When building a homemade fallout shelter in a basement, or on a cement slab inside the first floor, it is important to understand halving thickness and protection factors.
First of all, after a nuclear detonation, there will be light, heat, and a blast wave. This essay assumes that you will be out of that target area, with your home and roof intact. If you are close to targets, you may need better shelter than this improvised model. At the end of this essay I will list a few sources showing target maps, fallout maps, blast areas, etc.
Fallout is the mixture of the dirt and materials at the site of the blast, all mixed up with radioactive material. Every single piece is radioactive. Near the blast it can fall out like gravel, then farther away like rice grains, then like sand, and then like fine powder. And every fallout particle is sending out gamma rays.
You need to take almost immediate shelter for the gamma radiation from fallout. Gamma rays are part of the electromagnetic spectrum, like radio waves and X-rays and light. If you picture the fallout landing on trees and the ground being like tiny little light bulbs, you realize that even in a basement there will be dim, indirect light. If your basement walls stick up a couple feet above the ground level, there will be lots of little light bulbs all along the edge of the basement shining at you. As light and radio waves reflect off the atmosphere, in the same way the gamma rays scatter off the air. Little light bulbs will land on trees and the roof. You want to dim/block them as much as possible, on all four sides and overhead.
A halving thickness is the amount of material that will block half of the gamma rays passing through it. Any mass will block them, whether lead or feathers, sand or chocolate bars, as long as you have enough mass. You can use all of your survival foods and other items to add extra shielding.
Here is a list of materials and their approximate halving thicknesses. (References differ slightly when listing these figures.)
3.5” sand or dirt
7” books or magazines
4”hollow concrete blocks
3.2” red bricks
5” broken anthracite coal
5” wet peat moss
Here is where many items you are storing up can contribute to shielding. [JWR Adds: If in sealed containers, these foodstuffs will still be safe to consume after shelter emergence if any residual fallout dust is first washed off of the exterior of the container.]
7” navy or soy beans
7” butter or oil
7” shelled corn
12” coffee beans
Now, one layer of any item above will block half the gamma rays. That is 1/2, which is called a protection factor (PF ) of 2 (read only the denominator of the fraction). 1/2 of the rays are hitting you, 1/2 are blocked. By adding one more halving thickness, you block half of the remaining gamma rays, so now 1/4 are hitting you. So you have a protection factor (PF) of 4. Another layer blocks 1/2 of that remaining 1/2 of the radiation, so that means only 1/8 of the original total outside radiation is hitting you, and you have a PF of 8.
A fourth layer of anything listed above blocks half of that 1/8 radiation still entering, so now we only have 1/16 of the outside gamma rays hitting our body. ( PF 16)
5 layers= PF 32
6 layers=PF 64
7 layers=PF 128
8 layers=PF 256
9 layers=PF 512
10 layers=PF 1024
Now, how much of a PF do you need? The answer involves how much gamma radiation, or rads/ Roentgens, are in the fallout outside your house. They are called “R ”. The less R the better. 50 in one day is considered the most you can safely handle, or 10 a day for a week, or 100 over the course of two weeks. So your shelter must not let you get more than 100 R in two weeks. (It is far safer to get none or almost none.)
So, how many R will be outside after bombs, and how does PF relate? The first question depends on where you are and where the bombs are, how big the bombs are, and where the wind is blowing. If you are 25 miles from a total of two megatons blowing your direction, during the course of two weeks there will about 4,500 R total outside. If 200 Russian bombs go off nationwide, the East coast can easily get 20,000 R outside during two weeks. If you are 25 miles from a target that might get four big bombs you could easily have 20,000 R outside. Suitcase nukes would produce much less fallout. You have to decide if you expect limited suitcase nukes, a limited Russian strike with MIRV s (several bombs on one target area equaling one megaton total), or a “real” nuclear war with perhaps hundreds of big bombs of two or more megatons each. Sources below show fallout possibility maps.
Now, how does PF relate to the R outside?
Remember that the bottom denominator of the fraction is the PF, telling you what fraction of the fallout (R) is hitting you. A PF of 2 means half of it is hitting you. A PF of 16 means 1/16 of it is hitting you. A PF of 100 means only 1/100 is hitting you.
If it is 20,000 R total outside during two weeks, you don’t want to get more than 100 R, so you need a PF of 200. Makes sense? Divide the R by the PF. 20,000 divided by 200=100. If one 2 megaton bomb detonates near you, and the R over two weeks is 4,000, what PF do you need to only get 100 R? 4,000 divide by what equals 100? Answer is 40.So, a shelter with a PF of only 40 can save your life. This is the FEMA  minimum standard. PF 200 is much safer. The ideal is PF 1,000, which equals about 3 feet of dirt or sand, or 22” of cement. STRIVE TO GET AS CLOSETO A PF 1000 AS YOU CAN, OR AT THE VERY LEAST A PF 200.
Now the basement shelter should have a PF 1000 on all four sides. If you cover the exposed sides of the basement, outside, up and over the ceiling level, with ten layers of the halving thickness chart items (3 feet of sand or 4 feet of hollow concrete blocks) your basement will have an automatic PF of 10. That means 90% of the fallout is already blocked, and you need to only get a PF of 100 on the four inside walls and overhead for a total PF of 1000. That means seven layers of the materials listed above.
4 feet thick of old magazines and paper will work. Stagger some water barrels. You can get 5 gallon buckets of wheat and rice and beans, and stagger them so there is 4 feet total of wheat and beans on the sides. About 5 feet of wood works too. Personally, I think the 7 foot thick wall of coffee is a good idea.
The hardest part is the overhead shielding. A basement support with 10.5” of sand 3.5×3) has three halving thicknesses or a PF of 8. Add one more layer and you are up to PF 16. My first and second floor and roof are at least another halving thickness for a PF of 32.
(This is easily done with the steel shelving units at Home Depot that hold 2,000 lbs per each top shelf (20 cubic feet of sand): with two units that is a foot of sand over 40 square feet. [this was my method, but I don’t trust the specs and used more supports per cubic foot.] Or make your own supports with 4×4’s, or cinder blocks with 1/2 inch plywood. Try to get 4 layers ( PF 16) overhead, using sand, or maybe some cinder blocks with a waterbed on top of that. Hopefully the house floors and roof will then get you to PF 32.)
As soon as the bombs go off, you pile 7” of books and wheat and beans on the first floor directly overhead. That gives you a PF of 64. (The overhead PF of 32-64 will save your life if all four sides are PF 1000, even if fallout is severe.) Better to pile on more stuff though, another 7” of stuff- plenty of your cans and heavy items. Anything with mass. That gives you a PF of 128 just from last minute living room piles. This is for a worst case scenario. But if we have a limited strike, the fallout will be far less for most of us. Even one waterbed overhead on the first floor, with 9” of water, gives a PF of 4. That means you get 1/4 of the initial radiation. If it is 600 R overhead, with no shelter you will get severely ill and might die. Just using the waterbed over the basement with basement walls covered up outside all the way up, means that you get 150 R and will be basically OK.
So, the moral of this story is, start now and do what you can. Don’t feel like it is useless to only do a little, if you can’t do a perfect shelter. Do what you can now and build up the shielding as you get money. Start with a foot on all sides, and try and get to 18”. Then go for two feet next summer. Think about your stash of preps and books, and what can go overhead on the first floor. Mark off the first floor spot that will have last minute cans and buckets and books. Clear the basement area, and get the flashlights and bedding ready. Try really, really, hard to do something in the basement- overhead- now, even an old table you can lie down under covered with cans and buckets.
You can find lots of useful information here: http://www.radshelters4u.com/ , including a free download of Nuclear War Survival Skills, and all sorts of maps and diagrams.
Our favorite book for basement shelters is J Allan South’s “The Sense of Survival.” This wonderful little chart compares the mass of many items. Use sand and dirt as your standard for a halving thickness, and you can see how various things like beans and wheat and wood compare. http://www.reade.com/Particle_Briefings/spec_gra2.html 
JWR Adds: I consider a home fallout shelter a must for anyone that is serious about preparedness. The end of the Cold War–culminating with the breakup of the former Soviet Union–significantly increased the risk of the use of nuclear weapons. (Since traditional nation states are are much more responsible with their toys than are rogue states or terrorist groups.) Two SurvivalBlog advertisers (Safe Castle  and Ready Made Resources ) offer prefabricated shelters as well as consulting on shelter construction and HEPA  air filtration systems. Also, be sure to read the extensive information on fallout shelter design, construction and ventilation available for free download at Dr. Arthur B. Robinson’s Oregon Institute of Science and Medicine  web site.