Flood Resilient Construction

What This Requirement Covers

Buildings in flood risk areas require specific design measures to reduce the impact of flooding. The National Planning Policy Framework (NPPF) addresses flood risk at the planning stage, while Approved Document C and the CIRIA guidance on flood resilient construction address the design and construction of buildings to withstand flooding.

Key Requirements

Planning and Flood Risk

1. The Environment Agency designates Flood Zones:

- Flood Zone 1: Low probability (less than 0.1% annual probability) - Flood Zone 2: Medium probability (0.1-1.0% annual probability) - Flood Zone 3a: High probability (1.0% or greater) - Flood Zone 3b: Functional floodplain

1. A Flood Risk Assessment (FRA) is required for developments in Flood Zones 2 and 3, and for developments of 1 hectare or more in Flood Zone 1
2. The Sequential Test requires development to be located in the lowest-risk flood zone available

Flood Resilient Design Measures

Resistance measures (keeping water out):

1. Flood barriers at doors and air bricks


2. Waterproof renders and coatings on external walls below flood level


3. Non-return valves on drainage connections


4. Sealed service entry points

Resilience measures (accepting water entry and facilitating recovery):

1. Raised electrical sockets and switches (above predicted flood level)


2. Concrete or tiled ground floors (instead of timber)


3. Closed-cell insulation (instead of mineral wool, which absorbs water)


4. Water-resistant wall finishes (cement render, lime plaster, tiled surfaces)


5. Boiler and consumer unit mounted above predicted flood level

Finished Floor Levels

1. In flood risk areas, the finished floor level should be set above the predicted flood level (with a freeboard allowance, typically 300-600 mm above the 1-in-100-year flood level plus climate change)

Practical Compliance Tips

1. If buying or developing land in a flood risk area, obtain the Environment Agency flood risk data and commission a site-specific FRA
2. Design the building with both resistance and resilience measures; no resistance measure is 100% effective
3. Raise all electrical services above the predicted flood level; rewiring after a flood is one of the most expensive recovery costs
4. Use closed-cell insulation (PIR, XPS) below flood level; mineral wool must be replaced after flooding
5. Install non-return valves on all drainage connections to prevent sewage backing up into the building during a flood
6. Specify easily replaceable materials at low level (removable skirting boards, sacrificial plaster)
7. Consider the impact of climate change on future flood risk; current maps may underestimate future risk