Calculations for Gutters and Drainage in Central Missouri

Without adequate gutter capacity your home is at serious risk of foundation damage along with basement or crawl space flooding. Here are the detailed calculations.

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The weather here in Missouri includes warm, moist air from the Gulf of Mexico and cold fronts from Canada. All this results in true four-season weather. However, those seasons can often occur on the same day! 

Of particular concern is responding to heavy rainfall that can lead to basement or crawl space flooding. Even excess moisture can impact your foundation, causing cracking and leading to future flooding. 

Our focus here is to establish the needed drainage of rainfall from your home’s roof and routing it away from the foundation.

Rainfall and Drainage — Protecting Your Home’s Foundation

Here’s the starting list of what to consider when calculating gutters, downspouts, and overall drainage around your home.

  • Roof surface area, pitch, along with peaks and valleys
  • Capacity of gutters and downspouts based on size and shape
  • Gutter pitch and downspout number and placement
  • Grading of the landscape to move water away from the foundation

Rooftop Rainfall Accumulation

The example below shows the amount of rainfall that builds up on a 1,600-square-foot single-story home. With just one inch of rainfall, nearly 1,000 gallons of water are looking for somewhere to go once they leave your roof. Add four more inches, and it’s almost 5,000 gallons.

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 Structure and Water Volume

The example above is for a flat roof. A steeply pitched roof adds more surface area and moves the rain off the roof more quickly. 

If the roof structure has peaks and valleys, those will act to gather more water that will also be moving toward the gutters.

Wind is yet another element. It will blow the rain into the windward side of the roof, gathering still more water.

Here’s a list of key items used to calculate gutter and downspout dimensions.

  • Maximum rainfall intensity expected
  • Roof square footage and pitch
  • Gutter width dimension
  • Shape of gutters (K-style or half-round)
  • Slope of the gutters between downspouts
  • Downspout shape (round or rectangular)
  • Number and position of downspouts

Drainage Calculation for St. Louis

The NOAA Weather Service estimates that the expected five-minute burst of rain likely over a 10-year period for St. Louis is 0.623 inches. Calculating inches per hour is 0.623 x 12, which is 7.48 inches.                                                               

For a home of 850 square feet with a roof pitch of 9-in-12 (pitch factor 1.2), the roof watershed is 850 x 1.2 or 1,020 square feet. Multiplying by the rainfall intensity of 7.48 inches yields a drainage capacity of 7,629 square feet.

That barely fits within the capacity of a six-inch K-style gutter at 7,960 square feet. The slope of the gutters could be increased to move water more quickly. Another possibility is to increase the number of downspouts. 

Drainage On the Ground

Rainfall accumulates on the roof as well as on the ground. So all that water moving across the roof (and into the gutters and downspouts) meets up with quite a few more gallons of water that took the direct route to the ground.

All that results in water above ground and water under the ground. The above-ground water ideally will be flowing away from the foundation in response to landscape grading as well as downspout extensions. The underground water will be moving in the opposite direction toward your basement or crawl space.

This happens due to the clay bowl effect. During home construction, the soil was excavated and backfilled once the foundation was built. This disturbed soil has a different drainage factor and will result in water moving toward the foundation, seeking out any cracks or porous concrete, causing flooding. 

To prepare for this, you’ll need to consider basement waterproofing or crawl space encapsulation, both of which can include interior drainage systems and sump pumps.

The Costs of Home Flooding

FEMA has developed cost estimates on the impact of several different levels of home flooding. We’ve highlighted those in the graph 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.

You can readily see that taking steps now to prevent water damage can pay off when those storms send water rushing through gutters and downspouts, threatening your foundation.

For professional advice on drainage systems for your home, contact your local foundation experts Foundation Recovery Systems for a free inspection and repair estimate to identify any issues that could lead to foundation damage.

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