Gutter and Drainage Calculation for Northern Ohio

The summer of 2021 came close to breaking the all-time rainfall record in northeast Ohio. The record was set in 1972 at 17.34 inches. 2021 came in second at 16.76 inches, well above the third-place 15.33 inches recorded in 2011.

Heavy rainfall can cause not only outdoor flooding but flooding in your home’s basement or crawl space. One critical factor is a home’s gutters and downspouts and making sure they are sized correctly to channel rain from the roof. Once it flows through the downspouts, the water then needs to move away from the foundation to prevent foundation 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, then onto properly graded soil that allows the water to flow away from the foundation.

Here are the critical elements to consider:

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

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, hitting nearly 5,000 gallons at five inches and 12,000 gallons at 12 inches. 

From this example, it’s easy to see how gutters could be overwhelmed in a downpour. This results in the water running over the gutters and 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, along with the correct number and placement of downspouts. Here are the critical items to consider.

• Home square footage, roof pitch, and roof 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. We selected Cleveland WSFO AP as the location. For your location, choose one from the list of Ohio stations. Then check the table generated below the map.

Cleveland’s five-minute expected rainfall burst that’s considered likely over a 10-year period is 0.525 inches. From this data point, the 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 to 6.3 inches x 1,050, which is a 6,615-square-foot drainage capacity.

A K-style five-inch gutter has a capacity of 5,520 square feet. Unfortunately, this is far less than our required drainage capacity of 6,615. So 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. These should be positioned evenly around the roof to avoid any one downspout being overwhelmed.

Ground Saturation Challenges

That’s the required gutters and downspouts. However, once all that water is on the ground, it needs to move away from the foundation. The starting point for moving that water is sufficient landscape grading along with downspout extensions. That can get the water moving in the right direction. 

Even so, the rain is also falling directly on your lawn and around your foundation. This results in saturated soil, which forms an underground water flow that is moving toward your basement or crawl space.

This happens due to the clay bowl effect, which is the 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 it 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 along with a sump pump system.

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 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.

Correctly sized gutters and downspouts plus the needed 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 review the entire situation.

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