The average annual rainfall in Indianapolis is 42.2 inches, with 125 days of precipitation per year. But the record one-day rainfall was 7.2 inches on Sept. 1, 2003. Hopefully, we won’t be setting any records soon. But even so, it is wise to be prepared with properly sized rain gutters and downspouts as well as lawn drainage to help prevent basement or crawl space flooding.
Home Rainfall and Drainage Water Management
There are quite a few critical items to consider in the full sweep of drainage water management around your home.
- Total roof surface area and pitch, as well as hips and valleys
- Gutters and downspout capacity to safely route water off the roof
- Downspout extensions directing water away from the foundation
- Landscape grading to prevent rain pooling near the foundation
- Basement or crawl space waterproofing with drainage systems
- Dehumidifiers to prevent mold and mildew
Rainfall Volume on a Typical Roof
The graph below shows just how rapidly rainfall builds up on a typical home. One inch of rain results in nearly 1,000 gallons of water. That rapidly advances to almost 5,000 gallons with five inches of rain.
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
Rainfall Water Volume vs. Roof Structure
The overall roof structure, starting with total surface area, determines the amount of rainfall volume that accumulates. A steeply pitched roof has more surface area. Plus, the rain moves off the roof much more quickly down that steep slope.
Not only that, but any wind moves more rain onto the roof, at least on the windward side of the home. Roof hips and valleys can also gather and accelerate the flow of rain into your home’s gutters and drainage system.
Properly Sized Gutter and Downspout Considerations
Here’s a list of things that must be considered when calculating gutter and downspout dimensions.
- Anticipated maximum rainfall intensity
- Roof square footage, pitch, and any hip and valley rafters
- Width and shape of gutters (K-style or half-round)
- Slope of the gutters between downspouts
- Downspout number and position as well their shape (round or rectangular)
Drainage Calculation for Indianapolis
NOAA states that the most intense five-minute burst of rain that’s likely over a 10-year period for Indianapolis is 0.618 inches, selecting the Indianapolis WSFO AP location. Using that number to calculate inches per hour is then 0.618 x 12, or 7.4 inches.
Let’s start with a home footprint of 850 square feet. With a roof pitch of 8-in-12, the pitch factor is 1.1, which is a 10% increase over a flat roof. That brings the roof watershed to 935 square feet. Multiplying the maximum rainfall intensity for one hour of 7.4 by 935 yields a drainage capacity of 6,919 square feet. That fits within the capacity of a six-inch K-style gutter at 7,960 square feet. The builder will also need to take into consideration the pitch of the gutter, the number of downspouts, as well as their size, shape, and position.
Drainage Away From the Foundation
That gets the rain off the roof and onto the ground. But the journey should not stop there. With heavy rain, the ground will also be soaked with the water from the roof pooling up around the foundation.
Critical elements of the drainage system at that point will be downspout extensions that move the water away from the basement or crawl space. In addition, your home’s landscape should be graded to a slope that moves the water away from the foundation. Installing basement waterproofing solutions or crawl space encapsulation can greatly limit any chance of flooding.
The Costs of Home Flooding
FEMA has developed cost estimates 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.
These are horrendous numbers to take into account as you consider your home’s gutters, downspouts, and drainage. Getting everything set up correctly to prevent basement or crawl space flooding can certainly be worth the cost.
For professional advice on drainage systems for your home, contact your local foundation experts at Indiana Foundation Service for a free inspection and repair estimate to identify any issues that could lead to foundation damage.