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Gutter and Drainage Calculation for Northern Ohio

Make sure you have the correct size gutters to prevent basement or crawl space flooding. Here are all the calculations.

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We get plenty of extreme weather here in Northern Ohio. From thunderstorms and tornadoes to blizzards and ice storms, you can find it here. 

Of critical concern for home foundations is heavy rainfall. We need to make sure our home’s gutters and downspouts are sized correctly to channel that rain from the roof. That needs to be followed by moving water away from the foundation to prevent basement or crawl space flooding.

Rainfall Water Management Issues

Drainage management begins with collecting water from the roof through properly sized gutters, into downspouts that are free of obstructions, and then onto properly graded soil that allows the water to flow away from the foundation.

Here’s a list of the critical elements to consider:

  • Gutters with the capacity to handle the anticipated rainfall
  • Downspouts properly sized and positioned to handle that water
  • Lawn sloping that routes water away from the home
  • Downspout extension pipes to facilitate movement away from the foundation
  • Basement waterproofing or crawl space encapsulation
  • Interior drainage system and sump pump to collect and remove any leaking water

Single-Story 1,600-Square-Foot Home Rainfall

In the example shown below, a single-story 1,600-square-foot home accumulates nearly 1,000 gallons of water from just one inch of rain. Add a few more inches and the gallons advance rapidly.

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

From this example, it’s easy to see how gutters could be overwhelmed in a downpour. This results in gutter overflow and hundreds of gallons of water falling directly on your foundation.

Calculating Gutter and Downspout Capacity

To avoid overflowing gutters it’s critical to calculate the expected rainfall and then match that to properly sized gutters and downspouts. Here are the critical things to consider.

  • Home square footage, roof pitch, along with peaks and valleys
  • Gutter shape (K-style or half-round) and width
  • Downspout placement, slope, and shape (round or rectangular)
  • Expected rainfall intensity in your area

All those factors go into calculating the proper sizing of gutters and downspouts.

Sample Gutter Calculation for Cleveland

Our calculation begins with the expected rainfall intensity, which can be found at NOAA Weather Service in their detailed precipitation frequency estimates. Cleveland’s five-minute expected rainfall burst that’s likely to happen over a 10-year period is 0.525 inches. Expected inches per hour is 0.525 x 12, or 6.3 inches.

With a 1,000-square-foot home with a roof pitch of 4-in-12, a pitch factor of 1.05, the total roof watershed is 1,000 x 1.05, or 1,050 square feet. Taking into account the expected rainfall intensity computes at 6.3 inches x 1,050, or a 6,615-square-foot drainage capacity.

A K-style five-inch gutter has a capacity of 5,520 square feet. This is far less than our required drainage capacity. Instead, the six-inch K-style gutter with a capacity of 7,960 square feet must be used.

Downspouts would need to be sized and positioned to handle this level of rainfall. Rectangular 2 x 3-inch downspouts have a capacity of 600 square feet, while 3 x 4-inch can handle 1,200 square feet. If we choose the 3 x 4-inch rectangular downspouts, we’d need at least six to handle the expected water flow.

Ground Saturation Challenges

We’ve successfully calculated the required gutters and downspouts. But once all that water is on the ground, we need to move it away from the foundation. 

The starting point for addressing that challenge is sufficient landscape grading along with downspout extensions. That gets the water moving in the right direction. 

However, the rain is also falling directly on your lawn and around your foundation. All this water results in saturated soil, which forms underground water that is moving toward your basement or crawl space.

This happens due to the clay bowl effect. This effect is a natural result of removing soil when constructing the foundation and then backfilling around the finished basement or crawl space. That backfilled soil has a different drainage factor from the rest of your landscape. As a result, water has an easier time moving through that soil while seeking ways to enter through cracks or other openings.

Effective drainage around your foundation is the starting point. Your home may also require basement or crawl space waterproofing including interior drainage and sump pump systems.

Water Damage Repair Costs

FEMA has developed cost estimates on the impact of several different levels of home flooding. Those are highlighted 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)

  • 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.

Properly sized gutters and downspouts plus sufficient drainage around your foundation can go a long way toward preventing this type of costly flooding in your home. But it’s always a good idea to bring in the professionals to take in the whole picture.

To schedule a free inspection and repair estimate as well as identify any drainage issues, contact the experts at Ohio Basement Systems today.

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