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Calculating the Importance of Gutters and Drainage in Ohio

Heavy rain can lead to basement or crawl space flooding in Ohio. 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 heavy rain, thunderstorms with lightning and hail, and winter snow and ice storms. The critical issue is to be ready for them. 

Rainfall Considerations

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

Excess rainfall can lead to flooding. It accumulates on the roof and in the gutters, shedding off onto the ground. As rainwater collects on the ground, it can pool around your foundation if not properly diverted away from your home’s foundation.

Gutters, Downspouts, and Drainage Systems

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

  • Gutters with sufficient capacity to deal with excessive rainfall
  • Downspouts to direct the water away from the house and landscape grading that optimizes water drainage 
  • Extension pipes attached to 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
  • Materials options such as vinyl, aluminum, or copper 
  • Gutter shape (K-style or half-round), dimensions, and slope
  • Number of downspouts, placement, slope, and shape (round or rectangular)
  • Expected rainfall intensity and climate

Drainage Calculation for Cincinnati

Let’s take all this information 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. It’s important to install several downspouts around the home.

Rainfall Ground Saturation

Excessive precipitation runoff can pool on the ground and enter the soil, thus becoming groundwater. 

Once the soil is saturated,  the groundwater will start moving toward your foundation. The result is hydrostatic pressure, leading the pressurized water to find cracks in the foundation and start leaking into your basement or crawl space.

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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Serving Greater Columbus, Cincinnati, Dayton, and Northern Kentucky

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1270 Hillsmith Dr.
Cincinnati, OH 45215

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Obetz, OH 43207

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Beavercreek, OH, 45440