When it comes to gutters installation mostly people think about esthetic look, and only those who have had already problem with water in the basement or mold on the walls think about functionality and proper water drainage.

And a very first question is, what size of gutters and downspouts need to be used. Even if 5” gutters with regular 2×3 downspouts can be able to collect and distribute rain water to storm drainage on most residential houses of the country, there is a bigger 6” gutter or 3×4 downspouts needed in some situation (bigger or steeper roof, longer distance between two adjusted downspouts, bigger buildings -apartments or commercial construction, etc). Few factors can help us to make a right decision:

  1. Roof size. With simple two slope roof we would need just calculate one side (width x length) times 2. Intersecting or hip roofs have a multiple facets, and to get total roof square footage we need to add up all areas of each surface within water collection area.
  2. Roof-pitch factor. The steeper is roof’s pitch, the more a windblown rain it can collect. Roof-pitch factor is presented in table below:

Roof pitch to roof-pitch factor

0 to 3 pitch – 1.0 factor

4 to 5 pitch – 1.05 factor

6 to 8 pitch – 1.1 factor

9 to 11 pitch – 1.2 factor

12 and higher- 1.3 factor.

  1. Maximum rainfall intensity. The U.S. Weather Bureau records the maximum rainfall that could possibly happen in a 5-minute period, in inches per hour, for various regions. The higher the amount, the bigger a gutter has to be to keep from being overwhelmed in a storm burst
  2. Gutter sizing. Adjusted square footage roof area is equal roof size multiplied by roof-pitch and rainfall factors. Compare numbers to the chart below to determine gutter size.

5” gutter cover roughly 5,500 square feet roof, one foot of gutter can hold 1.2 gallons of water

6” gutter cover roughly 8,000 square feet roof, one foot of gutter can hold 2.0 gallons of water

For example: 2,500 square feet roof with pitch 7 at house located at Lexington, MA (maximum rainfall is 1.7inch) yields an adjusted square footage of 2,500×1.1×1.7=4,675 feet. This shows us that 5” gutter can properly collect and distribute rain water on this building.

Water distribution and drainage

There is not enough install gutter sufficient big to collect all participations, not less important is correct distribute and drain all water. Improper drainage can create gutter overflow, soil erosion, may cause structural damage and in some situations even flooded basement.

First, all gutters must be properly pitched at least ¼” for every 10 feet of gutter run. Sometimes is good idea increase pitch, especially on longer gutters (over 40-50 feet long), though not everybody likes how those gutters looks along a fascia.

Second, for proper drainage downspouts need to be installed not less than every 30-40 feet of gutter run to match or exceed expected runoff. Chart below will help to understand how many downspouts are needed to install.

2×3 inches for every 600 square feet drainage area

3×4 inches for every 1,200 square feet drainage area

5” gutters come usually with 2×3 inches downspouts, but for increased drain capacity or in situation when impossible to install more than one downspout for gutter run more than 40 feet, 3×4 downspout could be installed. But for long gutter runs (over 40 feet) it always true, that is better to have two smaller 2×3 downspouts on each end of gutter than just only one 3×4 inches.

6” gutters come with 3×4 downspouts.