
blocked by debris.
Placing the overflow drain or
scupper approximately 2 inches
above the low point of the roof
reduces the likelihood that debris
will flow into the overflow system
and become blocked. Additionally,
overflow drain outlets should be
located in a prominent location so
that maintenance personnel can
readily observe water flowing out
of the overflow. This is an indication
that the primary drains may
be blocked and maintenance
needs to be performed.
Sometimes it is difficult to
place overflow scuppers 2 inches
above the low point of the roof,
due to the location of the drains.
Today, there is no specific requirement
to locate the overflow
drain or scupper 2 inches above
the low point of the roof. The
requirement is to find out how
much water can accumulate above
the overflow system and
design the structure accordingly.
However, the overflow drain or
scupper should be located approximately
2 inches above the
low point to prevent debris from
blocking the overflow, but the
scupper can be located at a higher
elevation, provided the structure
is adequate to support the
General Requirements
The primary drainage system
is an important element in
drainage designs, but it is the
least important of the three elements
of drainage design. These
elements include the slope of the
roof and ponding instability, the
overflow drainage system, and the
primary drainage system. However,
it is important that roofs
drain freely, and the primary
drainage system is designed to
remove water efficiently. This
author has investigated roof
drainage systems that were
under-designed, resulting in
water depths deep enough to
cause leaks at low curbs and
expansion joints.
The primary drainage system
generally consists of either roof
drains or scupper drains. Most
roof drains today are manufactured
by companies like Josam,
Zurn, and J.R. Smith and have
standard flow rates, which are
reflected in the various plumbing
codes. These drains are designed
with sumps and strainers that
conform to the codes. Strainers
are important, as they block the
debris from getting into the drains’
lines. Also, strainers can actually
improve the flow into drains
by breaking up the vortex of the
water flowing into the drain.
Scupper drains, on the other
hand, are generally shop- or fieldfabricated,
and generally, flow
rates must be calculated. There
are no standard strainers
designed to promote water flow
and function with debris.
Scuppers also generally require a
greater depth of water to achieve
the designed flow, so the depth of
water at scuppers can be significant.
Scupper drains and overflow
drains should be separate and
should not be connected.
Proceedings of the RCI 22nd International Convention Patterson – 131
Drainage Rates for Roof Drains
The various plumbing codes
generally agree on the design criteria
in principle. However, there
are variations in the flow rates
between the rates provided in the
International Plumbing Code and
the other two national plumbing
codes – the National Standard
Plumbing Code and the Uniform
Plumbing Code. It should be
noted, however, that the flow rates
in the International Plumbing
Code are the same as the old
BOCA Plumbing Code, old Standard
Plumbing Code, and old
Uniform Plumbing Code (prior to
1997). Adjacent is a comparison
of the flow rates.
The National Standard and
Uniform Plumbing Codes (after
1994) are more conservative. All
these flow rates are based upon
Manning’s equations, but there
are slightly different assumptions
regarding the amount of open
area in the pipe, which results in
the differences in the charts. Roof
drain manufacturers also publish
drainage design literature, and
most of these manufacturers use
the same drainage design assumptions
as the International
Plumbing Code’s standards.
Drainage Rates for Scuppers
Scupper drains must be
designed, and there are also variations
in the formulas used for
calculating the flow through a
scupper. It is important to understand
that water has to build up
to a relatively high elevation in
order to achieve the design flow
rate through the scupper. The
depth of water that develops is
primarily an issue of the width of
the scupper. The wider the scupper,
the lower the head of water
that will develop at the scupper,
which is desirable even if the
structure is designed to support
the loads from a large head of water.
The flow rate of water through
scuppers is generally determined
by the derivation of an equation
known as the Francis Formula,
which is Q = 3.33LH1.5 where Q is
the flow rate, L is the length of the
weir (scupper), and H is the head
of water. Because experiments
have shown there is a contraction
in the water flowing through the
weir, the equation has been modified
to adjust for this reduction.
The modified form is Q = 3.33(L-
0.2H)H1.5. Below is a chart derived
from this equation.
The design of the primary
drainage system is relatively
straightforward. There are variations
in flow rates of drains and
scuppers, and further research
into these variables would be
helpful in establishing consistent
design guidelines.
Requirements to Modify
Drainage for Reroofing
The 1967 Uniform Building
Code added Chapter 32 to the
appendix of the code, which was
titled Re-Roofing, and the first
section (3209) in that chapter
stated that all re-roofing had to
comply with Section 32 in the
Building Code. This was a significant
change in the code. However,
the most significant change
came in 1988 with the addition of
the statement that roof systems
shall be sloped a minimum 1/4-
Patterson – 132 Proceedings of the RCI 22nd International Convention
inch in 12 inches for drainage.
This requirement to provide a
minimum slope in Chapter 32 of
the 1988 UBC in combination
with the changes made to Chapter
32 in the appendix for Reroofing
had major implications for reroofing
design. Chapter 32 in the appendix
required a re-roof to conform
to Chapter 32 of the code,
which required the roof be sloped
a minimum 1/4 in 12 inches.
There was no reference to
draining within 48 hours or allowing
1/8-inch per 12 inches for
coal tar pitch. From a fundamental
design perspective, this was
the most appropriate code dealing
with roof drainage and roof slope.
A minimum slope of 1/4-inch in
12 inches has long been recognized
as the most appropriate
minimum slope for low-sloped
roofs. A minimum 1/4-inch per
12 inches is also important from a
structural design perspective, as
any roof with less than this slope
has to be designed for ponding
instability. Deflections in structural
elements with less than 1/4-
inch per 12-inch slope can result
in a progressive collapse due to
deflection. In other words, the roof
deflects, allowing more water to
accumulate until the roof collapses.
Clearly the authors of the
1988 UBC were addressing the
issues that cause roof collapses.
Below is Section 3210 from
Chapter 32 in the appendix to the
1988 UBC.
Sec. 3210. New roof coverings
shall not be applied
without first obtaining an
inspection by the building official
and written approval from
the building official. A final
inspection and approval shall
be obtained from the building
official when the re-roofing is
complete. The pre-roofing
inspection shall pay particular
attention to evidence of accumulation
of water. Where
extensive ponding of water is
apparent, an analysis of the
roof structure for compliance
with Section 3207 shall be
made and corrective measures,
such as relocation of
roof drains or scuppers, resloping
of the roof, or structural
changes shall be made.
An inspection covering the
above-listed topics prepared
by a special inspector may be
accepted in lieu of the preinspection
by the building official.
These changes in 1988 were
met with less than an enthusiastic
response from elements of the
roofing community. In fact, this
change was a mind-altering event
for many in the roofing industry.
A great number of existing buildings
did not have a minimum 1/4-
inch per 12-inch slope. In some
cases, it was not only impractical
but it was virtually impossible to
provide the minimum slope. Then
there were the issues from the
coal tar pitch industry, where
1/4-inch per 12-inch slope could
be too much slope for the system.
The result of these issues and
others was a watering down of the
Current Design Standards
Today’s International Building
Code is relatively ambiguous
regarding positive drainage. As
defined, “positive drainage” is
based upon ensuring drainage
within 48 hours of precipitation.
As previously stated, water standing
for 48 hours does not constitute
good drainage. What does “48
hours from precipitation” mean?
Does it mean 48 hours in summertime
conditions or wintertime
conditions? A properly sloped roof
should drain freely. Other than
anomalies in the roof created
around penetrations or crickets
and valleys, there should be no
water ponding after a rain.
Clearly, the code requires reroofs
to have positive drainage,
but since the definition is ambiguous,
enforcement is difficult.
Some building officials have ruled
that the design professional is
responsible for making the determination
of what constitutes positive
drainage, but often there is
no design professional in the case
of reroofing. Certainly, positive
drainage is a benefit in terms of
roofing longevity and performance,
and providing 1/4-inch
per 12-inch eliminates many
structural concerns. In those
cases where achieving 1/4-inch
per 12-inch is not practical, care
should be taken to help limit the
amount of water that can accumulate
on the roof and involving a
structural engineer should be
It is important to understand
that simply re-sloping a roof
with tapered insulation may
not be adequate. It is imperative
that the drainage system
function properly after the tapered
insulation is installed.
Often, increases in insulation
thickness can restrict drains
and overflow systems.
One of the most important
requirements for reroofing is to
make sure there is a proper overflow
system. This is a code requirement
that is often overlooked
by many in the roofing industry,
but the overflow system is critical
in terms of limiting the amount of
water that can accumulate on a
roof and in preventing roof collapses.
As a rule of thumb, some
try to limit the depth of water to a
maximum of 4 inches, which prevents
the load from water buildup
from exceeding the minimum
20-psf live load used throughout
much of the southern regions of
the country. It is also important to
understand that even though
there is a minimum 20-psf live
load requirements in the code,
there are live-load reductions that
are allowed by code, which can
reduce live loads on certain structural
elements to 16 psf and even
12 psf in some cases.
Proceedings of the RCI 22nd International Convention Patterson – 133
Drainage design problems
come in all types, from poor
drainage deteriorating a roof to
causing major roof collapses.
Most of the roof collapses investigated
by this author have involved
defects in drainage design, usually
in combination with other factors.
The build-up of rainwater is
nature’s way of load-testing structure,
and sometime structural design
or construction deficiencies
are identified. Realize that most
drainage systems are designed
based upon the 100-year rainfall
occurrences, so it may take a long
time before the structure really
gets tested by one of these rainstorms.
Below are some examples
of some of the collapses investigated
by this author.
• The first roof collapse investigated
was a simple
case of the contractor roofing
over the scuppers during
construction. Unfortunately,
that roof received
its first load test before the
contractor had time to cut
in and flash the scuppers.
• One of the first really large
losses involved a computer
assembly facility. A looselaid,
ballasted, single-ply
had been installed over an
existing built-up roof without
evaluating the structure.
The extra dead load
for the roof dramatically
reduced the live-load
capacity of the structure,
resulting in deflection
between the drains. This
structure failed as a result
of a progressive deflection
of the joists as the water
depth kept increasing between
the drains.
• One of the most expensive
collapses occurred in a
building that had no overflow
scuppers. Instead of a
conventional strainer,
screens were installed in
front of the drainage scuppers
to catch debris. The
screens worked efficiently
and soon became blocked
with debris. One eyewitness
reported water lapping
over the top of the 12-
inch parapet wall just
before the roof collapsed.
• Another large collapse in
volved the irregular spacing
of the roof drains and a
defective joist girder. There
were enough drains if the
drains had been spaced
evenly, but the end bays
were 50% larger than the
typical bays, resulting in
50% more water. Additionally,
there were no provisions
for overflow. The head
of water over the drain was
high enough to cause the
defective joist girder to fail.
• One of the most dramatic
collapses involved a concrete
structure. The roof
was designed to drain to
an outside wall through
drainage scuppers, but
the roof had deflected as a
result of long-term plastic
deformation. The roof
sagged in the middle,
resulting in ponding. The
roofer decided to install
drains in the center and
add slope from the outside
walls to the center of the
roof. The drains were too
small, and the drain lines
were not sloped, restricting
drainage. There were
no overflow scuppers or
drains, and the roof collapsed
during a heavy
• One collapse was actually
predicted by this author
after a routine roof inspection.
The core sample indicated
that there were multiple
roofs, one on top of
the other, weighing more –
than 20 psf. The recommendation
was to remove
the roofs immediately, as
the roof could collapse
during a heavy rain. The
roof was flat, and the addition
of all the roofs over
the years reduced the live
load capacity to zero.
Unfortunately, the owner
waited, and the roof collapsed
several months
after the inspection.
This sampling of collapses
illustrates some of the issues
related to collapses involving roof
drainage design. Lack of overflow
is a common problem, along with
inadequate drainage and too
much weight from the roof(s)
installed. Sometimes the weight of
the water simply finds the weak
link in the structure.
The basic concepts of proper
roof drainage design have been
around for many years, and there
are extensive data and design
guides available. Roof Drainage,
published by the RCI Foundation,
provides a much more complete
discussion of roof drainage, and
every roof consultant should have
a copy in his or her library. There
are still issues that need clarification
and additional research is
needed, particularly in the area of
water accumulation on roofs and
the appropriate flow rates of
drains and scuppers.
The roof consultant can play
an important role in preventing
roof collapses. It is essential that
there is a properly functioning
overflow system on a building.
Checking roofs for an appropriate
overflow system and recommending
corrective action to add or
enlarge overflow drains and/or
scuppers should be a standard
part of the roof investigation
process. Further, improving the
drainage and the design and
installation of overflow systems
should be part of a reroofing project
where these systems are
Patterson – 134 Proceedings of the RCI 22nd International Convention