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Gutter Hanger Selection

In deciding which hanger style is most acceptable, please consider the following factors: appearance, expected life, ice loading, size of gutter, material and expansion.

When fascia board is less than 2″ thick, the hanger fasteners must be driven into the rafter ends.

In areas subject to snow slide, coordinate the front edge elevation of the gutter with with the extended roof line (See Table on Required Clearance of Gutter Below Extended Roof Line).

Recommended Gutter Hanger Spacing

Gutter should be spaced at maximum of 36″ on center. In areas where ice and snow are long lasting, hanger spacing should be reduced to 18″ on center.

Recommended Gutter Pitch And Position On The Eave

There are two factors that go into determining gutter position on the eave, the pitch of the gutter and the roof slope. Gutter should be pitched 1/16″ or greater per foot for proper drainage. Gutter can be run level for appearance but additional downspout will be necessary (See section on Proper Gutter and Downspout Sizing).

Gutters should be placed below the extended roof line so that snow and ice can slide clear. Steeper pitches require less clearance.

Required Clearance of Gutter Below Extended Roof Line Roof Pitch Clearance Below Extended Roof Line
0 in 12 – 2 in 12 1″
2 in 12 – 5 in 12 3/4″
5 in 12 – 8 in 12 1/2″
9 in 12 or Greater 1/4″

Lap Joint Specification

Joints in gutters must be overlapped 1″, riveted on 2″ centers and soldered if the metal can be soldered (See soldering instruction for details). With metals that cannot be soldered, a compatible sealant must be continuously applied within a 1″ lapped joint and the sealed lap must be reverted on 1″ centers. All rivet applications must receive sealant after installation. Lapping joints in the direction of flow is preferred; gutter outlet connections may not fit this condition.

Expansion Of Metals Used For Roof Drainage

Increase in 50 ft Length of Material Due to an Increase in Temperature of 100° Fahrenheit
Increase (in inches) Galvanized Copper Stainless Steel Aluminum Zinc
13/32″ 5/8″ 5/8″ 25/32″ 1 1/32″

Note: Material will decrease in length by the same distances due to a decrease in temperature of 100° Fahrenheit.

Gutter Expansion

It is essential to provide expansion joints in all gutter installations to allow for movement due to temperature changes. In planning expansion joint locations, it must be recognized that an expansion joint acts as a dam in the gutter, therefore, the number and placement of downspouts will be influenced (See section on Proper Gutter and Downspout Sizing).

No gutter length should exceed 50 ft without an expansion joint. Expansion joints should be installed to comply with the SMACNA Architectural Sheet Metal Manual 5th Edition page 1.16 to 1.17 – Allowances For Gutter Expansion.

There are two types of expansion joints commonly used in gutter installation, Lap Type and Butt Type. Both of these types of expansion joints are easily fabricated on jobsite using end caps and small amounts of sheet metal. Details for both Lap Type and Butt Type can be found in the SMACNA architectural Sheet Metal manual 5th Edition pages 1.1 to 1.21 – Lap Type Expansion Joint Figure 1-6 and Butt Type Expansion Joint Figure 1-7.

Proper Gutter And Downspout Sizing

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 never be any more effective than the outlet and downspout selected to drain it. Downspouts 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 it’s own velocity tend to overrun the front edge.)
  • Style of gutters to be used. (All gutters are not effective for their depth and width, see Figures 1-1 and 1-4 for design data.)
  • Maximum length of gutter. 50 ft. (15.2m) between ends or expansion joint is the limit unless the system is especially 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. Sloped gutters may be sized by Chart 1-3. Formula for flow in gutters 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 factors:

  • 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 experimentally determined by the National Institute of Standards and Technology (NIST) formerly National Bureau of Standards. It is plotted from W = 0.0106 M 4/7 L 3/28 (1A) 5/14 with W in feet (m).

Irregular Cross Section Gutter Sizing

The required sizes of gutters other than rectangular or round can be determined by finding 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 by NIST. It is based on W = 0.0182 (I.A.)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 building 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 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 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 it’s depth need be only 4.5 in. (144 mm). in.

Sample problem

To size a half round gutter for a building, 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 gutter to be located on an 80 ft. (24.4 m) side. Column A, Table 1-2, was used to determined rainfall conditions. Since the gutter run will exceed 50 ft. (15.2 m), two downspouts will be used with an expansions joint between. The area of the building is 3200 sq ft. (297 sq m). Thus each of the downspouts will serve an area of 1600 sq ft. (149 sq m) by 160 sq ft/sq in. (2.3 sq m/100 sq mm) to determined that each downspout should have the minimum area of 10 sq in. (6470 sq mm). From Table 1-3 it is found that a 4 in. (102 mm) downspout is required. From Chart 1-2 it is determined that a 9.5 in. (241mm) 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 rainfall rate in inches per hour to determine the actual roof area to be served by the gutter diameter. “The capacity of a sloped rectangular gutter may be approximated by using a gutter cross section area not less than that of a semicircular gutter and depth to width ratio of at least 0.75.

Proper Gutter And Downspout Sizing

Table 1-3
Dimensions of standard downspouts
Type Area “A” Size Nominal Size Actual
Plain Round sq.in. sq.mm. sq.in. sq.mm. in. mm. in. mm.
7.07 4560 5.94 3831 3 76 3 78
12.57 810 11.04 7120 4 102 4 102
19.63 12661 17.71 11422 5 127 5 127
28.27 18234 25.97 15737 6 152 6 152
50.24 32404 47.15 30411 8 203 8 203
Corrugated Round 5.94 3831 3 76 3 76
11.04 7120 4 102 4 102
17.72 11429 5 127 5 127
25.97 16750 6 152 6 152
Plain Rectangular 3.94 2541 3.00 1935 2 51 1.75 x 225 44 x 57
6.00 3870 4.80 8096 3 76 2 x 3 51 x 76
12.00 7740 10.31 6649 4 102 3 x 4 76 x 102
20.00 12900 15.75 10158 5 127 3.75 x 4.75 95 x 121
24.00 15480 21.56 13906 6 152 4 x 6 102 x 152
Rectangular 3.80 2451 3.00 1935 2 51 1.75 x 2.25 44 x 57
7.73 4985 6.38 4155 3 76 2.37 x 3.25 60 x 83
11.70 7621 10.00 6513 4 102 2.75 x 4.25 70 x 108
18.75 12213 16.63 10832 5 127 3.75 x 5 95 x 127

Assuming that using the fewest number of downspouts is desirable, their locations will be affected by:

  • gutter capacity and length. To limit the effects of thermal expansion in gutters 50 ft (15.3m) is a practical maximum length of gutter to be served as a downspout. Unless special provisions are made for flexibility and downspouts, gutters and their support systems, gutters should expand away from downspouts and downspouts should not be located near gutter expansion joints. See expansion coefficients in Appendix A-1 and expansion allowances in Figures 1-5 to 1-10.
  • the capacity of the inlet tube. See Table 1-3 and Figure 1-33. Also, a sharp bend at the inlet may clog.
  • potential for water freezing in downspouts and gutters. Open, partially open or corrugated styles downspouts are suggested for areas subject to icing. Locating downspouts on the north side of buildings is not recommended for such areas.
  • the appearance of the downspout system and potential need for concealment. See Figures 1-31 and 1-32.
  • the greater capacity of a pitched gutter.
  • the downspouts discharge location. Water disposal at this location should be acceptable. See Figures 1-31 and 1-36.
  • the risk of gutter overflow from from insufficient drainage capacity. See Figures 1-4, 1-21, and 1-23.
  • a scupper serving a designated roof area. See Figures 1-26 to 1-30. After the number and location of downspouts of 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 recommendations 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 26m) with a double pitched roof having a slope of 6 in./ft. (152 mm/m). The slope is toward the 100 ft. (30.5m) side. Maximum rainfall conditions will be used to determine downspouts size.

It is decided to drain the building with 4 downspouts located 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 downspouts will drain 170 sq ft. (16 sq m) of roof area. Divide 2338 (217) by 170 (16) to determine that each downspouts 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. (127mm) Plain Round, a 5 in. (127mm) Corrugated Round, a 5in. (127mm) Rectangular Corrugated, or 5 in. (127mm) Plain Rectangular downspout.

Design Of Roof Drainage Systems Roof Drainage

The roof is one of the most essential parts of the 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.

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 necessary fail 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.

Table 1-1
Design areas for pitched roofs
in./ft. PITCH mm/mm *B
Level to 3 76/305 1.00
4 to 5 102-127/305 1.05
6 to 8 152-203/305 1.10
9 to 11 229-279/305 1.20
12 305/305 1.30

*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 inches (square mm) for each downspout. From Table 1-3 select the downspout.

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 minutes 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 (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 i/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. (76mm); 911 (85); 4 in.(102mm); 1100 (102); 5in. (127mm); 1280 (119); 6in. (152mm); 1400 (130) and 8in.(203mm);1750 (163) sq .ft. (sq.m). Net drainage capacity from using Table 1-1 and 1-2 should be compared with local code requirements.

Downspout sizing

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. (12.2m) to admit air and prevent vacuum.
  • Offset of more than 10 ft. (3.0m) can affect drainage capacity.
  • The gutter outlet capacity should suit the downspout capacity.
  • The downspout size must suit the bottom width of the gutter.

Remember that...

During any installation, the following considerations apply:

  • 2″ round downspout can handle 75 gallons a minute
  • standard size or 2 x 3 downspouts can handle 186 gallons per minute
  • commercial size or 3 x 4 downspouts can handle 434 gallons per minute
  • 5″ gutters can use 2 x 3 and 3 x 4 downspouts
  • 6″ metal gutter systems use 3 x 4 downspouts
  • another way to increase gutter capacity is to add more downspouts
  • Increasing size of downspouts or number of downspouts are excellent recommendations for wide roof areas, heavy downpour areas, and for gutter systems that overflow
  • aluminum roof drainage system components can be painted.