Fill each sandbag two-thirds full of sand, soil or gravel, fold the top over without tying it, and stack the bags in offset rows like bricks, stamping every row flat before adding the next. A wall three bags high, about 45 cm, handles moderate flooding. Build it in a pyramid, with a wide base and a narrow top, and expect a 20 m wall to need about 240 bags and 3.4-4.3 tonnes of fill.
Part of Flood Control and Drainage
Sandbags are the emergency flood barrier: bags of local fill stacked into a temporary wall when a flood is imminent and no permanent barrier exists.
A Sandbag Wall Is A Race Against Weight And Time
Sandbags are cheap and flexible, and a crew can raise a wall in hours. They are also heavy, they leak, and they decay. Treat the wall as a short-term measure that buys time to move people and goods, not as a substitute for a berm.
What You Need
- Bags — burlap or woven plastic.
- Fill — sand, soil or gravel, the nearest to hand.
- Shovels and a funnel — a cut-off bottle or a rolled sheet of bark speeds filling.
- Plastic sheeting — if available, to line the water side.
- A crew — filling, carrying and stacking are three separate jobs.
Method: Stack The Wall
Step 1: Fill two-thirds full. A bag filled to the brim cannot be folded flat and does not seat against its neighbours. Two-thirds full lets it settle and mould itself to the gaps.
Step 2: Fold the top over; do not tie it. Folded bags lie flatter and seat better than tied ones.
Step 3: Prepare the line. Clear the ground, loosen the top few centimetres for the first row to key into, and lay plastic over the wet side if you have it.
Step 4: Lay the first row end to end. Place bags lengthwise along the wall line, folded side down and tight against each other.
Step 5: Stack in a pyramid. Offset each row like bricks so no joint lies above another, and make each higher row narrower than the one below.
Step 6: Stamp each row flat. Tread or tamp every row before adding the next to close gaps.
Step 7: Know the limit. Three bags high, about 45 cm, handles moderate flooding. Beyond that, a sandbag wall needs real design, and it should not be relied on.
Worked Example: How Much Work A Sandbag Wall Takes
Assumptions, all labelled: each filled bag lies about 0.5 m long along the wall and 0.25 m across, and a bag is about 15 cm high (the parent article’s 3 bags = 45 cm); a pyramid of three courses with 3, 2 and 1 bag rows across; a filled bag about 14-18 kg (the commonly quoted 30-40 lb for a bag two-thirds full, recalled from flood-fighting guidance).
- Bags per metre of wall: (3 + 2 + 1) rows x 2 bags per metre = 12.
- Bags for 20 m: 12 x 20 = 240.
- Mass: 240 x 14 kg = 3,360 kg to 240 x 18 kg = 4,320 kg, about 3.4-4.3 tonnes.
That is about 240 bags, each shovelled, carried and lifted at least once, so a crew of ten placing one bag a minute each needs 240 / 10 = 24 minutes of continuous stacking, and the filling takes much longer. Start early.
Common Mistakes
- Overfilled or tied bags — they do not seat, and the wall leaks.
- Aligned joints — a vertical joint is a leak path. Offset every row.
- Stacking higher without widening — the wall becomes a tall thin line that tips. Keep the pyramid.
- Skipping the stamp — gaps between bags remain open.
- Using contaminated fill — see below.
Safety
Floodwater is moving, deep and dirty
About 15 cm of fast-flowing water can knock an adult off their feet, and the edge of a rising flood hides holes and debris. Do not stack at the edge of moving water, work in pairs, and keep a retreat route. Floodwater carries sewage and chemicals: wear gloves and boots, cover cuts, and wash hands before eating. Assume any submerged electrical equipment is live.
Lifting
A filled bag of 14-18 kg is lifted hundreds of times in a wall. Lift with the legs, rotate crews, and fill the bags at a platform height where possible.
What’s Next
- Learn next: Earth Berms — the permanent version of the same wall
- Combine with: Flood Walls and Berms — choosing between barriers