Choosing the right Gutter size
Two sizes of gutters are the most popular in standard homes – five-inch K-style gutters and six-inch half-round round gutters. Each of these sizes are sufficient for most houses, in most areas of the country. It should be noted, however, that houses with roofs that are large and steep, or in some areas of the country where there are large downpours of rain, wider gutters and additional downspouts may be necessary to prevent the overflowing of rainwater.
Drainage Area Square Footage
So what size is necessary? First, the square footage of the drainage area of the gutter needs to be determined. For a standard gabled roof, only two calculations would be required – one for each slope. If your roof has multiple facets, however, the area of each surface that has a drainage area would need to be calculated in order to know the entire square footage.
The next step requires consideration of the roof-pitch factor for your house The roof pitch factor simply indicates the steepness level of your roof. A roof with a higher level of steepness will collect more rain on a windy day. To measure the pitch, use a two-foot level as well as a tape measure. With one end of the level against the roof, level it, then measure the distance between the middle of the level and the roof underneath it. This measurement is called the “twelve-inch run”. For example, a six-inch gap between the underside of the level’s midpoint would be considered a six-in-twelve pitch. Once you know the pitch of your roof, you can then determine the roof-pitch factor using the following table:
Roof pitch Roof-pitch factor
Twelve-in-twelve or higher 1.3
Nine-in-twelve to eleven-in-twelve 1.2
Six-in-twelve to eight-in-twelve 1.1
Four-in-twelve to five-in-twelve 1.05
Zero-in-twelve to three-in-twelve 1.0
Maximum Rainfall Intensity
For different regions, the maximum possible rainfall over a five-minute interval is recorded by the United States Weather Bureau. This measurement is expressed in inches per hour. For this measurement, higher numbers require a larger gutter to avoid overflow in the event of a large storm. Check this table to know the maximum rainfall intensity for your region.
Take the square footage of the drainage area and multiply it by the roof-pitch factor, as well as the maximum rainfall intensity. This number is called the adjusted square footage, and you can use it with the chart below to determine the recommended size of gutter for your home. If different drainage areas on your roof call for different sizes of gutters, choose the one that is the largest.
Five-inch 5,520 square feet
Six-inch 7,960 square feet
Five-inch 2,500 square feet
Six-inch 3,840 square feet
For example: A house in Los Angeles has a roof whose actual drainage area is 1,000 square feet. The pitch factor of five-in-twelve is 1.05. 1.05 multiplied by 1,000 gives us an effective 1,050 square feet of area. If we multiply that number by the local maximum rainfall intensity, which is 2.7 inches per hour, we get an adjusted square footage of 2835 square feet. Therefore, we should choose five-inch half-round gutters.