Oklahoma's red dirt has a lot to do with the lack of basements in Tornado Alley

If you're looking for a home in Oklahoma and wonder why it's hard to find one with a basement, there's a good reason.

They're an uncommon find in the Sooner State. For example, out of nearly 17,000 Oklahoma homes currently listed for sale on Zillow, only 429 of the listings mention having one. By comparison, Indiana had roughly the same number of listings but over 5,300 basements.

Why aren't there more basements in Oklahoma?

With Oklahoma being notorious for its tornadoes, however, the lack of residential underground shelter is striking. The safest place you can be during a tornado is underground, so why don't more homes here come with basements?

The answer has a lot to do with the state's geology and environment.

Over millions of years, sandstone and shale eroded and left behind a vast plain of clay tinted red by iron oxide. This made Oklahoma's characteristic "red dirt" into what it is today.

More: Does OKC have public tornado shelters? What to know about tornado safety

When it comes to building a basement into the earth, clay isn't ideal. Geologists will tell you that clay has a tendency to expand and contract depending on the amount of water it holds. That can cause damage to concrete structures and lead to flooding, although contractors can use waterproofing materials to mitigate the risk.

Oklahoma foundations aren't as deep

Economic and climate forces are also at work to keep basements off the menu.

When pouring a foundation, contractors must dig down below where groundwater can freeze. Oklahoma doesn't get very cold and when it does, the temperature doesn't stay below freezing for very long. That means any groundwater deep in the soil will have a hard time freezing.

According to the Oklahoma City building code, the frost depth is set at 18 inches.

In Chicago, the frost depth is 42 inches. That means foundations there have to be poured more than three and a half feet into the ground. Adding a basement to that home, when you've already gone down that far, wouldn't be much of a stretch.

This article originally appeared on Oklahoman: Oklahoma's red dirt has a lot to do with the lack of basements in Tornado Alley