Type your roof size and household: how much water that roof could catch in a typical year, which months fill the tank, how many days of water that is, and how much rain has actually fallen in the last 30 days.
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From Town Tools. For the current version, visit https://www.towntools.org/bonthe-southern-province-sierra-leone-sl/rainwater
Litres this roof catches each month of a typical year. The rains do the work in a few months — storage carries you through the dry ones. The rains here come May–October.
JanJan: 1,100 L
FebFeb: 928 L
MarMar: 1,900 L
AprApr: 3,700 L
MayMay: 9,600 L
JunJun: 15,800 L
JulJul: 18,200 L
AugAug: 19,700 L
SeptSept: 15,400 L
OctOct: 9,800 L
NovNov: 5,300 L
DecDec: 2,000 L
And right now
About 531 mm of rain has fallen in the last 30 days — enough for this roof to have caught roughly 17,000 litres.
These are estimates, not a promise
The catch is plain arithmetic from your roof size and average rainfall — real water depends on your roof, gutters, the first wash thrown off a dusty roof, and how big a tank you have to hold it. Treat it as a starting point for sizing a tank, not a guarantee.
Bonthe gets more than three metres of rain a year, and more than four-fifths of it falls between May and October. July, August and September alone bring about 1,666 mm. December through March bring about 187 mm in four months together — less than August delivers in ten days. What a roof catches and a tank holds through the rains is what a household has in March. Rain is also the one thing here that does not arrive by boat.
Put in your roof size and how many people live in the house. The figures below are plain arithmetic from thirty years of rainfall averages for this part of the estuary, and nothing you type leaves the page.
Live rainfall from Open-Meteo (CC BY 4.0) — a weather-model estimate for the area, not a rain gauge on your street. Updated .
Rain off a roof is not drinking water
Water off a roof is not drinking water. On its way to the tank it picks up dust, leaves, bird droppings and smoke residue, and the rains are also when diarrhoeal illness and cholera risk are highest. Three steps make it safe to drink. Send the first few minutes of each downpour away from the tank instead of into it, so the dirty first wash off the roof is lost. Pass what you keep through a clean cloth or a filter. Then boil it until it is bubbling hard, or treat it with chlorine at the dose written on the packet. Water you have not treated is still good for washing, cleaning and the garden.
Rainfall varies a lot around here
These are averages from a weather model on a grid roughly 11 km across, not readings from a rain gauge in Bonthe, and an average year is not this year. Two things follow. Some years are much wetter or drier, so a tank sized on a normal year can still run dry. And other published figures for Bonthe are higher than the 3,232 mm used here — around 3,665 mm a year, roughly 13 per cent more. The lower figure is used on purpose: when someone is sizing a tank, over-stating the catch is the harmful mistake. Your roof may well catch more than this page shows.
What goes into the numbers
Why not every drop is caught
The sum assumes a hard roof — corrugated iron sheets, the zinc pan roofing on most houses in Sierra Leone: 83.8 per cent of households in the 2017 Multiple Indicator Cluster Survey had a metal or corrugated iron roof, against 13.5 per cent thatch or palm leaf. Even on a good iron roof about a fifth of the rain never reaches the tank. Some blows off the edge, gutters spill in the heaviest downpours, and the dirty first wash off the roof is deliberately thrown away. That is the 0.8 in the arithmetic. A thatch or palm-leaf roof catches considerably less, and what it does catch is dirtier.
How much water a person uses
The days-of-water figure assumes 20 litres per person per day. That is a basic-needs figure, not a comfortable one: drinking, cooking, and washing hands and body. WHO's technical note on water in emergencies states that 20 litres per person per day is the minimum quantity of safe water needed to reach basic levels of health and hygiene. A house with a tap and a shower uses several times that. If this water also washes clothes, waters a garden or serves animals, count on more per person and the days will be fewer.
Worth knowing
The tank is the limit, not the rain
On these figures rain is not the scarce thing here — storage is. A 40 m² iron roof catches roughly 19,700 litres in August alone, so a 1,000-litre drum fills and overflows many times over in one wet month. What decides whether there is water in March is how much can be held. December to March bring about 187 mm in total, which off that same roof is about 6,000 litres for the whole dry stretch. Size a tank against the dry months you need to cross, not against the year's total.
Throw the first rain away
Between downpours a roof collects dust, leaves, bird droppings and smoke from fish-smoking fires, so the first water off it is the dirtiest of the storm. A first-flush diverter sends that away — in its simplest form a capped length of downpipe that fills first, and only then lets water carry on to the tank. Keep the tank covered and the inlet screened so mosquitoes cannot breed in it, and clear the gutters before the rains build in May.
The piped water people are still waiting for
Piped water has been a standing grievance in the municipality. At a 2021 inspection of the Bonthe Municipal Water Supply Project, council representatives told the Ministry of Water Resources and SALWACO that they were dissatisfied with the slow pace of work and described pipe-borne water as a critical need. Neither SALWACO's nor the ministry's website could be reached when this page was built, so nothing here reports where the project now stands — the council is the place to ask.
How water is carried and treated in Sierra Leone
A 2019 household survey in four districts, run by the Sierra Leone Field Epidemiology Training Program with WHO, found the jerrycan was the collection container for 39 per cent of households, and that most households treat nothing before drinking: 76 per cent treated no water in urban areas and 89 per cent in rural ones. UNICEF puts 58 per cent of people in Sierra Leone without a basic water service. A covered tank whose water you treat yourself is a step up from an open container, not a step down. the 2019 household WASH survey.