10. Gutters and Downspouts

• 10.1. Hung Gutters and Downspouts Basics
• 10.2. Hung Gutters
• 10.3. Built-in Gutter Linings
• 10.4. Water Diverters
• 10.5. Scuppers
• 10.6. Sumps and Drains
• 10.7. Downspouts
• 10.8. Downspout Hangers

Introduction

The design of gutter and downspout assemblies is an area of building design which demands special attention. Leaking gutters and downspouts can cause serious damage to a building's interior as well as exterior, and repairs can be expensive.

Maintenance, durability and longevity are important factors to consider when designing gutters and downspouts. Copper is an intelligent choice of materials because of its low maintenance, high resistance to corrosion and long life. Even in severe climates such as marine atmospheres, a well designed copper gutter and downspout assembly will provide many years of low maintenance service.

Other metals used in gutter and downspout assemblies require frequent repainting or recoating to maintain their durability. Copper is an inherently corrosion resistant material which does not require special coatings to maintain its durability or its appearance.

The ease with which a material can be joined to form a continuous, leak-free water conductor is also important. Copper's inherent properties make it an easy material to form and solder. Thus, strong leakproof joints are readily achievable with copper.

Design Principles for Roof Drainage Systems

The building type, its appearance and location have a direct influence on the design of the roof drainage system. They determine the roof area, slope and rainfall intensity. They also influence the use of gutters and downspouts, roof drains and scuppers.

The process of calculating the required size of gutters and downspouts involves:

1. Obtaining rainfall intensity for the building location.
2. Determining the spacing and locations of downspouts.
3. Calculating design roof areas.
4. Sizing the downspouts.
5. Sizing the gutters.

Rainfall Intensity

Rainfall intensity is measured over a 5-minute period. It is recorded, in inches per hour, as the resulting accumulation as if the intensity remained constant for a full hour. Table 10A shows the rainfall intensity for major U.S. cities. The table is divided into two sections, A and B. These sections represent the intensities which are likely to be exceeded once in 10 years, and once in 100 years, respectively.

The table also shows the calculated roof area which can be drained per square inch of downspout. It is based on the assumption that during a rainfall with an intensity of 1 inch per hour, each square inch of downspout can drain 1200 square feet of roof. If the intensity is doubled the downspout capacity is halved, or 600 sq. ft.; if it is tripled the capacity is one third, and so on.

Downspout Locations

The locations of down-spouts depends on the configuration, architectural features and appearance of the building. The technical considerations include:

1. Each downspout should drain a maximum of 50 feet of gutter. Gutter expansion characteristics may further limit the distances, since water cannot flow past an expansion joint.
2. Avoid locations where water must flow around a corner to reach a downspout.
3. In locations where icing occurs, downspouts on the north side of the building should be avoided, if possible.

Expansion Joint Spacing

Expansion joints in copper gutters must be provided to allow for the natural expansion and contraction of copper caused by thermal changes. In general, long straight runs should have joints spaced a maximum of 48 feet apart. Expansion joints may also be required at changes in gutter width or depth, at corners and at end conditions. Based upon the desired joint spacing, designers should consult Table 10B to determine the required gauge of copper gutter, width of gutter bottom and angle of gutter sides.

Expansion Joint Table

Determination of gauge and expansion joint location for various sizes and shapes of copper "U" sections

Design Area for Pitched Roofs

The roof area to be drained is a key factor in designing gutters and downspouts. The area of roof contributing runoff to each gutter and downspout should be determined. The maximum accumulation of rainfall occurs when it falls perpendicular to the roof plane. With flat roofs, it is a simple matter of calculating area, since the true roof area is equal to plan area.

When a roof is pitched, its plan area is less than its true area. However, using the true area in the calculations has typically resulted in oversized gutters, downspouts and drains. Table 10C shows the factors that should be used to determine the design area for pitched roofs. The plan roof area should be multiplied by this factor. The result is the design roof area that is used to calculate the required sizes of downspouts.

Downspout Sizing

Downspouts should have a cross-sectional area of at least 7 square inches, except for small areas such as porches and canopies. Their size should be constant throughout their length.

The design roof area is divided by the area of roof shown in Table 10A, column A2 or B2 (see discussion above), to give the minimum required area for each downspout. See Table 10.7A for standard downspout sizes.

Gutter Sizing

The minimum required size of a gutter is related to the intensity of rainfall and the area of roof that drains into the gutter. The latter depends on the length of the gutter, which is related to locations of downspouts, expansion joints, and gutter ends.

Other factors considered in the design of gutters, include the size and spacing of outlets, the shape of the gutter, and the pitch of the roof. The gutter size must be capable of handling even fast moving water from a steep roof.

Table 10D is used to determine the required width and depth of a gutter. To do this, a ratio, M which equals the depth divided by the width, is initially assumed. Starting with the length of the gutter, L, follow a vertical line until the ratio, M, is reached. At this point follow a horizontal line to the left until the vertical line of rainfall intensity x design area, IA, is crossed. The required gutter width can be read from the diagonal lines. If the intersection lies between two lines, use the higher value. Finally, the width is multiplied by the ratio, M, to determine the depth.

Table 10D. Gutter Sizes for Given Roof Area and Rainfall Intensity

The size of gutters with an irregular shape can be determined by calculating the required size of a rectangular gutter which closely matches in profile and cross-sectional area, the irregularly shaped gutter.

Table 10E shows an example of the complete process.

Table 10E. Example Calculation