The sandbag is the universal image of a business bracing for a flood. According to new laboratory testing, it is also a fairly mediocre way to keep water out of a building.
Researchers at the University of Hull found that duct tape, applied to the outside of a household door in 10 to 20 minutes, cut water leakage by 97-98% compared with an unprotected door. By contrast, a wall of 54 sandbags with plastic sheeting reduced leakage by at most 70%, with performance varying depending on water depth.
For a retailer, cafe or ground-floor office on a flood-prone street, that is the whole story. A product sitting in every hardware aisle beat the traditional answer on performance, labour, cost and clean-up.
The gap under the door is the real enemy
The baseline explains why this matters. Unprotected doors in the tests leaked “often… thousands of litres per hour at 60cm water depth“, according to Stuart McLelland, deputy director of the Energy and Environment Institute at the University of Hull. The same tester applied the tape every time to control for bias. Sandbags leak because they are stacked, not sealed. Tape closes the gaps where water actually gets in.
Two practical details stand out. Flood Re’s wider testing of low-cost temporary protection points to specialist flood putty tape as an even tighter seal, and notes that tape works best on the outside, flood-facing side of the door. That second point is worth remembering next time a business replaces an external door or shopfront. Sandless synthetic bags also perform well, but cost more, need pre-soaking and remain awkward to deploy and dispose of.
McLelland is careful not to oversell it. “Duct tape is not a substitute for professionally designed flood doors, barriers or a property-specific flood resilience plan“, he writes, noting it cannot stop water entering through walls, floors and drains. Fair enough. But most small firms are not choosing between tape and engineered flood barriers. They are choosing between tape and nothing.
Insurers are starting to price the difference
The more interesting signal is coming from the UK insurance market. Jonathan Kassian, head of flood resilience at Flood Re, says “sandbags remain a symbol of flood protection, but these tests turn that assumption on its head.” Brokers there are being encouraged to tell flood-risk clients to keep high-quality tape on hand and learn how to use it.
The bigger shift is structural. As the UK moves to risk-reflective flood pricing by 2039, “showing that a home has resilience measures, temporary or permanent, is likely to become a bigger factor in getting cover and price.” In other words, resilience is becoming an underwriting input, not just good housekeeping.
New Zealand’s exposure is large and poorly measured
New Zealand has every reason to watch. Treasury’s 2021 national flood risk assessment estimated 250,050 homes, or 14.5% of the national stock, were exposed to flooding. That figure is five years old, and the absence of a newer public national dataset is itself a problem for anyone trying to price risk sensibly. Commercial premises were not separately counted.
The insurance market has not been getting friendlier. A 2024 Treasury report found the cheapest online home premiums had risen about 24% between October 2022 and April 2024, and that insurers used inconsistent methods to assess and price flood risk, with around a quarter of high-risk homes paying at least $250 a year more for flood exposure.
Small businesses, meanwhile, were more confident than covered. MBIE’s 2024 resilience report, based on a late-2023 survey of 2,278 firms, found 71% scored well on a resilience scale but only 27% held contents or business continuity insurance. The same report noted New Zealand ranked second in the world for natural disaster exposure. That is a lot of self-assurance sitting on very thin protection.
A flood kit that costs less than lunch
None of this has been tested under New Zealand conditions, and the numbers are UK lab results, not field data from a Hawke’s Bay main street. But water pushing through a door gap behaves the same in Hull as it does in Gisborne. The cost-benefit logic transfers directly.
The practical response is not complicated. Keep a few rolls of quality duct tape or flood putty tape with the emergency gear. Make sure staff know it goes on the outside of external doors, and that it needs doing before the water arrives. Photograph it in your flood plan. Then ask your broker whether it counts for anything.
Today the honest answer is probably not much. But if New Zealand insurers follow the UK towards resilience-linked pricing, and their inconsistent flood models suggest they have plenty of room to evolve, the firms that can already show their homework will be first in the queue when it starts to matter. Sandbags and hope was never a strategy. Now it is not even the best cheap option.
Sources
- 1News: Worried about flooding? A test found duct tape better than sandbags (2026-10-25)
- The Conversation: Duct tape kept out more flood water than sandbags in laboratory tests (2026-10-16)
- Insurance Business: Tape beats sandbags at keeping floodwater out, tests find (2026-10-19)
- Flood Re: Testing low-cost temporary PFR solutions
- Treasury: A national flood risk assessment of NZ (2021-09-01)
- Treasury: Report T2023/2206 insurance update (2024-06-27)
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