Sandbagging and Flood-Proofing a House
Sandbags are the image of flood preparation, and they are widely misunderstood in both directions. People either believe a wall of them will hold back a river, or they dismiss them as useless.
The truth is narrow and practical: sandbags divert shallow, slow water and seal low openings. They do not hold back deep or fast water, and stacking them higher does not change that. Used for the job they can do, they are worth the effort. Used as a levee, they will fail and they may kill someone standing behind them.
The work that matters more is unglamorous: lifting things up.
The 10-minute version
- Sandbags are for shallow diversion and sealing openings — doorways, drains, toilets. Not for holding back deep water.
- Lifting contents beats defending the building. Water that gets in does less damage if everything valuable is above it.
- Never sandbag and stay if the forecast is above the level you can defend. Property is insurable; deciding to leave is the actual plan.
What sandbags do and do not do
They do:
- Divert shallow surface water away from a doorway
- Seal a low external door threshold
- Block toilets, floor wastes and drains to stop sewage backflow — genuinely the most valuable use, and the one most people skip
- Hold plastic sheeting in place against a wall
They do not:
- Stop deep water
- Stop fast-moving water
- Make a house watertight — water comes up through floors, through brickwork and through the ground
- Justify staying in a house that is going to flood
How to actually place them
- Fill to about two-thirds, not full. Under-filled bags conform and interlock; full bags are rigid and leave gaps.
- Tie or fold the neck under the bag so the weight seals it.
- Lay them like brickwork, long side along the direction of the water, each course overlapping the joints below.
- Two courses high is the practical limit for a doorway. Above that you need engineered barriers.
- Line the water side with plastic sheeting — heavy builder’s plastic — tucked under the first course and running up the wall. The plastic does the sealing; the bags hold the plastic.
- Seal the drains first. Toilets, floor wastes, laundry drains and shower outlets. A bag or a purpose-made bung inside a plastic bag, weighted down. Sewage backflow through a toilet is the most common and most disgusting form of internal flooding.
- Do not build a ring around the house. Water finds another way in and you have then trapped it.
Where to get them: many councils and SES units distribute sandbags ahead of forecast flooding, and hardware stores sell them. Do not wait for the warning — the queue forms fast.
The work that matters more
Lift everything. Raising contents above the expected water level does more for your total loss than any barrier. Furniture onto blocks, boxes off the floor of the garage, tools onto high shelving, documents already scanned and in the cloud.
Move the cars to high ground. Vehicles are frequently the single largest uninsured-by-choice loss in a flood, and moving them costs nothing.
Photograph and film everything before the water arrives — cupboards open — for the claim.
Turn off electricity at the switchboard before water reaches outlets, if you can do it safely and dry.
Roll up rugs, lift curtain hems, take out bottom drawers.
Put the go-bag in the car, on high ground, with documents and medication.
Fill water containers. Flooding contaminates supply and boil-water notices follow.
Longer-term measures
Worth costing against your insurance excess and premium if you are in a mapped flood area.
| Measure | Rough cost | What it changes |
|---|---|---|
| Storage shelving 1.2 m up in the garage | $150–400 | Removes most contents loss in a shallow flood |
| Non-return valve on the sewer line | $500–1,500 | Stops sewage backflow permanently. High value |
| Purpose-made door barriers | $400–2,000 | Faster and far more effective than sandbags |
| Raising power points and switchboard | $800–3,000 | Reduces electrical damage and reconnection delay |
| Tiled or concrete floors instead of carpet | Varies | Dramatically cheaper to recover |
| Raising the house | $50,000+ | The real solution in a high-risk area |
What actually goes wrong
Sandbags are treated as a levee, and people stay behind a barrier that fails.
Drains and toilets are not sealed, so the water arrives from inside.
Bags are overfilled, leaving gaps at every joint.
No plastic sheeting, so water flows straight through hessian.
The car is left in the driveway.
Contents are never lifted — the single highest-return action available.
People sandbag instead of leaving. The one genuinely dangerous error on this page.
Nothing was photographed, and the claim becomes a memory test.
Thresholds
| Situation | The rule |
|---|---|
| Forecast above your defendable level | Leave. Do not defend |
| Sandbag height | Two courses at a doorway. Above that, engineered barriers only |
| Bag fill | Two-thirds, folded under, laid like brickwork |
| Sealing | Plastic sheeting on the water side, tucked under the first course |
| Drains, toilets, floor wastes | Sealed first. Highest value use of a sandbag |
| Contents | Lifted above expected water level, garage included |
| Vehicles | Moved to high ground early |
| Electricity | Switched off at the board before water reaches outlets, only if safe and dry |
| Before anything | Photograph and film every room |
What it costs
| Tier | Spend | What it buys |
|---|---|---|
| Free | $0 | Lifting contents, moving cars, photographing rooms, switching off power, filling water containers |
| Solid | ~$60–150 | Sandbags from council or hardware, a roll of heavy builder’s plastic, drain bungs |
| Better | ~$400–1,500 | Purpose-made door barriers, a non-return sewer valve, high garage shelving |
| Structural | $5,000+ | Raised switchboard and outlets, hard floors, or raising the house |
The drill: the pre-season hour
- Twenty minutes. Check your council flood map and write down the expected depth at your address.
- Fifteen minutes. Walk the house and list what would need lifting, and to what height. Buy the shelving now, not during the warning.
- Ten minutes. Locate your toilets, floor wastes and laundry drains, and buy bungs for them.
- Ten minutes. Find where your council or SES distributes sandbags, and note it.
- Five minutes. Confirm where you would move the cars.
Frequently asked questions
Do sandbags actually work?
For shallow, slow water and for sealing low openings and drains, yes. They do not hold back deep or fast water, and stacking them higher does not help — above two courses at a doorway you need engineered barriers.
How do you fill and lay sandbags properly?
Fill to about two-thirds so they conform, fold the neck under, and lay them like brickwork with the long side along the water flow, overlapping the joints below. Line the water side with heavy plastic sheeting tucked under the first course.
What is the most useful thing to sandbag?
Toilets, floor wastes and laundry drains, to stop sewage backflow. It is the least glamorous use and the one that prevents the worst mess.
Is it better to sandbag or to lift my contents?
Lift contents. Water that gets in does far less damage if valuables are above it, and lifting is free. Barriers are a supplement, never the plan.
Should I stay to defend the house?
Not if the forecast is above the level you can realistically defend. Property is insurable and a sandbag wall failing with people behind it is a genuine danger.
Where do I get sandbags?
Many councils and SES units distribute them ahead of forecast flooding, and hardware stores stock them. Get them early — the queue forms as soon as a warning is issued.