A storm or fallout shelter uses earth cover for protection: below-ground burial shields from wind and flying debris, and soil mass shields from radiation. Build it on ground that cannot flood, give it two exits and a ventilation shaft sized for the occupants, and make the roof strong enough for the soil, because a shelter that floods, suffocates or collapses kills the people it was built to protect.
A shelter is an earth-sheltered room built to be occupied briefly under threat, so its failure modes are the ones that matter in an emergency.
A Shelter Has To Work Exactly When Conditions Are Worst
Storm shelters meet wind, debris and often heavy rain, so the first concern is water. A shelter in a flood-prone hollow is a trap, and the parent article’s rule applies without exception: if the water table is within 1 m of the floor level in any season, do not build underground there. Choose high, well-drained ground, and make sure the entrance cannot funnel surface water into the shelter.
For a fallout shelter, radiation shielding comes from mass: the more earth between people and the fallout, the lower the dose; see Nuclear Safety for the half-value-layer idea. Fallout activity also decays fast. A widely used approximation, the 7-10 rule, says the radiation level falls by a factor of 10 for every sevenfold increase in time since the burst, so at 7 hours it is about a tenth of the 1-hour level, and at about 49 hours about a hundredth. The rule is a rough guide for the first two weeks after a single detonation (Glasstone and Dolan, The Effects of Nuclear Weapons).
For tornadoes and storms generally, see Shelter Protocol.
What You Need
- Ground that cannot flood, and a drain.
- A strong roof, designed for the full soil load; see Roof Structures for Earth Loading.
- Two exits, so a blocked door does not trap people. This is a general safety principle rather than a figure from the parent article.
- Ventilation: an intake low and an exhaust high, sized from the parent article’s 150 cm² per 20 m² of floor as a minimum; more occupants need more air than that floor-area rule suggests. See Ventilation Shafts and Chimneys.
- Water and sanitation for the length of stay.
Method: Design Around Failure
Step 1: Check the site for flood and water table.
Step 2: Dig with sloped or shored sides and plan the work to avoid leaving a pit open.
Step 3: Build a roof for saturated soil with a three-times margin, as the parent article advises.
Step 4: Build two exits, one of which is not under the main structure.
Step 5: Fit the ventilation and keep intakes clear of debris and flood water.
Step 6: Test the shelter with the intended number of people for a few hours before depending on it.
Common Mistakes
- Sealing it up tight. People need air; carbon dioxide pools at floor level.
- One door. Debris or a collapsed entrance can bury it.
- Ground that floods.
- Cooking or heating with combustion inside. See Carbon Monoxide Risks.
Safety
Shelters kill by water, bad air and collapse
Do not shelter in a space that is filling with water. Never use a flame to test air. If anyone is drowsy or has a headache, get out into fresh air. Excavate with sloped or shored walls for any depth beyond about 1.2 m. Never rely on a shelter you have not inspected for roof damage and flooding.
What’s Next
- Learn next: Fully Underground Structures
- Combine with: Earth Load Calculations