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Home Gutters and Drainage Calculations in South Carolina

Rainfall runoff from your roof can cause serious flooding and foundation problems. Here's how to calculate the downspout capacity your home needs to protect it.

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South Carolina gets its fair share of rain. The average annual precipitation in Charleston is 51 inches, and it’s not much different in Columbia at 46 inches. That can quickly move into flooding, with Charleston ranking at the top in the state with over 29,000 properties at risk, which is 59% of its total number of properties.

That’s a good starting point for assessing your home’s ability to move all that rain off your roof and away from your foundation. Unfortunately, basements or crawl spaces are the normal places for all that water to accumulate. That, in turn, can cause a great deal of damage.

Major Water Challenges

Rainwater quickly accumulates during storms. That includes the home’s roof but also the water on your soil and landscaping. Groundwater is another challenge as the rain saturates the soil and moves toward a basement or crawl space. That’s hydrostatic pressure searching out cracks or porous concrete, and that can lead to major flooding.

Managing Home Water and Drainage

There’s more to managing all this water than you may realize. Here’s the list:

  • Roof surface area and pitch that collect rainfall
  • Gutters and downspouts that move the water to ground level
  • Downspout extension pipes that direct the water away from the foundation
  • Property grading to move water away from the home
  • Basement or crawl space waterproofing that can include building a secure barrier to water as well as interior drain systems to collect leaking water along with sump pumps
  • Dehumidifiers to remove moisture and prevent mold and mildew

Rainstorm Water Volume 

The chart here shows the amount of water that accumulates on the roof of a 1,600-square-foot single-story home. A mere one inch of rain results in nearly 1,000 gallons of water. With a storm of 12 inches of rain, the roof sees almost 12,000 gallons. All that’s added to the water that falls on your yard, saturating the soil.

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

Roof Impact on Water Volume

The steeper your roof, the greater will be its surface area. A 45-degree slope increases the rain collected by 30 percent over the numbers above. That would result in 15,000 gallons for 12 inches of rain in the example above. 

Other factors include any hips and valleys on your roof. These can channel more water into the gutters. Not only that, but wind can force more water onto the windward side of your roof, bringing still more water into those gutters.

Here’s the full list of things to take into consideration when calculating gutter and downspout dimensions.

Calculations for Properly Sized Gutters

  • Maximum rainfall intensity of your location
  • Roof square footage and pitch along with any hip and valley rafters
  • Shape of gutters (K-style or half-round) and width dimensions
  • Gutter slope to move water into the downspouts
  • Number of downspouts and their shape (round or rectangular)

Once the water is on the ground, it’s essential to move it away from the foundation. It doesn’t help that the surrounding soil has also been collecting rainwater. That’s where proper grading of the soil to encourage flow away from the foundation is critical.

Drainage Calculation for Charleston County

Let’s choose a home with a footprint of 1,200 square feet. With a roof pitch of 8’x12’, the pitch factor is 1.1, or a 10% increase over a flat roof. That brings the roof watershed to 1,320 square feet. The maximum 10-year one-hour rainfall intensity in Charleston is 2.92 inches. Multiplying 2.92 by 1,320 yields a drainage capacity of 3,854 square feet. That fits well within the capacity of a five-inch K-style gutter at 5,520 square feet.

Drainage Away From the Foundation

Pooling water around the foundation can shift the soil, cause cracks, and as a result, cause basement or crawl space flooding. Plus, during heavy rain, the soil will already be saturated even before the water runs off the roof.

To start, downspouts should have extensions that help move the water away from the foundation, also pointing it in the right direction. Basement waterproofing measures or crawl space encapsulation can also be enormously helpful in preventing flooding and damage. 

Home Flood Costs

If you needed further motivation, FEMA established cost estimates of several levels of home flooding 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)

  • One inch of water in the home: $26,807
  • One 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.

Those figures look pretty daunting. To avoid that sort of impact on your finances, make sure your gutters and downspouts along with the drainage around your home are set to handle the type of rainfall we get here in South Carolina.

Hopefully, the information we’ve provided above can help you get started. But it’s always best to consult professionals for this critical aspect of protecting your home. Contact us today for a free inspection and repair estimate to help ensure your home avoids any water damage or flooding.

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