Septic Tank Storm Shelter: Is It Safe to Use One?

The short answer

A new, never-used concrete septic tank is heavy and made to sit underground, which is why people have used them as cellars for decades. But a tank is designed for a different job and is not tested or labeled as a storm shelter, its openings are small, and it needs a rated door, vents and anchoring added. A precast unit made as a shelter costs about the same and comes with a tested design.

Why people use septic tanks as shelters

In rural parts of the Plains and the South, a new concrete septic tank buried in the yard has long been a practical storm cellar. Tanks are made to sit underground, they are heavy, concrete does not rust, and a precast plant nearby can deliver one on a truck. Some precast firms that make septic tanks also make purpose-built shelters on similar molds, which adds to the confusion.

It is worth being clear about two very different things: a new tank that has never been connected to a drain, and an old tank that has been in use. Never use a tank that has held sewage. Gases from waste, such as hydrogen sulfide and methane, can build up in it, and no amount of cleaning makes it a sensible place to shelter. The rest of this post is about new, unused tanks.

What a septic tank is designed to do

A septic tank is designed to hold liquid and to carry the weight of the soil and, sometimes, a vehicle passing over it. Its walls, lid and joints are made for that job. It has small access openings at the top for pumping and inspection, often with risers and lids at ground level.

A storm shelter has other jobs. It has to resist the forces a tornado puts on it, take the impact of flying and falling debris at its entry, let people in quickly and out again if debris blocks the door, and supply air for everyone inside for as long as they are there.

The parts a tank is missing

A tested design and label

FEMA’s homeowner guide, P-320, says prefabricated safe rooms must be listed and labeled to show they comply with ICC 500 and FEMA P-361. ICC 500 is the national standard for storm shelters, and test labs, including those accredited by the ICC Evaluation Service, test shelters and their parts against it. A septic tank has not been through that process as a shelter, so there is no label to check.

That matters for rebates too. Alabama’s storm shelter tax credit, for example, asks for a shelter specification sheet with an engineer’s seal dated 2021 or later, showing compliance with FEMA P-320, FEMA P-361 and ICC 500.

An entry and a door

People need to get in fast, often at night and in the rain. A pumping hatch is too small for that. Cutting a larger opening into a tank changes how it carries load, and the new opening needs a door, frame and hardware that have been tested for debris impact and that open from inside.

Air

ICC 500 sets a minimum ventilation rate for tornado shelters, and vents have to be protected from debris. A tank has no vents unless you add them, and adding them means more holes in the shell.

A way out

If a tree or part of a house falls on the entrance, people inside need a way out, or at least confidence that responders know where to dig. Shelter designs deal with this; a tank does not. FEMA recommends registering any shelter’s location with the local fire department and emergency management office for the same reason.

Water and floating

Like any sealed box in the ground, an empty tank can be pushed upward by groundwater. A septic tank in normal use is full of liquid, which keeps it heavy. Used as a shelter, it is full of air. Installers of purpose-built shelters plan for this with concrete collars, anchors or extra weight. Our guide to waterproof underground storm shelters explains how.

What a proper concrete shelter costs

The usual reason for choosing a tank is price. But Angi puts concrete shelters at $3,000 to $7,000, which is in the same range as a new tank once you add a rated door, vents, digging, anchoring and backfill. A precast concrete storm shelter made for the job arrives with a design meant for people, a proper entry, benches and vents.

Use our cost calculator to see typical installed prices for underground shelters by size.

If you already have one

If a previous owner left a converted tank or homemade cellar in the yard, treat it with caution. Check that it was never connected to a drain, look for cracks, water and rust at any steel parts, and see whether the door opens from inside and the vents are clear. A shelter installer can tell you whether it is worth keeping or should be replaced. Our page on storm cellars lists the signs that an old cellar needs attention.

We can find an installer who covers your area to look at it, or to quote for a new shelter, and we name them to you before anyone gets in touch.

Can a septic tank be used as a storm shelter?