Filling and placing sandbags: the basics

A sandbag wall works with two properties: weight and mouldability. The weight keeps the wall in place against the water pressure. The mouldability means that partly filled bags conform to each other and to the ground and close gaps. A bag filled to bursting, by contrast, stays round and hard like a ball – water easily finds a way between bags like that.

The staggered joints are just as important. If the joins of two layers lie directly on top of each other, a continuous gap forms through which water seeps. Laid staggered, like bricks in a wall, the water has to take a detour and is slowed down.

Because sand lets water through, a wall only reduces inflow. A sheet on the water side seals it further, and pumps on the dry side remove the water that still gets through. That is how the three parts complement each other.

Filling works best in pairs: one person holds the bag open, the other shovels. The bags are filled only about half to two-thirds, so that they stay mouldable. The open end is folded over or loosely closed, as the local guidance says. Many municipalities hand out sand and bags when flooding threatens and explain how they want it done.

Before laying, clear the area of stones and branches so that the bags sit snugly. If possible, lay out a sheet first that you can later pull over the wall on the water side. The bags go lengthwise in the direction of flow, with the folded end underneath. Press and tread each bag down firmly before the next one comes, and stagger the joints of the next layer.

For multi-layer walls, build a pyramid-shaped base that is clearly wider than it is high – like a mountain, not a tower. A single row in front of a door is only enough for very low water depths. How high a wall can be and how many bags it needs depends on terrain and water level; for larger structures, emergency services decide.

Sandbags are suitable for low flood depths and for diverting water away from doors, basement windows and light wells. But they take time: filling, carrying and laying take hours even with several people. So they only make sense when there is timely warning and enough help.

In high, rapidly rising floods or flash floods, sandbags are not the right answer – then all that counts is getting to higher ground in time. Defending dykes is a job for emergency services. And high water pressure from outside can damage basement walls if they are sealed off completely. For deeper water, therefore, get professional advice beforehand on whether sealing off makes sense at all.

Water does not only come through doors. It can push back from the sewer through drains and toilets – this is called backflow. A backflow valve is part of home preparedness and is installed by professionals. For doors there are also flood boards, mobile barriers and water-filled barriers that go up faster but must be acquired and tried out beforehand.

Evacuation instructions always take priority. Do not keep working when the water is rising or starts to flow. Flowing water can sweep you off your feet, and a house can be replaced; a person cannot.

Filled bags are heavy, and after hours your back and arms ache. Lift with your legs, not your back, and pass the bags along in a chain from hand to hand rather than carrying each one. Plan breaks and drink enough. Gloves and sturdy shoes protect hands and feet.

Bags that were in floodwater are contaminated, because floodwater can contain sewage, oil and chemicals. Handle them only with gloves and dispose of them as the municipality instructs. Bags and sand from a flood do not belong in the sandpit or garden.

A finished wall is no reason to sit back. Watch where water seeps through and add bags there. Small trickles are normal; if more and more water is being pushed through at one spot, the wall is too low or too narrow there.

Keep pumps ready on the dry side if you have them and can use them safely, and make sure electrical equipment does not end up in water. And through it all, stay alert to warnings: as soon as there is danger, stop immediately and get to safety.