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.
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.
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.