This guidance explains:  

• what the risk of flooding from surface water (RoFSW) map is
• the information contained in the map
• how it can be accessed and used

It does not give guidance or advice on spatial planning and development.

1. Overview

Surface water flooding occurs when the volume and intensity of rainfall overwhelms local drainage systems.

Unlike flooding from rivers, surface water flooding can happen many miles from a river. This means it can happen in places that people would not expect. It happens because there is nowhere else for the rainwater to go.

There are over 4.6 million properties in areas at risk of surface water flooding in England.

Lead Local Flood Authorities (LLFAs) are responsible for managing the risk of flooding from surface water. LLFAs are the unitary authority or, if there is no unitary authority, the county council for the area. They manage local flood risks and work in partnership with other organisations.

These include:

• the Environment Agency
• district councils
• internal drainage boards
• water and sewerage companies

In 2025, the Environment Agency updated the national map showing the risk of flooding from surface water.

The map uses the latest improvements in data, technology and modelling to help you understand how likely it is to flood where you:

• live
• travel
• work

2. The RoFSW map and what it shows

We produced the RoFSW map on behalf of government. It includes information and input from LLFAs. The RoFSW map is an assessment of where surface water flooding may occur. This happens when rainwater lies on or flows over the ground, instead of draining away through the normal drainage systems or soaking into the ground.

We have developed the map using cutting-edge technology. Flood experts use models to observe how rainwater flows and collects. Using these models, we produced a map that accounts for:

• local topography
• rainfall patterns
• historical data

We produced the map using national-scale modelling. It was then enhanced with compatible, locally produced modelling from LLFAs.

The RoFSW map shows flooding that is likely to occur as a result of rainfall with a 3.3% (1 in 30), 1% (1 in 100) and 0.1% (1 in 1000) chance of happening in any given year. It includes information about flooding extents and depths.

As well as present day risk of flooding, we have created new data to show the potential impact of climate change on the risk of flooding. We use estimates of future rainfall called climate change allowances.

Climate change allowances are based on the latest UK Climate Projections (UKCP18) from the Met Office. They use a mid-range allowance within Representative Concentration Pathway (RCP) 8.5. Representative Concentration Pathways are emissions scenarios that describe possible future greenhouse gas emissions based on assumptions about human activity. These estimates are based on projections for the middle of the century.

We plan to publish further maps accounting for climate change in future.

2.1 Depth of flooding

The maps provide information on flood depths to show the chance that different depths of flooding could be reached or exceeded. The chance of flooding beyond the stated depth (for example 20cm) is given as follows:

• ‘High’ - means more than 3.3% (1 in 30) chance of a flood each year
• ‘Medium’ - means between 1% (1 in 100) and 3.3% (1 in 30) chance of a flood each year
• ‘Low’ - means between 0.1% (1 in 1000) and 1% (1 in 100) chance of a flood each year
• ‘Very low’ - means less than 0.1% (1 in 1000) chance of a flood each year

3. How the RoFSW map was created

The RoFSW map uses the best available surface water flood risk information. Wherever possible it uses outputs from local detailed surface water models.  We developed a new national model for surface water flooding using up to date data and techniques. This provides flood risk information where no detailed local data is available.

3.1 New national model (NNM)

The new national model (NNM) for surface water represents a significant improvement over previous national-scale models. We used computer modelling techniques to model surface water run-off when there is excessive rainfall.

3.2 Rainfall and losses (drainage)

We used industry standard techniques to specify rainfall inputs, depending on the geographic location. Rainfall data is then mapped onto a 1km grid. Each grid cell receives rainfall based on its location, to represent how much rain falls in different areas during a storm.

We estimate how much rain would run off the land surface, depending on the surface type. In urban areas (identified using Ordnance Survey data), the surface is represented as 100% impervious. However, we do make allowances for subsurface drainage systems.

In non-urban areas, we model natural surfaces like fields and parks using industry standard techniques. These consider the soil moisture and other factors to predict how water flows over the land.

We apply a drainage rate in millimetres per hour (mm/hr) as an ongoing loss within all model cells classed as manmade. The default loss rate is 18 mm/hr. This varies between 13 and 25 mm/hr if local information is available or for calibration/validation purposes.

3.3 Digital terrain model (DTM)

Our Integrated Height Model (IHM) 2019 dataset provides the basis for the Digital Terrain Model (DTM) used in the hydraulic modelling. This assigns a ground level to every 2m x 2m model cell.

3.4 Local knowledge and information

We asked LLFAs to review the NNM results and compare them with their local and historic knowledge. Where LLFAs could provide more detailed local maps, we included these to create the RoFSW map. You can find out whether an area is covered by local modelling or the NNM using the Model Origin Layer on data.gov.uk.

3.5 Identifying areas at risk

Once we combined the data from local detailed models and the national model, we removed flooding to a depth of less than 75mm. This is because in surface water flood modelling, most land is covered by some water as rain falls across the area. While this very shallow floodwater may be an inconvenience, it is generally unlikely to prevent access or cause flooding to properties.

Building footprints are raised above their surroundings as part of the modelling process. We have used an uplift of 30cm above the ‘zero’ elevation of the building. This means that in the raw model outputs, buildings can look like they are not at risk of flooding.

The map does not provide property-specific data and cannot specify whether individual buildings are themselves at risk. However, it is important that the RoFSW maps indicate where buildings are considered to be in a flood risk area.

To do this, we infill building footprints, based on how much of the building’s exterior wall and area of the property is wet.

We used building outlines from the OS MasterMap layer to do this.

Finally, we infill small dry areas (less than 50m²) surrounded by floodwater and remove small wet areas (less than 100m²). This is an important step in cleaning our data.

4. Accessing and using the map

4.1 Access to the map

You can view the RoFSW map on the check your long term flood risk service on gov.uk. You can also download the data from data.gov.uk.

If you don’t have access to the internet you can get a copy of the map posted or emailed to you. You can request by calling Floodline on 0345 988 1188.

4.2 Using the map

You can search for a specific address on check your long term flood risk where you will be provided with further information. This includes:

• detail about the level of risk for the area around your property
• future risk associated with climate change
• the name of your LLFA
• access to flood depth information

You can also search the risk of flooding from surface water map on the check your long term flood risk website. You can zoom in or pan out depending on the area you are interested in.

The map does not contain enough information to determine flood risk to individual properties. It does give an indication of whether an area may be affected by surface water flooding and to what extent.

Floods can be devastating, so it is important to find out if you are at risk now rather than wait until it is too late. There are simple steps you can take to prepare for flooding:

• check your long term flood risk service to see if you are at risk from other sources, such as the river, sea or reservoirs
• create a simple personal flood plan - it only takes 10 minutes and doing this in advance will save you valuable time and reduce dame if you’re flooded
• check the 5-day flood risk on the check for flooding in England service
• find out how to prepare your property for flooding - flood defences cannot eliminate all risks of flooding but there are some practical steps which will help to protect your home

We do not currently provide warnings for flooding from surface water. The sporadic and intense nature of rainfall makes it very difficult to accurately predict when surface water flooding will occur.

5. Professional users of the RoFSW map

Flood risk mapping and observed flooding can both influence decision making by financial institutions such as insurers or banks. The RoFSW map and data is available to these institutions who may use it alongside their own independently produced datasets to inform their decisions.

The Environment Agency and local authorities have no role in determining insurance cover. Insurers are free to use whichever data they like to determine whether to offer insurance and at what price.

6. Questions about the map and more information about your local flood risk

Contact your LLFA if you would like more information about:

• your local flood risk from surface water
• the management of surface water flood risk.

In some cases, your LLFA may not have any additional information to offer.

You can find out who your LLFA is by:

• clicking on the highlighted area of the surface water map
• using the postcode search on find your local council

6.1 If you disagree with the map

You may want a review of evidence to potentially update the flood risk rating in your location. If so, you should first contact your LLFA.  A change to the mapped flood extent will only be considered if a flood risk model is submitted for review. There is guidance available on what to consider as part of surface water modelling. If you do not have a flood risk model to submit, you can submit other evidence for review in relation to risk ratings. This can include:

• topographical surveys
• engineers report

Any approved changes to the mapped outputs will take at least 3 months to be published. Without appropriate evidence, LLFAs won’t recommend changes to the risk rating or map. Anecdotal evidence will not be considered.

7. Contact the Environment Agency

Email RoFSW@environment-agency.gov.uk if you have any queries or feedback relating to this guidance.