Gutter and Drainage Calculations for Minnesota and the Dakotas

It’s an understatement to say that we get weather extremes here in Minnesota, North Dakota, South Dakota, and eastern Montana. As one example, Fargo averages 49 inches of snow per year compared to the U.S. average of 28 inches. Rainfall is a bit less at a 24 inch average in a year versus the U.S. average of 38 inches. 

All that rain and snowmelt collects on a home’s roof and needs to be routed to the ground and away from the foundation. Our focus here is to determine the gutters and drainage your home needs to avoid basement or crawl space flooding that can cause foundation cracking and flooding. That includes preparing for the occasional year with extreme rainfall.  

Water Issues To Consider

Flooding is all about excess water and how we deal with it. Here’s a quick overview:

• Rainwater accumulating on your roof
• Snowfall on your roof that melts as temperatures rise
• Surface water from rain or snowmelt
• Groundwater from soil saturation

All this can collect around your basement or crawl space, causing a buildup of hydrostatic pressure finding entry through foundation cracks and other openings.

Managing Water and Drainage

Since the roof is collecting a great deal of water, the challenge is moving that water off your roof and away from your foundation. Here are the critical aspects to consider.

• Gutters installed along the roof edge
• Downspouts to channel the water to the ground
• Extension pipes to direct the water away from the foundation
• Grading the soil around your home to move surface water away
• Basement waterproofing measures or crawl space encapsulation
• Interior drainage systems to gather leaking water
• Sump pumps to remove the water
• Dehumidifiers to reduce moisture, preventing mold and mildew

Rainfall on an Average-Sized Home

In the example shown in the chart below, a 1,600-square-foot single-story home accumulates nearly 1,000 gallons of water from just one inch of rain. Add a few more inches and the gallons advance rapidly, with five inches of rain reaching nearly 5,000 gallons of water. Then there’s an incredible 11,962 gallons from 12 inches of rain. 

Rainfall Intensity and Water Drainage

The chart above provides an idea of how much water can collect on your roof. With that in mind, it’s a matter of managing the overall flow of that water. Dealing with a steady, gentle rainfall over the course of a day is much easier than dealing with a downpour. When that happens, the water can overflow the gutters with hundreds of gallons of water pouring directly onto your foundation.

From that, you can see that it’s particularly important to install correctly sized gutters and downspouts to deal with the expected amount of water.

Drainage Capacity Factors

Quite a few factors come into play when calculating drainage capacity. For example, the pitch of the roof can add more surface area and accelerate the flow off the roof. In addition, roof peaks and valleys collect water as well. Plus, any wind can direct even more rain to the windward side of the roof.

Here are the key items to consider:

• Roof size and pitch along with peaks and valleys
• Gutter dimensions and shape (K-style or half-round)
• Slope of the gutters
• Downspout number, position, and shape (round or rectangular)
• Expected rainfall intensity

Drainage Calculation for Fargo

Let’s dig into a sample calculation. The NOAA provides extensive tables of precipitation frequency estimates. For Fargo, the expected five-minute burst likely over a 10-year period is 0.59 inches. Inches per hour is 0.59 x 12, or 7.08 inches.

Let’s choose as an example an 850-square-foot home with a roof pitch of 9-in-12 (pitch factor 1.2). The roof watershed is then 850 x 1.2, or 1,020 square feet. Applying the rainfall intensity of 7.08 inches by 1,020 equals a drainage capacity of 7,222 square feet.

A six-inch K-style gutter has a capacity of 7,960 square feet, so that would be a good fit. Downspouts would need to be placed 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 downspouts can handle 1,200 square feet. If you chose the 2 x 3-inch downspouts, you’d need 14. The 3 x 4-inch downspouts could handle the drainage requirements with half that number, or seven.

Ground Saturation and Water Flow

Our calculation example gives you an idea of the amount of rain collecting on your roof, but that rain is also falling on the ground around your home. Then it sinks in, saturating the soil and setting up underground water flow.

With effective grading, water from your downspouts and direct rainfall will flow away from your foundation. But underground water will quite often be flowing in the opposite direction, toward your basement or crawl space as a result of the clay bowl effect.

During construction, the soil is removed to build the foundation and then backfilled. This soil’s drainage factor is different from the surrounding soil since it is not packed tightly. This results in water flowing toward the foundation, seeking out cracks and causing flooding.

The best way to deal with this is basement or crawl space waterproofing, including interior drainage and sump pump systems.

Water Damage and Repair Costs

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 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.

Those are serious costs that should prompt you to take the steps needed to prevent water damage to your home’s foundation. 

That’s where the professionals can help. The experts at Innovative Basement Authority provide free inspections to identify any drainage issues that could lead to foundation flooding and damage.