Some things to consider when building fallout shelters

I’ve long had an interest in the construction of nuclear blast and fallout shelters. Most preppers will be familiar with Cresson Kearny’s guidebook, “Nuclear War Survival Skills” (NWSS), which is available as a free pdf here (save a copy!). Anyone interested in the construction of a fallout or blast-resistant shelter should start by reading this guide, which is excellent.

I have some reservations about strict adoption of NWWS to mitigate contemporary nuclear threats. Kearny’s book was largely a response to underfunded civil defence. He operated on the premise that prior to a Russian first strike there would be a significant exodus of civilians from Russian cities. Having detected this migration via satellites, the US civilian population would have 3 days to migrate to rural areas and construct fallout shelters. There are many open questions regarding this strategy. Some thoughts on how contemporary shelters could be modified:

  • Today, threats are more likely to be sustained at low probabilities for extremely long time periods. Consequently, shelters don’t have to be constructed hastily but should be highly durable.
  • There are several historic examples of accidents nearly resulting in a nuclear disaster. Obviously these scenarios wouldn’t have provided sufficient time for people to construct a shelter.
  • Independent of their durability, the materials and techniques used in NWSS were quite crude – i.e. tools and materials that could be taken from most suburban homes. In addition, we’ve had ~35 years of building science and product advancement since the last version of the book was published.
  • Scientists seem to be converging on the idea that a 5-10 year nuclear winter will be likely after a full-scale exchange, and perhaps even after a smaller exchange such as the sort that would occur between Pakistan and India. This would result in the near total collapse of the industrial agricultural system. Launching a full-scale reboot of technological civilization is outside the scope of this post. However, it might be worth considering the integration of protected food storage, tools, and technology.
  • Although Kearny did provide details for high capacity shelters, I’m of the opinion that in many areas the default shelter size should have at least moderately high capacity. If you’re going to the trouble of constructing a shelter, the marginal cost of adding some neighbors is low. And this isn’t strictly a bleeding heart perspective. Just think of how smug you’ll be in the middle of armageddon, surrounded by people who dismissed you as a survivalist crank! How many times can you pointedly glance at someone and say “good thing I built this, right?” in 3-14 days?

We should divide shelters into 2 major types, those that are within an existing building – usually the basement of a home – and those that are separate. How should you decide between the two? Kearny believed that freestanding shelters were significantly more robust, mainly due to fire risk. On the other hand, basement shelters are considerably less expensive to construct. In addition, it’s somewhat of an open question whether basement shelters could add reasonable fire protection – or even shockwave protection – given contemporary materials, back-up power systems, filtration, ventilation, and airtightness levels. Overall, my sense is that people would ideally have some idea of the threat of fire risk, and gravitate towards basement shelters in the majority of cases where the fire risk isn’t extraordinary, due to the cost savings. I’ll try to cover both types, but will start with the basement.

I’m going to start with a basement shelter that modularly moves through a descending order of priorities. The basement would ideally have a clearance to the bottom of the first floor joists of 7’6″ (or more), which should provide an interior headroom clearance of ~6’. I’ll add an asterix where where I have a lot of uncertainty.

  • Overhead ceiling mass;
  • Wall mass;
  • Water (possibly integrated into ceiling/walls massing);
  • Food (possibly integrated into ceiling/walls for massing);
  • Fire protection;
  • Ventilation;
  • Air quality and radiation monitoring*;
  • Sleeping;
  • Hygiene;
  • Entertainment and activities;
  • Shockwave protection *.

The driving principle to reduce gamma radiation exposure is to place a high mass between you and the source. We have to assume that gamma-laden fallout dust will be distributed everywhere outdoors, so this means placing mass between you and all lines of sight to outside (gamma doesn’t tend to bounce around corners). In fallout literature, people often refer to a material’s “halving thickness” which is the thickness required to reduce gamma radiation by 50%. Concrete’s halving thickness is 2.4”. In the case of a basement shelter, a realistic target is to place 12″ of concrete overhead, which is 5 halving thicknesses. In conjunction with other materials in the joists directly overhead, or other layers of the building, we could likely achieve 6 halving thicknesses, equivalent to roughly a 98.5% reduction in gamma penetration (2).

Here are some instructions for building a shelter with an interior of roughly 7′ wide & whatever length you specify (although working in modules of 4′-8′ is sensible). The frame consists of a supported ledger and a parallel stud wall. These support joists that support ceiling joists that support a ceiling mass (ideally concrete). I have a spreadsheet cutsheet in the works but it’s not ready for primetime.

  1. Select an ideally windowless corner of a basement with minimal overhead obstacles. The selected corner should also be as far below ground as possible. Wiring is OK if stapled to the joists, but avoid significant plumbing (esp waste) or duct-work if possible.
  2. Ideally the width of the shelter (short axis) would run parallel to the overhead ceiling joists.
  3. Measure the height from the floor to the bottom of the existing basement ceiling joists. Deduct 20″ from this. This is the stud length for support wall. Cut one stud for every 16″ of length + one for the end.
  4. Cut 5 pc 2×4 at the specified length of your shelter. These are your ceiling ledger, 2 ceiling rim joists, wall bottom plate, and wall top plate.
  5. Mark a level line on the long side of the wall at a height above the floor of (stud length + 6.5″). This is the top of the ceiling ledger.
  6. Back the ledger with ice and water shield or sill gasket and attach it to the wall using construction adhesive and masonry anchors.
  7. Individually measure and cut studs for placing beneath the ledger 16″ on center.
  8. Attach ice and water shield or sill gasket to the flat side of the studs and attach them to the wall in their measured locations using masonry anchors.
  9. Frame the wall by connecting the wall plate to studs placed vertically every 16″. Set the wall well back from the work area.
  10. Attach ice and water shield or sill gasket to the bottom plate of the wall.
  11. Cut the ceiling joists to the specified width
  12. Deduct 4 1/2″ from the overall width of the shelter. This is your ceiling joist length. Cut the same number of ceiling joists as wall studs.
  13. Frame the ceiling by attaching the ceiling rim joists to the ceiling joists placed horizontally every 16″.
  14. Install 1/2″ sheathing over the ceiling using glue and construction adhesive. The sheathing should be flush on 3 sides, but project over one rim joist by 1 1/4″. This projection will be attached the ledger.
  15. Apply either asphalt impregnated 15# felt or ice and water shield over the sheathing. This will serve as a capillary break between the concrete and the plywood to prevent rot.
  16. Optional: Drill 1 1/8″” holes 12″ on center ~6 1/2″ above the ledger. Epoxy 1″ dia rebar exactly equal to the length and width of the shelter long into each hole, with 2″-4″ embedment in the wall (this will leave the rebar 2″-4″ short of the penetrating the outside edge of the concrete). Connect each point in the rebar grid using a tie wire loop.
  17. Run a bead of construction adhesive along the top of the ledger.
  18. Measure from the base of the ledger wall (overall width minus 10″) and mark a line the length of the shelter.
  19. This part will suck and will require several pairs of hands. Place the ceiling assembly over the top of the ledger and connect several screws through the plywood into the ledger. Quickly place the support wall under the ceiling assembly on the opposite side.
  20. Align the outside edge of the wall with the line you struck in 13. Fasten the wall to the floor slab using masonry anchors. Plumb the wall at the two outside studs and fasten the plate at these two locations to the ceiling joists above. Straighten the wall in between these points by sighting the wall along the top. Fasten the top plate 2x at every joist.
  21. The ideal overhead mass is 12″ of concrete, but you can use other heavy objects if you prefer. For concrete, prepare plywood edge forms roughly 16″ wide to be attached to the perimeter joists. Don’t install these yet.
  22. This part will also suck. Mix concrete and shovel it into the deck. As you approach the nearest edge of the form, install the plywood edge forms so the concrete doesn’t spill out.
  23. Once the concrete has set, you can apply more heavy things into the overhead joist cavity to provide even more protection.

This should complete a reasonably well protected overhead mass. If folks are interested I’ll try to work on the wall section. Hope you enjoyed this!


  • Comments (8)

    • 2

      That was a very interesting read. It got me fantasizing about what I would do if I had the time, money, and skill to build such a shelter.

      Is this something you would like to build for yourself someday?

      I wrote a very crude and basic post compared to yours on surviving a nuclear attack. There are some of the things you mentioned that I talked about in that post like placing as much material between you and the fallout as possible.

      • 2

        Excellent post! You cover a lot of ground really effectively!

        My intent is to provide a simple, inexpensive blueprint that most people could complete over the course of 1 or 2 weekends for <$200. Adding a concrete ceiling (vs just massing food/water/sand) & masonry walls to the open sides would add another few days & $300, but seriously increase the level of protection. To me this is worth evaluating for people who are somewhere between 2-100 miles from a likely nuclear target. My personal sense is that we’re currently running ~1%/yr likelihood of nuclear exchange, so for most middle class people this makes such a project considerably more worthwhile than eg. standard medical screenings such as colonoscopies.

        You may have already found this in your search, but there are a couple of Cresson Kearny vids on youtube. Here’s one on hasty basement fallout protection: https://youtu.be/XLyZAVtObLU

        I believe Kearny also developed the idea of fallout protection via a vehicle over a hastily dug trench using the following steps: 

        1. Dig a slit trench in front of your vehicle at minimum of 2′ less than the wheelbase and ideally long enough to lie down in. While digging, ensure that the earth is thrown just beyond the width of the vehicle.
        2. Drive the vehicle over the trench. 
        3. Open the doors to the vehicle and pile the fill so that it’s mounded over the openings to the ground, leaving a small crawl access).
        4. Pile several inches of dirt on the floor of the vehicle (yes you will need detailing after). 
        5. Climb in the trench (ideally w food and water). 
        6. If possible, backfill the opening to the trench. You can also partly block this with water. 
        7. Don’t emerge for a minimum of 3 days.

        The target time on this sort of project is probably 1-2 hours. Given sufficient warning this would make it accessible to people several miles from a detonation.

      • 1

        That is a smart way to improvise some sort of shelter while in the middle of nowhere. Truly a scare situation to find yourself in though. 

        You say that digging that trench for the vehicle protection would take 1-2 hours. While I like the concept and having a backup plan, my first thought would be to find the wind direction and drive perpendicular to that and the fallout and take those 1-2 hours to find a concrete shelter. But… if the blast fried my vehicle and I wasn’t going anywhere, I could push it into neutral and do that digging strategy.

      • 1

        A big problem for nuclear defense seems to be high situational and regional variations in appropriate response. There’s a fair bit of ambiguity, but it’s conceivable that US residents would have up to 30 minutes warning from the point of launch to detonation. I think the car shelter might work for someone who 1. has a lot of warning and can dig quickly, possibly with multiple people. or 2. is stranded with their vehicle many miles from the detonation, but facing possible vehicle gridlock.

        I agree that where viable, fleeing is preferable. For the gridlocked, seeking nearby shelter on foot might be less prudent where one is uncertain of proximity to quality shelter sites, or even where the majority of foundations are slab on grade.

    • 1

      Adding detailed risk estimates via Samotsvety. This is somewhat narrow imo, but noteworthy that they perceive the relative risk as having recently jumped 5x: 


    • 2

      Weight on slab

      I have read many articles about a basement fallout shelter and noticed that no mention is made on the strenght of the basement slab.  Especially the older cold war examples of building concrete walls on the slab.  Just building a concrete wall on a slab seems to me silly without consulting a architect.  The weight is significant of the wall alone but adding a 12″ concrete sealing is just a big load on just 4″ of slab.  I like to hear if there is anyone thinking of the consequences and how to mitigate this concern.

      • 0

        You’d need to reinforce the slab by cutting and removing portions of the slab, to dig and pour footings for walls to be built upon. 

        It’s typical in residential construction to have areas under the slab that are poured thicker to support a point load from above, (load bearing), essentially a footer under the slab. You could probably build a footer on top of a slab that could support the heavy loads you mention. In any case, a consultation from a structural engineer would be more useful than an architect. An architect simply designs, an engineer makes sure it can be built safely.   

    • 2

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