Photo: Jaggery · CC BY-SA 2.0Flood risk in Mountain Ash: under 0.1% of homes from rivers or the sea, 0.9% from surface water
On Natural Resources Wales’s maps, which allow for flood defences, “high or medium” means at least a 1 in 100 chance of flooding from rivers or the sea in any year (1 in 200 for the sea in Wales). Mountain Ash grades B for hazard risk: #7 of 32 in Mid Glamorgan.
What stands out about Mountain Ash’s hazards
From Natural Resources Wales’s maps and records and the BGS radon atlas, below.
What Mountain Ash faces
Each hazard against its own yardstick; meters run from low to high on that yardstick.
Rivers and the sea
Surface water
Past floods
Radon
Mountain Ash grades B for hazard risk
Flood risk from rivers and the sea, against all UK towns.
The share of homes at high or medium risk from rivers and the sea, ranked against every UK town of 1,000 or more: fewer homes at risk is better. Towns with none at high or medium risk are ordered by their share at low risk, so they don’t all share one score. Surface water and radon aren’t graded: they are described below.
Where the water goes in Mountain Ash
Turn layers on and off. Blue is the risk from rivers and the sea; pink is where heavy rain collects. Both allow for flood defences.
The river and sea risk follows the River Cynon. Surface-water risk is spread across the town, in dips, underpasses and along small streams, which is why more homes are exposed to it.
Under 0.1% of Mountain Ash’s homes are at high or medium risk from rivers or the sea
Natural Resources Wales’s national map, allowing for defences. A further 0.5% are at low risk.
Share of each UK town’s homes at high or medium risk from rivers and the sea. 35% of UK towns have none; Mountain Ash has more than 40% of towns.
| # | Town | People | High or med. | |
|---|---|---|---|---|
| 1 | Clifton Greater Manchester | 17,748 | 0% | |
| 2 | Oswestry Shropshire | 17,500 | 0% | |
| 3 | Royston Hertfordshire | 17,443 | 0% | |
| … | ||||
| 129 | Egglescliffe County Durham | 10,250 | under 0.1% | |
| 130 | South Woodham Ferrers Essex | 16,029 | under 0.1% | |
| 131 | Mountain Ash Mid Glamorgan | 13,002 | under 0.1% | |
| 132 | Shelf and Northowram West Yorkshire | 9,191 | under 0.1% | |
| 133 | Caister-on-Sea Norfolk | 9,095 | under 0.1% | |
| … | ||||
| 405 | Bentley South Yorkshire | 12,044 | 40% | |
| 406 | Dunscroft and Hatfield South Yorkshire | 14,000 | 51% | |
UK towns of 9,000 to 18,000 people.
Natural Resources Wales’s Flood Risk Assessment Wales map (May 2026) tested against every address in the town (ONS NSUL), weighted by Census 2021 households. Rivers and sea can overlap; the worse band counts.
0.9% of Mountain Ash’s homes are at high or medium risk from surface water
Surface water is rain that falls faster than drains and the ground can take it. It is mapped separately from rivers and the sea, and it is not in the Hazard risk Grade.
About 0.8% of homes are at high risk of surface-water flooding (a 1 in 30 chance or more each year) and 0.1% at medium risk: roughly 50 households. A further 1.2% face a low risk from surface water. That is more than 9% of UK towns.
Our estimate: every address in the town (ONS NSUL) read on Natural Resources Wales’s Flood Risk Assessment Wales map of surface water and small watercourses, each postcode’s share weighted by its Census 2021 households. Neither agency publishes a count of homes for surface water, so treat it as a guide; one street can differ from the next.
Mountain Ash has flooded in 3 recorded years since 1977, most of all in 1979
Every flood Natural Resources Wales has mapped inside Mountain Ash’s built-up area, newest first. The record is fullest for big river floods; small and surface-water floods are often missing.
- 2020February 2020: River flooding recorded3 hectares inside today’s built-up area · Mountain Ash Storm Dennis
- 1979December 1979: River flooding recorded36 hectares inside today’s built-up area
- 1977November 1977: River flooding recorded1 hectare inside today’s built-up area
Floodwater covered 36 hectares of what is now Mountain Ash in 1979, the largest flood the record holds here (December).
169 of Mountain Ash’s households today live inside the outline of a recorded flood.
Since 1979, the largest recorded flood inside the town covered 3 hectares.
Natural Resources Wales warns 1 area in Mountain Ash
Natural Resources Wales issues flood alerts and warnings for Wales; the Environment Agency’s live service covers England only. Both run Floodline, 0345 988 1188.
Show the 2 warning and alert areas
| Area | Kind | River or sea |
|---|---|---|
| River Cynon | Alert area | River Cynon |
| River Cynon at Mountain Ash | Warning area | River Cynon |
Parts of Mountain Ash are in radon Affected Areas, up to the 10–30% band
Radon is a natural radioactive gas from the ground; long exposure raises the risk of lung cancer. The atlas gives the chance that a home in each 1 km square is above the UK action level (200 Bq/m³).
100% of Mountain Ash’s built-up area is in a radon Affected Area (1% of homes or more above the action level), and 61% in squares of 5% or more.
The squares are indicative: each takes the highest band found anywhere inside it, so most homes in a coloured square are below it. Only a three-month test in the home gives a real answer.
BGS/UKHSA Indicative Atlas of Radon in Great Britain, version 3. Check an address at UKradon.org.
Flood risk in the places around Mountain Ash
Share of homes at high or medium and at low risk from rivers and the sea. Tap a town for its page.
Troedyrhiw and Pentrebach, 2.9 miles north-east, has the most homes at risk around Mountain Ash (8.7% at high or medium).
Get ready for what Mountain Ash faces
Free warnings, what to do in a cloudburst, and what to check before you move.
- Sign up free for Natural Resources Wales flood warnings by text, call or email.
- Floodline: 0345 988 1188, 24 hours.
- Mountain Ash’s warning areas include River Cynon at Mountain Ash.
- Surface water rises in minutes, often with no warning issued: stay out of underpasses and dips.
- Keep the gullies and drains outside your home clear of leaves.
- Report a blocked road drain to the council.
- Test over three months, in a living room and a bedroom.
- Above 200 Bq/m³, a small sump and fan usually fixes it.
- New homes in radon areas are built with protection: basic from 3%, full from 10%.
- Check the address on Natural Resources Wales’s flood map: it shows rivers, the sea and surface water.
- Ask whether the home has flooded or had a flood insurance claim; the TA6 property form asks the seller.
- Homes built before 2009 can get flood cover through Flood Re, the government-backed scheme; newer homes can’t.
- Check radon for the address on UKradon.org (a small fee) or ask the seller for a test result.
What this data can and can’t tell you
- Defences are counted. Both agencies’ maps allow for flood defences and the chance they are overtopped or fail, so a town behind a defence scheme can show few homes at high risk and many at low risk.
- Surface water is our estimate. Neither agency publishes a count of homes at risk from surface water, so we read each address on the map. A flat on a hill and a basement in a dip can share a postcode.
- Past floods are what was recorded. The flood outlines come from surveys, aerial photographs and reports; many small floods, and most surface-water floods, were never mapped.
- One town, many streets. Risk is street by street. Check a specific address on Natural Resources Wales’s flood risk map before you buy or rent.
- Wales is measured slightly differently. NRW publishes no count of homes at risk, so each address in the town is tested against its map. Addresses include garages, chapels and shops, and NRW’s medium band for the sea runs to 1 in 200 (England’s stops at 1 in 100), so Welsh figures lean a little higher than English ones.
Questions about flooding and hazards in Mountain Ash
Does Mountain Ash flood?
The last flood Natural Resources Wales recorded inside the town was in February 2020. Under 0.1% of homes are at high or medium risk from rivers and the sea on Natural Resources Wales’s maps, which allow for flood defences. About 0.9% of homes are at high or medium risk from surface water, when heavy rain overwhelms the drains. Mountain Ash grades B for hazard risk against UK towns.
Which parts of Mountain Ash are at risk of flooding?
From rivers and the sea, the places Natural Resources Wales warns directly: River Cynon at Mountain Ash. Surface water can collect in low spots anywhere in the town. Check a specific address on NRW’s flood risk map.
When did Mountain Ash last flood?
The recorded flood outlines inside the town run from 1977 to 2020 (3 years); the largest was in 1979.
Is Mountain Ash at risk of surface water flooding?
About 0.8% of homes are at high risk and 0.1% at medium risk from surface water, against under 0.1% at high or medium risk from rivers and the sea. Surface water floods come with little warning, and the agencies don’t issue warnings for them.
Is radon a problem in Mountain Ash?
100% of the town’s built-up area is in a radon Affected Area on the BGS/UKHSA atlas, and 61% is in 1 km squares where 5% or more of homes may be above the action level. Most homes are below it; a three-month test is the only way to know.
Where can I find flood warnings for Mountain Ash?
Natural Resources Wales issues flood warnings and alerts for Wales on its flood warnings service, and runs Floodline (0345 988 1188) with the Environment Agency.
Contains Natural Resources Wales information © Natural Resources Body for Wales and database right. Contains OS data © Crown copyright and database right 2026. Contains Royal Mail data © Royal Mail copyright and database right 2026. Contains British Geological Survey materials © UKRI 2026.
Sources: Environment Agency Risk of Flooding from Rivers and Sea, postcodes in areas at risk (June 2026); Natural Resources Wales Flood Risk Assessment Wales (May 2026); ONS Census 2021 households by postcode · Environment Agency Risk of Flooding from Surface Water (NaFRA2, 2025) and Natural Resources Wales Flood Risk Assessment Wales surface water and small watercourses, read at every address (ONS National Statistics UPRN Lookup, September 2026) and weighted by Census 2021 households by postcode · Environment Agency Recorded Flood Outlines and Natural Resources Wales historic flood map, inside the ONS 2022 built-up-area boundary · BGS/UKHSA Indicative Atlas of Radon in Great Britain, version 3, by 1 km square · TownRatings grades: our own summary of the data, ranked against UK towns only. All under the Open Government Licence v3.0.
Contains public sector information licensed under the Open Government Licence v3.0. Contains HM Land Registry data © Crown copyright and database right 2026. Contains OS data © Crown copyright and database right 2026 (ONS boundaries). Northern Ireland: Source: NISRA; contains Land & Property Services data and Ordnance Survey of Northern Ireland information, © Crown copyright, licensed under the Open Government Licence v3.0. Scotland: contains National Records of Scotland data, Registers of Scotland data and SEPA data, © Crown copyright / © SEPA 2025, licensed under the Open Government Licence v3.0.