Underground storm shelters for Huntsville-area homes

Ask longtime residents of north Alabama what a storm shelter looks like and many will describe a hatch in the garage floor. The in-ground shelter is popular here for good reason: once you are below grade with a few feet of soil around you, flying debris has very little to hit. FEMA's homeowner guide notes that in-ground safe rooms get that debris protection from the surrounding soil, which above-ground units have to earn through testing.

Buried shelters do bring their own set of problems, though, and most of them come down to water, rock and the people who have to climb into them.

Garage floor or backyard

FEMA lists two common places for an in-ground unit at a house without a basement: under the garage slab, or in the ground within 150 feet of travel from the house. That 150-foot limit comes from the ICC 500 standard for residential shelters outside the home.

Under the garage slab

• You reach it without stepping outside, which matters when hail and rain arrive ahead of the rotation.
• The work means sawing into the slab and excavating beneath it, so ask how long the garage will be out of use.
• FEMA flags two garage-specific details: a parked car must never block the hatch, and the design should keep runoff, or water from a burst water heater, from draining into the opening.

In the yard

• Can be easier when the garage is too small, finished, or set on a slab that should not be cut.
• You have to leave the house to reach it, so the family must move early, while there is still a watch rather than a warning overhead.
• FEMA advises grading the ground so it slopes away from the door, so storm runoff doesn't pour in.

Water, rock and Madison County ground

Much of the Huntsville area sits on limestone that dissolves over time. A 2006 study of Redstone Arsenal presented to the Geological Society of America counted 1,886 mapped sinkholes and 424 springs on the arsenal and found solution cavities in about 70 percent of the bedrock boreholes drilled there. Conditions vary a lot from one lot to the next, but it explains why an installer may hit rock a few feet down on one street and dig easily on the next.

Groundwater is the other concern. FEMA points out that a buried shelter can be pushed up out of the ground by rising water if it is not anchored against buoyancy, and recommends a sump pump inside in-ground units to clear any water that collects. If your lot holds water after heavy rain, or you are in a mapped flood zone, an above-ground safe room may be the better fit. FEMA's guidance does not allow a below-ground shelter inside a FEMA Special Flood Hazard Area at all under ICC 500.

Before anyone digs

1. Utility locates. Alabama 811 asks anyone digging, homeowners included, to submit a free locate request at least two full working days ahead. Ask your installer whether they file it.
2. Engineering paperwork. Ask for the shelter's specification sheet and the engineered drawings. You need a spec sheet with an engineer's seal dated 2021 or later to apply for the Alabama tax credit, and the drawings show how the unit is anchored.
3. Listing and labeling. FEMA's current P-320 notes that, at the time it was written, no in-ground or precast concrete units had yet been listed and labeled under ICC 500. That makes sealed engineering documents especially important for buried units. Ask the installer which standard and edition the design was checked against.
4. Access. In-ground units mean steps. FEMA reminds homeowners to think about anyone in the house who uses a wheelchair or crutches, for whom stairs could slow them down badly.

Sizing

FEMA's minimum for a home tornado shelter is 3 square feet of floor space per person. The size guide works through common unit dimensions.

Good questions

Can an underground shelter flood?

Yes, if it is poorly sited or poorly sealed. FEMA recommends sloping the ground away from the entrance and putting a sump pump inside in-ground units, and it does not allow below-ground shelters in a FEMA Special Flood Hazard Area under ICC 500.

Do I need to call 811 for a storm shelter install?

Alabama 811 asks anyone who digs, homeowners included, to request a free utility locate at least two full working days before digging. Ask whether your installer handles that request.

How far from the house can a yard shelter be?

ICC 500 caps it at 150 feet of travel distance from the nearest entrance of the home, and FEMA uses the same figure in its homeowner guide.