PROPER GUTTER AND DOWNSPOUT SIZING
DESIGN OF ROOF DRAINAGE SYSTEMS ROOF DRAINAGE
The roof is one of the most essential parts of a building as it protects occupants, contents, and interior of the structure from the elements. Once an architect has determined the kind of roof he intends to use, he must give equal attention to the design of the roof drainage system.
Factors to be considered in the design of roof drainage systems are the area to be drained, size of gutters, downspouts, outlets, slope of roof, type of building, and appearance.
ROOF AREA TO BE CONSIDERED
The design capacity for a roof drainage system depends on the quantity of water to be handled. The quantity of water in turn depends on the roof area, slope, and rainfall intensity.
In considering the roof area, it must be remembered that rain does not necessarily fall vertically and that maximum conditions exist only when rain falls perpendicular to a surface.
Since the roof area would increase as its pitch increases, then it would not be advisable to use the plan area of a pitched roof in the calculation of a drainage system.
Experience has taught that use of the true area of a pitched roof often leads to oversizing of gutters, downspouts, and drains. To determine the design area for a pitched roof, Table 1-1 is used.
1-1 DESIGN AREAS FOR PITCHED ROOFS
To determine the design area multiply the plan area by
the factor in B column
These areas are then divided by the proper factor given in
Table 1-2, thus obtaining the required area in square
(square mm) for each downspout. From Table 1-3 select
PROPER GUTTER AND
RAINFALL INTENSITY - DOWNSPOUT CAPACITY
Rainfall intensity is usually given in inches per hour for a
five minute duration or one hour duration based on U.S.
Weather Bureau records. Table 1-2 based on records
through 1978, gives five minute intensities for selected
cities. New Orleans, Los Angeles, for example, may have
8 in./hr.(203 mm/hr) for a five minute duration yet record
only 4.8 in. (121 mm) in an hour over a 100 year period.
These rates correspond to 0.133 in./min.(3.4 mm/min.)
and 0.08 in./min.(2 mm/min.). Local codes may require
that drainage systems only be designed for the latter. It
takes 96.15 square feet(8.93 square meters) of surface
with 1 inch per hour(25 mm/hr) of water to correspond
with 1 gpm (0.063 l/s) flow rate. Downspouts and gutters
are sized in relation to rainfall on this basis.
Plumbing codes typically use the vertically projected roof
area for drainage design and they often use a square foot
allowance per square inch of downspout for 1 in./hr.(25
mm/hr) rainfall that varies with diameter, for example, 3
in.(76 mm): 911(85); 4 in.(102 mm): 1100 (102); 5
in.(127 mm): 1280 (119); 6 in.(152 mm): 1400 (130) and
8 in.(203 mm): 1750 (163) sq. ft.(sq. m). Net drainage
capacity from using Table 1-1 and 1-2 should be compared
with local code requirements.
In sizing downspouts, the following considerations apply:
- Downspouts of less than 7.00 sq in.(4515 sq mm)
cross section should not be used except for small areas
such as porches and canopies.
- The size of the downspout should be constant
throughout its length.
- Downspouts should be constructed with conductor
heads every 40 ft(12.2 m) to admit air and prevent
- Offset of more than 10 ft(3.0 m) can affect drainage
- The gutter outlet capacity should suit the downspout
- The downspout size must suit the bottom width of the
1-2 RAINFALL DATA AND DRAINAGE FACTORS
1-3 DIMENSIONS OF STANDARD DOWNSPOUTS
"A" = area of 1/4 in.(6.4 mm) undersized inlet
See Figures 1-31 and 1-32 for gage
- Assuming that using the fewest number of downspouts
is desirable, their locations will be affected by
- a. gutter capacity and length. To limit the effects of thermal
expansion in gutters 50 ft(15.3 m) is a practical maximum
length of gutter to be served by a downspout. Unless
special provisions are made for flexibility in downspouts,
gutters and their support systems, gutters should expand
away from downspouts and downspouts should not be
near gutter expansion joints. See expansion coefficients
in Appendix A-1 and expansion allowances in Figures 1-5
- b. the capacity of the inlet tube. See Table 1-3 and Figure
1-33. Also, a sharp bend at the inlet may clog.
- c. potential for water freezing in downspouts and gutters.
Open, partially open or corrugated styles downspouts
are suggested for areas subject to icing. Locating
on the north side of buildings is not recommended
for such areas.
- d. the appearance of the downspout system and a potential
need for concealment. See Figures 1-31 and 1-32.
- e. the greater capacity of a pitched gutter.
- f. the downspout discharge location. Water disposal at
this location should be acceptable. See Figures 1-31 and
- g. the risk of gutter overflow from insufficient drainage
capacity. See Figures 1-4, 1-21, and 1-23.
- h. a scupper serving a designated roof area. See Figures
1-26 to 1-30.
After the number and location of downspouts have been
determined, the areas to be drained by each downspout
should be figured. In making this calculation for a pitched
roof, the plan area should be adjusted according to
given on Table 1-1.
SAMPLE PROBLEM: Select downspouts for a building in
Boston, Mass. The building is 100 x 85 ft.(30.5 x 26 m) with
a double pitched roof having a slope of 6 in./ft.(152 mm/m).
The slope is toward the 100 ft.(30.5 m) side. Maximum
conditions will be used to determine downspout size.
It is decided to drain the building with 4 downspouts
at each corner of the building. An expansion joint will be
installed in each gutter between the downspouts.
The plan area of this building is 8500 sq ft.(790 sq m).
Since the slope is 6 in./ft.(152 mm/m), factor 1.10 is used (Table
1-1), making the design area 9350 sq ft.(868 sq m). Thus
each of the four downspouts will serve a 2338 sq ft.(217 sq
m) area. From column B, Table 1-2, opposite Boston, it is
found that 1 sq in.(645 sq mm) of downspout will drain 170
sq ft.(16 sq m) of roof area. Divide 2338(217) by 170(16) to
determine that each downspout should have a minimum
area of 13.56 sq in.(8746 sq mm).
From Table 1-3, it is found that there is a choice of; a 5 in.
(127 mm) Plain Round, a 5 in.(127 mm) Corrugated
a 5 in.(127 mm) Rectangular Corrugated, or 5 in.(127 mm)
Plain Rectangular downspout.
In sizing gutters, the following considerations apply for
typical section lengths of 8 to 10 feet( 2.41 to 3.0 m):
- Spacing and size of outlet openings. (The gutter can
be any more effective than the outlet and downspout
to drain it. Downspout sizes must not exceed the bottom
width of the gutter.)
- Slope of the roof. (The gutter must be of such a design
and location that water from a steep pitched roof will not by
its own velocity tend to overrun the front edge.)
- Style of gutters to be used. (All gutters are not effective
for their full depth and width, see Figures 1-1 and 1-4 for
- Maximum length of gutter. (50 ft.(15.2 m) between ends
or expansion joints is the limit unless the system is
designed to accommodate the greater expansion, the
larger flow and the need for special supports.)
- Gutter support capability. (Supports should be based on
full capacity of the gutter. Ice load capacity also affect the
size and strength of the system.)
Level gutters may be sized by Charts 1-1, 1-2, or 1-3.
gutters may be sized by Chart 1-3. Formulae for flow in
with different pitch are not available. The capacity of a
gutter with 1/16 in./ft.(5.21 mm/m) or less pitch is taken as
that of a level gutter even though it is somewhat greater.
RECTANGULAR GUTTER SIZING
The size of rectangular gutters depends upon these
- Area to be drained. (A, Chart 1-1)
- Rainfall intensity per hour. (I, Chart 1-1)
- Length of gutter in ft.(m) (L, Chart 1-1)
- Ratio of depth to width of gutter. (M, Chart 1-1)
Chart 1-1 is based on level gutter capacity as
determined by the National Institute of Standards and
(NIST) formerly National Bureau of Standards. It is
plotted from W = 0.0106 M-4/7 L3/28(1A)5/14 with W in
IRREGULAR CROSS SECTION GUTTER SIZING
The required sizes of gutters other than rectangular or
can be determined by finding the semicircle or rectangular
area that most closely fits the irregular cross section.
HALF ROUND GUTTER SIZING
Chart 1-2 is based on level gutter capacity as determined
NIST. It is based on W = 0.0182 (lA)2/5. W is the width in
in.(mm). I denotes rainfall intensity (Table 1-2) and A is the
roof area in square feet(sq m) (Table 1-1).
SAMPLE PROBLEM: To size rectangular gutter for a
120 x 30 ft.(35.6 x 9.1 m) located in Buffalo, NY. This
building has a flat roof with a raised roof edge on three
sides. A gutter is to be located on one of the 120 ft.(35.6 m) sides.
So that each section of gutter will not exceed 50 ft.(15.2
m), three downspouts will be used with 2 gutter expansion
joints. The area to be drained by each section of gutter will be
1200 sq ft.(111.5 sq m), the rainfall intensity from Table 1-2, col
A is 6 in/hr(152 mm/hr), the length of each gutter section is
40 ft.(12.2 m), and the ratio of gutter depth to width is 0.75.
On Chart 1-1 find the vertical line representing L = 40(12.2
m). Proceed vertically along this line to its intersection with
the oblique line representing M = 0.75. Pass to B vertically
to the intersect the horizontal line representing IA =
7200(16948). The point of intersection occurs between the
oblique line representing gutter widths of 5 and 6 in.(127
and 152 mm). The required width of gutter is, therefore, 6
in.(152 mm) and its depth need be only 4.5 in.(114 mm).in.
SAMPLE PROBLEM: To size a half round gutter for a
located in Kansas City, Mo., with a flat roof 80 x 40
ft.(24.4 x 12.2 m). This building has a parapet wall on three
sides and a gutter to be located on an 80 ft.(24.4 m) side.
Column A, Table 1-2, was used to determine rainfall
Since the gutter run will exceed 50 ft.(15.2 m), two
downspouts will be used with an expansion joint between.
The area of the building is 3200 sq ft. (297 sq m). Thus
of the downspouts will serve an area of 1600 sq ft. (149 sq
m). From column A, Table 1-2, opposite Kansas City, Mo.,
is found that 1 sq in.(100 sq mm) of downspout will drain
160 sq ft.(2.3 sq m/100 sq mm) of roof area. Divide 1600
ft. (149 sq m) by 160 sq ft/sq in. (2.3 sq m/100 sq mm) to
determine that each downspout should have a minimum
of 10 sq in. (6470 sq mm). From Table 1-3 it is found that
in. (102 mm) downspout is required. From Chart 1-2 it is
determined that a 9.5 in. (241 mm) half round gutter should
be used. Area and flow in Table 1-4 are based on 1 in. (25
mm) of rainfall per hour; divide these areas by the local
rate in inches per hour to determine the actual roof area
to be served by the gutter diameter. "The capacity of a
rectangular gutter may be approximated by using a gutter
cross section area not less than that of a semicircular
and a depth to width ratio of at least 0.75.