The use of sandbags to stop flooding has been around for centuries, with their formal adoption for flood control following their military use in fortifying trenches.

Every year, thousands of people rely on sandbags to protect homes, businesses, roads and other structures—especially during tropical storms and hurricane season.

But do they actually work at preventing inundation? The short answer is yes. However, they are most effective against shallow flooding, and you must use them properly for them to work.

According to FEMA, shallow flooding is defined as inundation that is 3 feet deep or less. Sandbags function by forming a physical barrier that absorbs water energy and diverts floodwaters away from vulnerable areas.

Now let’s break down how to properly prepare sandbags, critical common mistakes to avoid and alternative flood defenses to consider for the best protection during an impending storm.

How to prepare and use a sandbag

According to the U.S. Army Corps of Engineers (USACE), St. Paul District, filling a sandbag typically requires two people.

When filled to capacity—typically about halfway—the bags weigh approximately 35 pounds, and using a funnel during the filling process can speed up the process.

Once your sandbags are ready, it is time to prepare the site by clearing away any debris or obstacles from the layout area.

“Site preparation begins with scraping away ice, snow and topsoil, if possible. A shallow trench sometimes helps hold the levee in place,” the USACE notes.

While sandbags do prevent flooding, FEMA warns they only work for water 3 feet deep or less. Los Angeles Times via Getty Images

When building sandbag levees, the USACE recommends constructing them at least a foot higher than the water’s expected peak crest. Regarding levee dimensions, the USACE advises:

“…that a levee have a base three times wider than its height. Other sources say a 1:2 ratio is adequate for a solid levee. Bags are staggered, like bricks in a wall. The water’s weight helps hold a levee in place.”

Lastly, to prevent water seepage through the structure, plastic poly sheets can be wrapped around the water-facing side of the levees.

Proper preparation and placement are everything when it comes to these significant weather events.

The U.S. Army Corps of Engineers informs that sandbags should be filled halfway to capacity while usually requiring two people to complete the process of doing so. Los Angeles Times via Getty ImagesCommon sandbagging mistakes to avoid

Tying bags too low: Avoid tying the knot tight against the sand inside. Secure the bag near the top to allow the sand to spread out flat. Tight bags create lumps and allow water to seep through.

Filling bags too much: It’s important to only fill bags about halfway. Filling the bag to its maximum capacity will not allow the sand to spread out, as it will, once again, create more of a lump for water to pour through.

Stacking vertically: It’s crucial to use a pyramid shape and staggering build, so the structure does not fall over from the pressure.

Mistakes to avoid include tying the bags too low, overflowing the sandbag, and stacking them vertically instead of horizontally before a storm arrives. Anadolu via Getty ImagesAlternatives to using sandbags

When it comes to techniques for preventing flood damage beyond sandbagging, Flood Risk America highlights key engineered options including custom flood panels, passive automatic floodgates, water-filled and fabric barriers and permanent flood doors.

As major storms barrel through the Gulf during hurricane season, taking the time to prepare proper flood defenses before landfall is vital.