Properly Size Roof Drainage (Gutters and Downspouts)

Properly designed roof gutters reduce the amount and frequency of roof run-off water that wets the above-grade walls or the foundation. A list of recommendations and a simplified design approach are presented below to help in the proper use of gutters. Further, an example problem is provided to illustrate the simplified design approach.

• Ensure that gutters have a slight downward slope toward downspouts.
• Downspouts that discharge directly to the ground surface should do so at least 2’ outward from the building. Splash blocks or plastic corrugated pipe are recommended to prevent erosion and to give further extension of discharge water away from the foundation, particularly for downspouts located at inside corners of building, if poor (such as expansive clay) soil conditions may exist, or where the ground surface slope away from the foundation is minimal.
• Where foundation backfill grade does not provide positive drainage away from the foundation, it should be corrected rather than simply relying on extension of downspout discharge. Should gutters become clogged causing water to spill over, the surface grade away from the foundation will be the only means of preventing potential water intrusion or damage to foundation walls.
• Downspouts that discharge water below grade should do so into nonperforated corrugated or smooth plastic pipe. The pipe should be run underground to a suitable outfall. Do not connect the gutter drain pipe to the foundation drainage system; this practice will soak the foundation.
• If local storm water management regulations require roof water infiltration on-site, do so at a maximum practicable distance from the building foundation; minimum separation distances required by local storm-water regulations may not be adequate.
• Gutters and downspouts should be resistant to corrosion and abrasion from flowing water; material choices include aluminum (most popular), vinyl or plastic, copper, and coated metal (baked enamel or galvanized).
• Use a gutter splash shield at inside corners (i.e., valleys) where fast moving water in a roof valley may “overshoot” the gutter.
• Gutters, downspouts, and splash blocks must be cleaned and properly maintained by the homeowner; where significant sources of roof and gutter debris exist (e.g., trees) a high-quality gutter guard system may be helpful in reducing the frequency of unclogging gutters.

A typical gutter installation uses a 5” deep K-style gutter with 2” by 3” downspouts at one or both terminal ends of gutters. While this “field practice” may be adequate for many homes in typical climate conditions, the following simplified design approach is useful to determine situations where such designs may be inadequate.

Only two steps are required to properly design a steep-slope roof drainage system using standard guttering products. Of course, this design method (drawn mostly from a 1999 industry reference15) assumes gutters are properly installed for positive drainage and that gutters are regularly cleaned to maintain a “clog-free” condition.

Step 1: Determine Design Rainfall Intensity

The design rainfall intensity for roof drainage design is sometimes based on a 10-year return period and 5-minute duration (Figure 4–8). However, other design return periods and durations may be used effectively or required by code (e.g., commercial building roof drainage). Conversion factors for other acceptable design conditions are included in Figure 4–8. A standardized design criterion in U.S. model building codes for homes does not exist, so practical experience and judgment are important.

Step 2: Determine Roof Drainage System Spacing and Layout

Based on a selected gutter size and type as well as the design rainfall intensity from Step 1, determine the maximum allowable plan (horizontal) area of the roof that the gutter can adequately serve from Table 4–5, including any adjustment required by the table notes. Based on this area and the roof geometry, downspout spacing and locations can be determined as shown in the example below. With use of the suggested minimum downspout sizes in Table 4–5, the gutter size will control the spacing of downspouts. It is also generally recommended that downspouts should serve no more than 50’ of gutter length.

Example: Sizing roof gutters and downspouts in two steps

For the house located in Savannah, GA, as shown in Figure 4–9, the following example is provided to illustrate this best practice:

Step 1: Determine Design Rainfall Intensity

From Figure 4–8, a design rainfall intensity of 7 in/hr is determined for the site.

Step 2: Determine Roof Drainage System Spacing and Layout

A 5” K-style gutter is selected from Table 4–5 with a maximum allowable roof tributary plan area of 600 ft2. Because the roof slope is 6:12, the allowable tributary roof area is 0.85 x 600 ft2 = 510 ft2. The actual horizontal (plan) roof area for the side shown is (14’x34’) + (14’x12’) = 644 ft2.

The number of downspouts required is 644 ft2 / 510 ft2 = 1.3. The number of downspouts should always be rounded up, so two downspouts should be used, one at each end of the L-shaped gutter layout. The downspout size may be 2”x3” or 3”x4” as suggested in Table 4–5. Use of the larger size downspout, if architecturally acceptable, may help reduce the potential for clogging from debris.