Rainwater · Norney Farm

Harvestable · last 12 months

—litres

Draw your roof section on the map below to see what it collects.

🌧

Next 24 hours

—L

—

now+12h+24h

The month either side of today

Last 30 days

—L

—

today

Next 30 days

—L

—

Your roof

The roof is divided into catchments, one for each section that drains to its own gutter or downpipe, so you can see how much rain each one gathers through the year.

Roof surface

—

0 m² footprint

Loading…

The orange dashed outline is the Ordnance Survey wall line, which usually leaves out the eaves overhang that also catches rain. Rain falls on the footprint seen from above, so roof pitch makes no difference to the area.

Catchments

What each section of roof gathers. The numbers use the last 12 months of rain at the farm, and a typical year from 30 years of records at nearby rain gauges.

Catchment	Area (m²)	Last 12 months	Share	Typical year
Total	—	—	100%	—

Month by month

Litres each catchment can gather every month over the last year, stacked, with a typical month, from 30 years of rain-gauge records, as the orange line. The current month is shaded lighter because it is not finished. The big number at the top counts the last 365 days, so it can differ slightly from the bars.

Day by day

Daily rainfall with the running total of litres collected over the period.

Rain (mm)

Forecast (mm)

Cumulative litres

Could the roof cover the household?

Worked out per person, then scaled to your household. Open to see the detail.

Everything here is worked out for one person first: a typical person in the UK uses about 140 litres a day. Enter the number of people in the household and it scales up. If every drop from the roof were captured, it shows how much of that need it could meet. Choose how far the water is treated: the tap and appliances you connect share the rainwater equally, and mains water tops up the rest.

People in the household

Rainwater is plumbed to

One person needs

—L a year

—

The household needs

—L a year

—

The roof gathers

—L a year

in a typical year, before any storage losses

Rainwater would meet

—% of need

—

Use	One person, a day	One person, a year	Household, a year	Treatment	Rainwater share
Total	—	—	—		—

Month by month, against what the household needs

Bars are what a typical month of rain would gather. The lines are what the household would use each month. Where a bar clears a line, that month's rain alone could cover those uses. Storage, which carries wet months over to dry ones, is the next piece.

How this is worked out

Litres = roof footprint (m²) × rainfall (mm) × runoff coefficient. One millimetre of rain on one square metre is one litre.
The runoff coefficient allows for water lost to wetting, splash and evaporation. Real systems also lose some to the first flush and to overflowing a full tank, so treat this as the most you could gather.
Where the rainfall comes from. The last 12 months, the last 30 days and the forecasts use weather-model data for the roof's coordinates from Open-Meteo, which is an estimate and not a rain gauge. The typical year and typical month use 1991 to 2020 averages from the four nearest Environment Agency rain gauges (9 to 16 km away), listed in rainwater-normals.json (Environment Agency hydrology data, Open Government Licence). Run tools/rainfall_normals.py to build the same file for another roof in England.
How accurate is it? Rain varies a lot over short distances: those four gauges average 727 to 875 mm a year, so the yields here could easily be 10% out either way. Over 1991 to 2025 the weather-model data ran about 7% below the gauges on average, and single years ranged from 15% below to 4% above.
The household calculation: one person's yearly need is their daily litres × 365. The household need is that × the number of people. At each treatment level the uses plumbed to rainwater add up to a need, and rainwater supplies the smaller of that need and what the roof gathers in a typical year, since mains tops up any shortfall and surplus rain has nowhere to go without storage. Every plumbed-in use gets the same share, because they all draw on the same tank. The roof's yearly gathering is the three-year average, so a dry year will do worse.
Storage size and how the water gets used come next, so this page only covers what arrives from the roof.
Using this for your own house: the catchments are not edited on the page. They live in rainwater-catchments.geojson, one polygon per catchment in British National Grid (EPSG:27700), each with a name, a color and an optional override area in m². The roof covering is set at the top of the same file: surface (a label) and runoff (the share of rain that reaches the gutters, about 0.9 for clay tiles, 0.8 for flat membrane, 0.6 for gravel, 0.4 for a green roof). Replace that file (and norney-farm.geojson, the outline drawn dashed) with your own. Draw the catchments so they share exact corner points. The page works out the rainfall location from where the catchments are, and checks that they fit the outline.

🌧 Rainwater

How this is worked out