Gutter downspouts are vital parts of the gutter system, but do you know how to figure the right gutter downspout sizes?

In general, the right gutter downspout size is based on:

  • The size of the roof,
  • The gutter volume, and
  • The level of rainfall during the rainy seasons.

The size calculation should be done with accuracy and is crucial to prevent overflow during heavy or prolonged rainfall or intense precipitation. If math isn’t your thing, rest assured, gutter professionals will help you find the right size for your gutter system and roof design. Read on to learn more about what should be considered when determining the size of a gutter drain.

Gutters and Gutter Downspout Sizes

Gutters are typically round-shape or K-style, but other designs are available. They are made fromaluminum, vinyl, zinc, copper or steel. 5 inches and 6 inches gutters are the most common sizes installed for residential use. However, 7, and 8 inches are available although the larger sizes are more suitable for larger buildings or business properties.

As far as downspouts go, they run at 2″x3″ and 3″x4″ in size or 3 or 4 inches in diameter. Gutter professionals have experience matching the gutters and the downspouts. For example, a 6-inch K-style gutter requires a 3″ x 4″ downspout while a 5-inch K-style gutter uses a 2″ x 3″ downspout. The need to call in the gutter experts may be even greater if you have a rectangular or an irregular cross-section gutter, a steeply sloped roof, or live in an area that experiences heavy rainfall.

Different Types of Downspouts

In general, there are three different types of downspouts:

  • K-Style: This is the standard downspout style used on homes with the 5” K-style gutter system. Available sizes are 2″×3″ and 3″×4″.
  • Round: Round downspouts are typically used with half-round gutters. They come in various colors and sizes of 3″, 4″, 5″ and 6″.
  • Rectangular: This is the second most common downspout style seen in homes constructed in the city. Rectangular gutter downspouts are available in 2″×3″, 3″×4″, and 4″×5″ sizes.

How to Figure the Right Downspout Size?

Installing the right-sized gutter downspouts not only promotes efficient drainage, but it can also save you money in expensive repairs by having the capacity to drain quickly. The following guide could help take the guesswork out of the selection process. It takes into consideration these important factors:

  • Area to be drained
  • Gutter system design
  • Roof shape, pitch and design
  • Gutter style and size
  • Gutter outlet capacity

Size of the Gutter Drainage Area

One downspout is typically enough for every 20-25 feet of guttering. But more could be required based on the local weather or rainfall intensity. Using 2 downspouts doubles the capacity of the gutter to manage water runoff without overflowing.

You’ll need to measure the roof to determine the watershed area. This is the area where the water will run off. Once you get the peak to eave length multiply it by the width to determine the square footage.

Rainfall Intensity

Rainfall intensity is measured in inches per hour at a 5-minute interval. You should be able to obtain the rainfall intensity figures from your local Weather Bureau. The gutter and downspout capacity is, therefore, tested based on how much rainwater they can collect and remove and how quickly.

This is measured in gallons per minute (GPM). A larger-sized gutter and downspout will work better in locations that experience heavy or prolonged rainy seasons. Together, they can remove water from the gutter faster thus improving the efficiency of your gutter system.

Gutter Outlet Capacity

The slope of the roof and size of the gutter affect the gutter drainage capacity. That is to say, how many gallons of water per minute will be channeled off the roof to the gutter and into the downspouts.

To prevent overflow and water damage to the building structure, e.g., the sidings, or the foundation, the gutter outlet capacity must be in tune with the speed at which the water will be flowing off the roof. Water will likely run off with more velocity from a roof with a steeper slope. Similarly, the gutter downspout sizes should be large enough to handle the amount of water fast enough to prevent overflow.

Additional Gutter Downspout Sizing Factors

Other important factors to keep in mind:

  • The downspout size should be consistent throughout its length
  • Gutter downspout sizes must not be larger than the bottom width of the gutter
  • The gutter and downspout outlet capacity should match
  • Offsets longer than 10 ft can interfere with the drainage capacity
  • Downspouts should not be spaced more than 30’ apart

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