When engineer Tim Marshall viewed the damage left behind by a dozen tornadoes in North Texas, he soon realized the region had a very big problem that might put residents in danger for years to come.

Many of the homes in the path of the tornadoes were shoddily built , he told the Dallas Morning News. Even worse, the storm survey took him to an elementary school where one side of the building just fell down, shocking the experts.

“We saw a tremendous number of improper attachment of the walls to the foundations, which just made walls fall either in or out,” Marshall, a forensic engineer and meteorologist who volunteered to join the National Weather Service's damage survey, told the Dallas Morning News.

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At Donald T. Shields Elementary School in Glenn Heights, where an EF3 tornado was in progress Saturday night, Marshall said some of the building's exterior just fell away, exposing the interior to the storm's strong winds and projectiles. Had there been children inside the school at the time a violent storm hit the area, the result could have been very bad.

“We saw problems at [Donald T.] Shields Elementary School that were horrific in my view as an engineer,” Marshall told the Dallas Morning News. “Walls not attached properly, and they’re just falling down like a house of cards.”

Once he saw the inside of the building, Marshall said he was floored to discover the walls were hardly connected together, or to the roof. Additionally, the walls were simply nailed to the concrete foundation, with no other fortification, the report added.

“You don’t need to be an engineer to understand this stuff,” Marshall told the Dallas Morning News. “The school officials, they saw this, and they knew immediately that this was a real problem.”

The school is only seven years old, Red Oak Independent School District spokesperson Adi Bryant told the Dallas Morning News, but she did not know the name of the school's builder.

After his alarming survey of Glenn Heights, Marshall also saw problems with the way some homes were constructed in Garland, Midlothian and Sunnyvale. He posted photos to his Facebook page to show the construction errors he saw in each city.

“The vast majority of houses we looked at did not have proper attachments,” Marshall told the Dallas Morning News. “It didn’t matter what size of house. It didn’t matter what city it was in.”

The NWS said in its storm report that 12 tornadoes were confirmed in North Texas on Dec. 26, and 11 people were killed by those twisters. Those tornadoes caused at least $1.2 billion in insured losses , the Insurance Council of Texas told the Fort Worth Star-Telegram.

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