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Gutters and Drainage Calculations for Your Home in North Carolina

Controlling water runoff from your roof is critical to avoid foundation problems and flooding. Here’s how to calculate the gutters and downspouts you need.

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North Carolina gets its fair share of precipitation. For example, average rainfall in Raleigh varies from a low of 2.8 inches in April to a high of 4.29 inches in July. That’s pretty consistent precipitation falling on your roof and trying to find its way to the lowest spot in your home, which is typically your basement or crawl space. 

Instead of that happening, you’ll need to establish the appropriate capacity for gutters, downspouts, and drainage away from your foundation. Let’s run through the numbers.

Volume of Water During a Rainstorm

The rainwater collection area, or catchment area, is your home’s roof. The chart shown here highlights how much water accumulates on the roof of a 1,600-square-foot single-story home. 

Just one inch of rain gathers almost 1,000 gallons of water. During a storm with 12 inches of rain, the roof collects nearly 12,000 gallons. Then there’s the rain accumulation on your yard that also impacts overall drainage away from your foundation.

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

Roofline and Water Volume

The steeper your roof slope is, the more surface area it will have. Plus, the rain will run off faster, and during winter, a steeper slope will minimize ice build-up. As just one example, a 45-degree slope will increase the rain collected by 30 percent, resulting in 1,296 gallons for the 1,600-square-foot example above.

Yet another factor is the wind. If the rain is blown onto the slope of the roof, more water will collect and run off the windward side of your home.

In addition, home roofs are often made up of hips and valleys. This is where two slopes come together, forming either a peak or a valley. That can act to channel still more water onto the roof and into the gutters.

All of these factors must be considered when calculating gutter and drainage capacity. If the rainfall exceeds their capacity, the gutters will overflow with water that will then run onto your foundation and into the basement or crawl space. That is not what we want to see.

Considerations for Properly Sized Gutters

  • Maximum rainfall intensity of your location
  • Home roof square footage
  • Roof pitch along with any hip and valley rafters
  • Shape of gutters (K-style or half-round)
  • Gutter width dimensions
  • Gutter slope
  • Number of downspouts and shape (round or rectangular)
  • Different types of materials for gutters

Even after all this, the water is still trying its very best to follow gravity. That often results in finding cracks in the foundation, flowing into basements or crawl spaces. That makes drainage around your foundation the critical next step. 

Drainage and Ground Saturation

Cecil soil is the official state soil of North Carolina. It’s a mix of red clay and clay loam. That means it’s critically important to ensure adequate drainage after the water moves off the roof into gutters and through downspouts. Pooling water around the foundation can shift the soil and cause cracks, and as a result, cause basement or crawl space flooding. Cecil soil is only moderately permeable. Plus, during heavy rain, the soil will already be saturated even before the water runs off the roof.

To help the rain move away from the foundation, the landscape needs to be graded to facilitate this flow. Plus, downspouts should have extensions that help that water movement, and these lines also can be placed in the ground. You can further benefit from basement waterproofing or crawl space encapsulation. Likewise, an interior drainage system and a sump pump can protect your foundation.

The Cost of a Home Flood

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.

To avoid these costs, the first step as noted above is to make sure your downspouts and gutters can carry sufficient water off your roof and into drainage away from your home. You also need to ensure your home has effective interior waterproofing measures and strategic exterior water management.

While all this information is helpful in establishing proper drainage, it’s always best to bring in the experts. Give us a call today for a free inspection to help you ensure your home avoids any water damage or flooding.

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