The likelihood and depth of water reaching the building footprint

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Flood Factors scores are based on the likelihood and depth of water reaching a building's footprint, which is the literal outline a home would make if it were situated directly on the ground.

Property boundaries and footprints

Flood Factor® scores come from First Street Foundation’s Flood Model. The model includes publicly available and 3rd party data to identify property boundaries and building footprints. A property boundary can be thought of as an outline of an individual lot of land while building footprints refer to the outline of building structures on a property.


While this home has two large structures on the property, the pin is located on the larger building. Therefore, the Flood Factor for this property is based on the larger structure, where the pin is located. 

Along with elevation data from the National Elevation Dataset (NED) managed by the United States Geological Survey (USGS), the flood model uses property boundaries and footprints to determine the likelihood of water on the ground reaching the lowest point of a building’s footprint. Where possible, the Flood Model calculates flood statistics based on the footprint of a home. 


The black box indicates the home’s footprint, while the red outline signifies the property’s boundaries. A Flood Factor score is analyzed based on the depth of likelihood flooding will reach any point of the black box, or the home’s footprint.

Building characteristics 

It’s important to note that water reaching this boundary doesn't necessarily equate to water entering the home. How quickly the water reaches the building, how long it stays, and the building’s foundation type has a lot to do with the actual damage the home may experience. 

For the majority of homes in the US, the elevation of the first floor of living space in a home corresponds to its footprint, but this varies from home to home. Homes with basements may experience more flooding at lower depths, while raised homes may be better protected from higher depths of flooding. Because the First Street Foundation Flood Model looks at risk to the building footprint, not the first-floor elevation, elevated homes may still have high Flood Factors, and homes with basements may still have low Flood Factors.


Homes with basements may experience more flooding at lower depths, while raised homes may be better protected from higher depths of flooding.

The changing environment 

Flood Factor also takes into account how flood risks are likely to change over time with sea level rise and atmospheric changes over the next 30 years, which means even a property that hasn’t flooded in the recent past could still face flood risk in the future. 

As the atmosphere warms, there is more evaporation and more water available for rain, which contributes to changing weather patterns and flood risks. Extreme rain events are increasing in duration, intensity, and frequency, which causes more urban and flash floods, and more flooding from overflowing rivers and streams. 



Properties that have not experienced flooding previously, may flood over the next 30 years due to the changing climate.

A warmer atmosphere also means warmer oceans. Higher ocean surface temperatures fuel hurricanes and offshore storms with more water and power, so these systems reach further inland and further north, are more intense, and last longer.

Even a property located high atop a hill could have a relatively high Flood Factor if it’s in an area where heavy rain events are expected to become worse, and more frequent, over the next three decades.

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