I have said that you need to think of many Deer hunters with hi-powered rifles shooting at you. From every angle those very hi-powered bullets trying to hit you. That is like Gamma Rays in the fallout. Density of your shelter is an absolute must. Just think of all those Deer hunters on your roof shooting down. Hunters out in your yard shooting through the walls for two weeks. You would get killed. That is what you need to protect yourself against. You need DENSITY to stop those bullets.
You can see that your wood and plaster walls would not protect you. Neither the wood and shingles on your roof. Bullets would just come through. Again I say you need DENSITY of shelter material to protect you. DENSITY shielding is the answer for your protection. Bullets would fly through a mattress. Sheetrock or plywood is denser. Hollow cement cinder blocks are denser. Solid Bricks are denser. Packed earth is denser. Concrete is denser. Solid steel or lead is the best. But few people can have a steel and concrete Bank vault shelter.
You need to think in what the scientists call HVT thickness. HVT = Half Values Thickness. That means only ½ of the gamma rays will get through, because the DENSITY has stopped them. The reason for getting into a basement is that the rays only come down, from the fallout on the roof to you. You are below ground level and the soil, dirt, outside has absorbed and stopped the rays, it is DENSE enough that two feet of earth gives protection.
Again realize, IF hunters were shooting, then a two-foot bank of earth would stop their bullets.
Every 3.6 inches of packed earth will cut the gamma rays in half. The HVT (half value thickness) of dirt packed is 3.6 inches. To be protected you need two (2)feet of earth. The diagram below shows this principle. This shows 18 inches of dirt.