Storm Shelter

A storm shelter is a small hardened room that people go into while a tornado or hurricane passes over and come out of once it has gone. It is also called a safe room. It is not a stronger corner of an ordinary house: it is a separate box, usually below ground or buried in an earth bank, built for one job and tested against it. A piece of timber weighing about 7 kg (15 lb) is fired at the walls, roof and door at 160 km/h (100 mph) to test it, and the finished assembly is rated to stand winds of 400 km/h (250 mph).

Every rating belongs to the whole assembly - shell, door, frame, hardware and anchors together - so a tested shell behind a badly fitted door has no rating at all. The structure needs an unbroken path from roof to foundation so wind cannot lift it, which makes this engineered work rather than a weekend build. Floor area is set per person - about 0.3 to 0.5 square metres (3 to 5 sq ft) each for a tornado, 0.65 to 0.9 (7 to 10 sq ft) for a hurricane, which holds people inside far longer - and ventilation openings the same way, 13 sq cm (2 sq in) per person against 26 sq cm (4 sq in). The governing standard caps a household shelter at 16 people.

Two site conditions can rule it out however well it is built: flood risk, and a high water table, since a buried box in saturated ground pushes against water pressure permanently and can drown the people it was meant to save. Drainage is a standing job - a drain outside the wall running to a sump pump, and, since the grid usually fails in the storm, a battery already charged rather than a generator somebody must go out and start. The commonest failure is human. People are slow to interrupt what they are doing, so a shelter that is far off, awkward to open or unpleasant inside is not reached in time; the guidance keeps the door within about 45 m (150 ft) of the house and five minutes' reach. For the same reason it makes a poor food store: shelves and produce take up floor the occupants were counted onto and clutter a route there is no time to clear.