Air raid shelters differ significantly from blast shelters and fallout shelters. Blast shelters must be airtight and fallout shelters must be airtight and able to sustain their inhabitants for 2 weeks or more. Air raid shelters only have to protect their inhabitants from flying shrapnel. An air raid shelter need only be a hole in the ground with at least 2 feet of "overhead cover." One well-known design is the "Anderson Shelter" used as a backyard shelter by Londoners during the Battle of Britain. With a prefabricated roof, sandbags and a shovel, it is possible to build an approximation of an Anderson Shelter in an afternoon.
Measure and mark a 4-by-7 foot rectangle in your backyard using a steel measuring tape and white spray paint.
Excavate the rectangle to a depth of about 4 feet using a shovel. The depth of the excavation should allow a sitting person to be protected by many feet of dirt.
Shovel the excavated dirt into sandbags. Enter and leave the hole using a stepladder.
Cover the excavation with an 8-foot length of 72-inch diameter, half round, corrugated culvert turned open side down. Build a rear wall for the shelter, at the opposite end of the hole from the ladder, by stacking sandbags filled with dirt from the floor of the excavation to the top of the overturned section of culvert.
Build a 4-foot high, 7-foot long, 2-foot thick blast wall 3 feet in front of the excavation by stacking sandbags filled with dirt. This wall will protect the occupants of the shelter from a blast directly in front of the shelter. Dig additional holes to obtain more dirt as needed.
Cover the culvert and the rear wall with a heavy plastic tarp. Cover the culvert and the front blast wall with a second tarp. Cover the culvert with a 2-foot thick layer of sandbags or dirt.