In places, and the river is the smaller of the two risks. Only a small part of the town is in the river flood zones. Around one postcode in five shows high risk on at least one flood measure. The last river flood on record was decades ago. Most of the risk comes from rain running off roofs and roads with nowhere to drain.
The Environment Agency's river-and-sea mapping puts about 2% of the area's postcodes in flood zone 3, the highest-probability band. Another 2% are in flood zone 2, the medium-probability band just beyond it. The two zones don't overlap, so together they cover a small part of the town. The agency's records hold two floods, the latest in September 1968, when a main river overflowed a channel that had no raised defences. The River Wandle rises here, and near its source it is a small stream.
Surface water is the bigger worry. There is no separate surface-water figure for Croydon, but the gap between the small river zones and the wider postcode picture below is mostly rain. Heavy downpours can pool on low streets and on paved-over front gardens wherever the drains can't keep up. The low ground around Broad Green is the obvious place to look first. "Flood zone 1" on a Croydon listing deals with the river and says nothing about the drains.
With the Environment Agency's climate change allowance added, the share of postcodes in the flood zones grows to about 5%. That is the agency's modelled picture of a wetter future, and it shows which way the zones are expected to shift.
Insurance follows the mapping. A home in a mapped flood zone, or with a past claim, can find cover harder to get and more expensive, and some insurers want more detail before they quote at all. Get a quote for the exact address early, because the answer can change which flats are still worth chasing.
For a fuller picture, the postcode indicators fold surface water in with rivers and sea. About 19% of postcodes rate high on at least one flood measure, about 23% medium and about 42% low. Those bands overlap with the zone figures above, so the two sets can't be added together. They measure related things in different ways.
Counting homes is more useful than counting postcodes. The Environment Agency's rivers-and-sea mapping lists about 4,470 properties in the 148 postcodes that carry any risk at all, out of the whole town. About 1,460 of those are at high risk, 410 at medium, about 1,120 at low and 65 at very low. Postcodes with no river or sea risk don't appear in this count at all.
Defences barely change the picture. About 2% of postcodes fall inside the high-risk flood extent when the model ignores defences, and about 2% still do when it includes them. "Defended" here means only that a defence appears in the model, so it shouldn't be read as a promise that a particular house stays dry.
The nearest tide gauge, at Portsmouth, recorded sea level rising by about 2 millimetres a year between 1962 and 2023. That is a measured past rate at a gauge a long way south, and for an inland town it is the least of the worries here.
Flood risk is one of Croydon's weaker points, though the cause is drains and downpours more than rivers. Most homes are fine, and the risk falls on a limited number of streets.