We are often asked to quote for higher flood barriers and told that another supplier has a product tested to a greater height. This creates the impression that the barrier is the limiting factor. It is not.
Once water is held back against a building or wall it applies a load to the structure. The higher the water level, the greater the force. Force is also transferred through the barrier into the fixings, into the wall and ultimately into the foundations. This is a structural issue, not a product issue.
Where the 600 mm H Guidance comes from
The 600 mm level did not come from barrier testing or from a manufacturer’s limitation. It comes from UK Government flood resilience research that was first brought together in 2003 in the document Preparing for Floods – Interim guidance for improving the flood resistance of domestic and small business properties. This was produced by the Office of the Deputy Prime Minister with the Environment Agency, the insurance industry, CIRIA, BRE and construction bodies and represented the industry’s agreed technical position at the time.
That same principle now sits within the Environment Agency Flood Risk Assessment Standing Advice on GOV.UK, which states:
“Flood water can put pressure on buildings, causing structural issues. If your design aims to keep out a depth of more than 600 mm of water, you should get advice from a structural engineer.”
Standing advice is not a casual guidance note.
It is the default technical position used by Local Planning Authorities when they assess flood risk for development. In practice it means this is the nationally accepted baseline unless a site specific, evidence based assessment justifies something different.
The 600 mm threshold is not the maximum height a barrier can achieve. It is the point at which national guidance requires the structure itself to be checked.
Why “the wall survived the last flood” is not evidence
In many flood events the water level inside and outside the property equalises. When that happens there is little or no pressure difference across the wall. The wall is wet, but it is not being asked to hold back a retained head of water.
Installing a high flood barrier completely changes the load case. You are now asking that wall to act as a dam.
This is not something that can be waived by a customer. If a structure fails under flood loading the consequences are not limited to that opening. Masonry can fail, walls can be fail and collapse can occur. In extreme situations there is a life safety risk.
This is no different from installing an RSJ. When a wall is removed the beam size is not chosen by the homeowner because they would prefer a smaller one or because it is cheaper. It is calculated by a structural engineer based on the loads it has to carry and how those loads are transferred to the supports. If the beam is undersized the structure can move or fail. The same principle applies here. Once you start holding back water you are applying a load to the building and that load has to be checked.
It is often misunderstood as a barrier strength issue
The barrier is rarely the weak point. The load is governed by the depth of water and the structure that resists it.
Where higher protection levels are required the solution is often structural.
Industry guidance already reflects this. Property Care Association guidance, for example, limits barriers on single skin walls to around 300 mm unless structural design is undertaken. The wall governs the safe height.
When we are asked to quote for protection above 600 mm we ask for confirmation that the wall or building has been assessed and deemed structurally suitable for the loading.
There are two clear routes:
• Remain within 600 mm relative to internal floor level, which aligns with current national guidance for typical resistance measures
• Obtain structural confirmation for higher protection levels
This approach ensures the whole system performs as intended and that responsibility sits in the correct place.