Gutter & Downspout Drainage Calculations for Homes in NC

rendering of gutters and downspouts installed on a house

As a homeowner, you have the responsibility of protecting your home from damage like flooding. Home essentials like properly functioning gutters and downspouts are your first line of defense in controlling water runoff from our wet weather.

Instead of holding your breath when it rains and hoping water won’t end up in your basement, you must establish the appropriate capacity for gutters, downspouts, and drainage away from your foundation. Let’s run through the numbers.

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A Look At The Weather

North Carolina is classified with a humid subtropical climate and gets its fair share of precipitation. For example, average rainfall in Raleigh varies from a low of 2.99 inches in April to a high of 4.49 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.

Volume of Water During a Rainstorm

The rainwater collection area, or catchment area, is your home’s roof. The chart below 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 in your yard, which also impacts overall drainage away from your foundation.

how many gallons of water after rainfall

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; during winter, a steeper slope will minimize ice build-up. For 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 roof’s slope, 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 the capacity of the gutters, the water will overflow, running onto the foundation and into the basement or crawl space.

What Are Gutters?

Gutters are half-pipes placed on the edges of your roof that help keep water away from your home’s foundation. The most common gutter options are K and U types, available in different materials, including aluminum and stainless steel. A comprehensive gutter system consists of two main parts: the channel through which water runs and the downspout that directs it into the ground.

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 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 a critical step.

What Are Downspouts?

installer carrying a section of a downspout

A downspout is a tube that is placed vertically and connected to the horizontal drains (gutters) that direct water away from your home. The three most common types of downspouts are K-style, round, and rectangular. Downspouts are an important element of any gutter system and help prevent soil erosion and foundation damage. Although they are just simple pipes, they must be properly maintained to do their job well.

Downspout Considerations

Rainfall intensity
Size of area to be drained
Design and size

Number needed (approx. every 30-40 feet of guttering)
Roof slope
How far the downspouts should extend (at least 10 feet)

Additional Drainage Considerations

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

The landscape needs to be positively graded to facilitate rain movement away from the foundation. Downspouts should have extensions that help water move; these lines can also be placed underground.

The Cost of a Home Flood

Flooding and water damage result when your home lacks proper water management solutions.

FEMA established cost estimates for several levels of home flooding in the chart below.

cost of water damage, repairs, and flood cleanup

(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: Causes up to 30% loss in home value. That’s $105,000 for a $350,000 home.

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

Need Drainage Help? Contact Tar Heel Basement Systems Today!

Proper drainage with gutters and downspouts is just the beginning of water management. Interior drainage, sump pumps, and dehumidifiers are crucial to protecting your home. Tar Heel Basement Systems is the go-to source homeowners in North Carolina trust for the latter. Call us today for a free inspection to help you ensure your home avoids any water damage or flooding.

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Gutter and Downspout FAQs

Are gutters considered part of the roof structure?

While gutters are an integral component of the overall roofing system, they are distinct from the roof itself. They work in conjunction with the roof but are separate structures.

Can faulty gutters contribute to roof leakage?

Yes, a malfunctioning gutter system can lead to roof leaks. If water can’t drain properly, it may accumulate in the gutters, potentially damaging your roof. Additionally, the weight of stagnant water or excessive debris can harm your home’s fascia, possibly causing leaks along the roof’s edge.

What are the signs that gutter or downspout replacement is necessary?

Consider replacing your gutters and/or downspouts if you notice signs of decay, such as rotting, paint peeling, cracking, or mildew growth. These indicators suggest your gutters are failing or on the verge of complete failure.

Related Resources

6 Reasons Why You Have Water in Your Basement

Discover how and why water gets in your basement in this guide. Tar Heel Basement Systems can help you prevent basement water!

How Poor Foundation Drainage Causes Basement Damage

If your foundation drainage is subpar, it can result in damage to your home, your health, and your budget.

Hands are installing pipe in the exterior tunnel.

Underground Downspouts: Worth The Investment?

Underground downspouts redirect rainwater away from your basement and foundation – but do you need to invest in them?

Holly Richards-Purpura

Content Writer

Holly is a Content Writer for Groundworks who has written and edited web content for the foundation services industry for almost 10 years. With a background in journalism, her passion for the written word runs deep. Holly lives in Columbus, OH, with her husband. Along with educating homeowners, she also has a big heart for the Big Apple.