In 1961 he year of the fallout shelter had begun. As TIME—which generally took a rah-rah tone on shelter-building—tracked the trend over the next months, a major public effort began to mark and equip existing public shelters and encourage individuals to build their own versions. A Gallup poll that August revealed that 3 million families had made changes to their homes for the purpose of protection, and another 9 million had stocked up on food. Some builders reported being overwhelmed by orders for shelters that would meet Office of Civil and Defense Mobilization standards: concrete or metal, under either three feet of soil or two feet of concrete or three inches of lead, with enough space for its occupants. A Department of Defense booklet about preparation was a runaway hit. Though it could be expensive to get a regulation prefab shelter installed, many people made do with simple DIY options.

https://time.com/4245126/cloverfield-lane-real-backyard-shelters/