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 first catchment 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. During a heavy rainstorm with 12 inches of rain, the roof collects nearly 12,000 gallons. Note that the rain also accumulates on your lawn, which further compounds drainage issues.
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
A steep roof effectively sheds water but is more prone to gutter clogs. 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 run onto the foundation and into your basement or crawl space.
Considerations to Properly Size Gutters
- Regional rainfall intensity
- Home square footage
- Roof pitch
- Shape of gutters (K-style or half-round)
- Materials (aluminum, vinyl, copper, zinc, steel)
- Gutter dimensions
- Gutter slope
- Number of downspouts and downspout direction
- Leaf Screens
Note: Denver has exceeded its annual halfway mark of rainfall in late spring of 2021.
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 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 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 gutters and downspouts 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.