Slough does have a flood risk, and most of it comes from rain with nowhere to drain. Only a small slice of the town is in the river flood zones. Far more postcodes are flagged once rain run-off is counted. The most recent recorded flood came up from a sewer. The Thames is near enough to worry about, but the drains are the bigger problem.
On the Environment Agency's river-and-sea mapping, about 3% of Slough's postcodes are in flood zone 3, the highest-probability band, and about another 3% are in flood zone 2, the medium-probability ring around it. The two zones do not overlap, so zone 2 is only the extra fringe beyond zone 3. For a town that lies between the Thames, the Jubilee River and the Slough Arm of the Grand Union Canal, that is a small exposure to rivers.
Surface water is where the risk sits. The Environment Agency holds five recorded flood events for the area, and the latest, on 12 January 2003, came from a sewer after the channel ran out of capacity, with no raised defences involved. A town that paved over brickfields, nurseries and a cattle market to build a trading estate has given heavy rain fewer places to soak away.
The Environment Agency's climate-adjusted mapping puts about 12% of postcodes inside the flood zone once a changing climate is allowed for. That shows how far the modelled zone could spread, and it gives no date for any actual flooding. The edge of the river zone is the part of town where that wider line matters most.
Insurers price individual properties, not the town as a whole. A house in one of the river zones, or on a postcode flagged for surface water, can expect more questions, a higher premium or a larger excess than one a few streets uphill. Getting a quote on the exact address early is the one check worth making, because the answer varies so much from street to street.
Across all the measures together, about 16% of postcodes rate high on at least one flood indicator, about 20% medium on at least one and about 44% low on at least one. These bands fold in surface water alongside rivers and sea, so they overlap with the zone figures and cannot be added to them. They answer a wider question than the river zones do, and the much higher shares come mostly from rain run-off.
Counted as properties, the Environment Agency's rivers-and-sea mapping lists 10,259 across the 404 postcodes that carry any river or sea risk at all. Of those, 1,375 are at high risk, 1,265 of them homes, 459 at medium risk, 1,778 at low risk and 1,413 at very low risk. Postcodes with no river or sea risk do not appear in the count at all.
About 5% of postcodes fall inside the modelled high-risk extent with no defences counted, and about 5% fall inside it with defences included in the model. The two numbers are almost the same, which suggests the modelled defences change very little about where the high-risk water reaches. A defence that appears in the model is a structure on a map, and it does not keep water out of any particular kitchen.
The nearest tide gauge is at Portsmouth, where sea level rose by about 2 millimetres a year over its record from 1962 to 2023. That is the rate measured at Portsmouth over those years. For Slough, the sea is a long way off and comes well down the list of things to worry about.
Slough's flood risk is below par. The river zones are small, but too many streets flood from the drains for buyers to ignore it.