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Gutter and Drainage Calculation for Central and Southern Ohio

Heavy rain can lead to basement or crawl space flooding. Learn how to calculate gutter capacity to help prevent damage to your foundation.

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Central and Southern Ohio expects and gets extreme weather. That includes thunderstorms with lightning and hail, tornadoes, and winter snow and ice storms. The critical issue is to be ready for them. 

Rainfall Considerations

One aspect of that preparation is to ensure you have the right gutters and drainage to move water off your home’s roof and away from the foundation.

Excess rainfall brings along with it flooding. It accumulates on the roof, rushing off and onto the ground. It also collects on the ground and can accumulate around your foundation if the ground isn’t graded correctly to move it away. 

In addition, once the water has saturated the soil, underground water will start moving toward your foundation building up hydrostatic pressure. That can find any cracks and start leaking into your basement or crawl space.

Gutters, Downspouts, and Drainage Systems

Effective drainage management is all about routing water off your roof and away from your foundation. Here are the key items to consider:

  • Gutters with sufficient capacity to deal with the expected rainfall
  • Downspouts that can handle that water and move it to the ground
  • Landscape grading that moves water away from the foundation
  • Extension pipes on the downspouts to help move the water
  • Basement or crawl space drainage system to gather and remove leaking water
  • Additional waterproofing measures for the home’s basement or crawl space

Amount of Rainfall on a Single-Story 1,600-Square-Foot Home

In the example shown in the graph below, a single-story 1,600-square-foot home accumulates nearly 1,000 gallons of water from just one inch of rain. Add a few more inches and the gallons advance rapidly.

how many gallons of water after rainfall

Rainfall on a 1,600-square-foot home

  • 1″ of rain: 997 gallons of water
  • 5″ of rain: 4,984 gallons of water
  • 12″ of rain: 11,962 gallons of water

This example shows just how much water can collect on your roof. Of course, if one inch of rain collects over a day, it’s a great deal easier to deal with than if the same amount happens in a few minutes. At that point, the rain can overflow the gutters and fall directly on your home’s foundation. That underlines the importance of installing the correct size gutters and downspouts.

Gutter and Downspout Capacity

Here are the critical considerations when calculating gutter and downspout capacity.

  • Roof square footage and the pitch or steepness
  • Gutter shape (K-style or half-round), dimensions, and slope
  • Downspout number, placement, slope, and shape (round or rectangular)
  • Expected rainfall intensity

Drainage Calculation for Cincinnati

Let’s take all this to build a sample calculation. For the rainfall intensity, the NOAA Weather Service provides tables of precipitation frequency estimates. For Cincinnati, the expected five-minute burst likely over a 10-year period is 0.58 inches. Calculating inches per hour is 0.58 x 12, or 6.96 inches.

We’ve chosen an 850-square-foot home with a roof pitch of 7-in-12, which is a pitch factor of 1.1. The total roof watershed computes as 850 x 1.1, or 935 square feet. Multiplying that by the expected rainfall intensity of 6.96 inches gives us a 6,507-square-foot drainage capacity.

A K-style five-inch gutter has a capacity of 5,520 square feet. That would not match our required drainage capacity. Instead, the six-inch K-style gutter with a capacity of 7,960 square feet should be used.

Downspouts would need to be positioned and sloped appropriately to handle this level of rainfall. Rectangular 2 x 3-inch downspouts have a capacity of 600 square feet, while 3 x 4-inch can handle 1,200 square feet. That’s why several downspouts are placed around the home.

Rainfall Ground Saturation

Not only does the water off the roof accumulate on the ground but the rainfall also goes directly onto the ground around your home. That can saturate the soil and set up an underground water flow.

Landscape grading and downspout extensions can help route water away from the foundation, but once underground, it can flow toward your basement or crawl space. That’s a result of the clay bowl effect.

During construction, the soil is removed to build the foundation and then backfilled. This makes the backfill soil drainage factor different from the surrounding soil. This results in water flow toward the foundation, seeking out cracks and causing flooding.

The best way to deal with this is with basement or crawl space waterproofing including interior drainage and sump pump systems.

Repair Costs from Water Damage

FEMA has developed cost estimates on the impact of several different levels of home flooding. Those are highlighted in the chart below.

cost of water damage, repairs and flood cleanup

Cost of Water Damage and Repairs

(2,500-square-foot single-story home)

  • 1 inch of water in the home: $26,807
  • 1 foot of water in the home: $72,163 
  • Damaged foundation: Lose up to 30% of your home value. That’s $105,000 for a $350,000 home.

That level of expense should help motivate any homeowner to install gutters and downspouts with sufficient capacity for expected rainfall intensity. That should be pursued along with any other means of preventing costly flooding.

That’s where help from the experts comes in. The professionals at Ohio Basement Authority can identify any drainage issues with a free inspection and repair estimate.

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Cincinnati, OH 45215

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