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Rainwater and Exact Calculations for Gutters and Drainage in Colorado

Rainwater from your roof: How much is there? How do I get it to drain instead of flooding my basement or crawl space? Here’s the math and further insight.

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During a heavy rainstorm, you’ve probably noticed the water running off your roof, into the gutters, through the downspouts, and out onto your lawn. With any luck, it continues its journey and doesn’t find its way into your basement or crawl space. But instead of relying on luck, let’s examine exactly how much water to expect and how to move it off the roof into suitable drainage.

What Determines the Volume of Water During a Rainstorm?

The overall collection area is the home’s roof. In the chart below we show how much water will accumulate on the roof of a 1,600-square-foot single-story home. 

You can see that with just one inch of rain almost 1,000 gallons of water will accumulate. With a full-blown storm with 12 inches of rain, the roof has collected nearly 12,000 gallons. Note, too, that the rain is also accumulating on your lawn, which further compounds drainage issues.

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

Roofline and Water Volume

The steeper your roof, the more rain it will collect and the faster it will run off. For example, a 45-degree slope will increase the rain accumulation by 30 percent, resulting in 1,296 gallons for the 1,600-square-foot home above rather than 997 gallons.

An additional factor is the wind. Rain is seldom falling straight down. If the wind blows the rain into the slope of the roof, more water will be collected on that side of your home. 

All this needs to be taken into account when calculating gutters and drainage capacity. If the rainfall exceeds the capacity, the water will overflow the gutters, and rather than following your carefully planned drainage, it will just run onto the foundation and into your basement or crawl space.

Considerations for Properly Sized Gutters

  • Home square footage
  • Roof pitch
  • Maximum rainfall intensity of your location
  • Shape of gutters (K-style or half-round)
  • Gutter dimensions
  • Gutter slope
  • Number of downspouts

Even though water is moving on the surface away from your foundation, it could also be moving below the surface into your basement or crawl space. It’s essentially seeking the easiest way to follow gravity. That can include finding cracks in your foundation and flowing into your basement or crawl space. 

Sample Drainage Calculation

The U.S. Weather Bureau collects data on rainfall intensity by region based on historical averages. For the Denver area, they show the following one-hour precipitation values based on the averages for each historical period: 100-year, 2.31 inches; 50-year, 1.99 inches; and 25-year, 1.69 inches.

If we select a home footprint of 1,800 square feet, a 45-degree roof slope, and a pitch factor of 1.3, the effective area is 2,340 square feet. Choosing the worst-case rainfall intensity of 2.31 inches yields an adjusted square footage of 5,405. Checking the charts for K-style gutters, the five-inch size is rated for 5,520 square feet, which works nicely. 

Downspouts are normally positioned every 40 feet. In our sample calculation, two-by-three-inch rectangular downspouts with a capacity of 600 square feet should work fine. Options include installing larger or round downspouts as well as installing extra downspouts.

Ground Saturation and Drainage 

Now that the rainwater is off the roof and onto the ground, the journey doesn’t stop there. The surrounding soil needs to be graded to ensure that the water flows away from the foundation. Not only that, but during the rain, the soil may already be soaked. 

Plus, there is water flow beneath the surface often focused on the clay bowl effect around your foundation. This is because the excavation when building the foundation set up different drainage properties. That, in turn, leads to a hydrostatic pressure buildup, which can cause basement or crawl space flooding.

The Cost of a Home Flood

FEMA provided cost estimates of various levels of home flooding 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)

  • One inch of water in the home: $26,807
  • One 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.

To avoid these costs, the first step as noted above is to make sure your downspouts and gutters can carry sufficient water off your roof and into drainage away from your home. You can further benefit from basement waterproofing or crawl space encapsulation. These waterproofing measures can include an interior drainage system, a sump pump, and much more to protect your foundation.

All this is a small price to pay to avoid the major costs and disruption of flooding and foundation repair. Learn the best way to protect your home from water damage with a free inspection from the Denver area’s leading basement waterproofing and foundation repair experts.

Serving Greater Denver area, Colorado Springs, Eastern Colorado, and parts of Nebraska and Wyoming

Complete Basement Systems, Colorado Springs, CO

5695 Parachute Cir
Colorado Springs, CO 80916

Complete Basement Systems, Denver, CO

11795 E. 45th Avenue
Denver, CO 80239