A house roof in Nepal collects about 1 litre of water for every millimetre of rain on every square metre, less 15 to 25% losses. In Kathmandu, with about 1,500 mm of rain a year, an 88 m² concrete roof gives roughly 105,000 litres a year. But almost 80% of it falls from June to September, so a normal 2,000 to 5,000 litre tank fills many times in the monsoon and stays nearly empty in winter. Size the tank for the monsoon months, send the overflow to a recharge pit, and do not expect rainwater alone to carry you through Poush and Magh.
This guide shows the sum step by step with real rainfall normals for Kathmandu and Pokhara, works out the tank, first flush, downpipe and recharge pit for a typical two-storey house, and lists the mistakes we see most often on Nepali sites.
The basic formula
Water collected (litres) = rainfall (mm) × roof plan area (m²) × runoff coefficient.
- Rainfall: use monthly averages for the nearest station, not the yearly total alone. 1 mm of rain on 1 m² is exactly 1 litre.
- Roof plan area: measure the roof as seen from above, including the parapet line or eaves. A sloping roof counts by its plan area, not by the length of the slope.
- Runoff coefficient (C): the share of rain that actually reaches the downpipe after splash, wetting of the surface, evaporation and leaks.
The technical guideline by Shakya and Thanju in Hydro Nepal (Issue 12, 2013) follows the same method: available water = rainfall × horizontal roof area × runoff coefficient, then compare it with demand month by month.
Runoff coefficients for roofs in Nepal
| Surface | Runoff coefficient | Source |
|---|---|---|
| CGI sheet roof | 0.85 | Hydro Nepal guideline |
| Concrete (RCC flat roof) | 0.80 | Hydro Nepal guideline |
| Cement tile | 0.75 | Hydro Nepal guideline |
| Hand-made clay tile | 0.40 | Hydro Nepal guideline |
| Roof tops in general | 0.75 to 0.95 | India National Water Mission |
| Paved yard | 0.50 to 0.85 | India National Water Mission |
| Bare ground | 0.10 to 0.20 | India National Water Mission |
For an RCC flat roof with a parapet, 0.80 is a fair value. If the roof has a terrace garden, water tank stands and lots of clutter, use less.
Rainfall in Kathmandu and Pokhara by month
The table uses the 1991 to 2020 climate normals for Tribhuvan Airport (Kathmandu) and Pokhara Airport, as published by the World Meteorological Organization from data of the Department of Hydrology and Meteorology. For another town, ask DHM for the nearest station's monthly data; rainfall changes a lot over short distances in the hills.
| Month | Kathmandu (mm) | Pokhara (mm) |
|---|---|---|
| January | 15.2 | 21.6 |
| February | 23.6 | 35.1 |
| March | 38.0 | 63.7 |
| April | 62.9 | 141.7 |
| May | 128.8 | 354.7 |
| June | 241.4 | 648.2 |
| July | 384.1 | 941.0 |
| August | 342.7 | 839.4 |
| September | 207.8 | 635.0 |
| October | 43.3 | 159.2 |
| November | 6.4 | 17.2 |
| December | 7.8 | 11.5 |
| Year | 1,502 | 3,868 |
June to September brings 78% of the year's rain in Kathmandu and 79% in Pokhara. That one fact decides how big a tank is worth building.
How much a 100 m² roof collects in a year
| Roof type (C) | Kathmandu (litres/year) | Pokhara (litres/year) |
|---|---|---|
| CGI sheet (0.85) | 1,27,670 | 3,28,806 |
| RCC flat (0.80) | 1,20,160 | 3,09,464 |
| Cement tile (0.75) | 1,12,650 | 2,90,122 |
| Clay tile (0.40) | 60,080 | 1,54,732 |
These are averages. A dry year can give much less, and the monsoon can arrive two or three weeks late.
Step-by-step method
- Measure the roof plan area that drains to the downpipes you will connect. If only half the roof drains to the tank side, use half the area.
- Pick C from the table above (0.80 for RCC, 0.85 for CGI).
- Get monthly rainfall for the nearest station and multiply each month by A × C. This is the monthly supply in litres.
- Decide what the water is for. In Kathmandu most households use harvested water for toilets, washing, cleaning and the garden, not for drinking. A survey of 244 households in Kathmandu Metropolitan City (study on nepjol) found 58.2% practising rainwater harvesting and 95.8% of them not drinking it. Flushing alone is often taken as about 45 litres per person per day.
- Work out the monthly demand = people × litres per person per day × days in the month.
- Run a month-by-month balance: start with an empty tank, add the month's supply, take out the demand, keep whatever is left up to the tank size and count the rest as overflow. Repeat for two or three years of the same normals so the start does not matter.
- Try several tank sizes and see how much of the year's demand each one meets. Stop where a bigger tank adds very little.
- Size the first flush, filter, downpipes and recharge pit for the roof area.
A monthly balance is a simplification. It assumes the month's rain arrives evenly, so it slightly flatters small tanks. The Hydro Nepal guideline suggests 10-day periods if you want a closer answer; the conclusions below do not change much.
Worked example 1: two-storey house in Kathmandu
This is the same 12-column house used in our whole-house cement and steel estimate: roof slab 7.85 m × 11.15 m = 87.53 m², RCC with a parapet, so C = 0.80. Six people live there and the water will be used for toilet flushing only: 6 × 45 = 270 litres per day.
- Yearly supply = 1,502 × 87.53 × 0.80 = 1,05,176 litres.
- Yearly demand = 270 × 365 = 98,550 litres.
On paper the roof gives more than the toilets use. The month-by-month view shows the real picture. With a 5,000 litre tank:
| Month | Supply (L) | Demand (L) | Demand met (L) | Overflow (L) |
|---|---|---|---|---|
| Jan | 1,064 | 8,370 | 1,064 | 0 |
| Feb | 1,653 | 7,560 | 1,653 | 0 |
| Mar | 2,661 | 8,370 | 2,661 | 0 |
| Apr | 4,405 | 8,100 | 4,405 | 0 |
| May | 9,019 | 8,370 | 8,370 | 0 |
| Jun | 16,904 | 8,100 | 8,100 | 4,453 |
| Jul | 26,896 | 8,370 | 8,370 | 18,526 |
| Aug | 23,997 | 8,370 | 8,370 | 15,627 |
| Sep | 14,551 | 8,100 | 8,100 | 6,451 |
| Oct | 3,032 | 8,370 | 8,032 | 0 |
| Nov | 448 | 8,100 | 448 | 0 |
| Dec | 546 | 8,370 | 546 | 0 |
| Year | 1,05,176 | 98,550 | 60,119 (61%) | 45,057 |
From May to September the toilets run fully on rainwater, and October nearly so. From November to April the roof gives only 13,809 litres against 57,240 litres of demand, and no ordinary tank fixes that. Here is what changing the tank size does:
| Tank size (L) | Share of yearly flushing demand met | Overflow to recharge (L/year) |
|---|---|---|
| 1,000 | 56.9% | 49,057 |
| 2,000 | 58.0% | 48,057 |
| 5,000 | 61.0% | 45,057 |
| 10,000 | 66.1% | 40,057 |
| 20,000 | 76.2% | 30,057 |
| 40,000 | 96.5% | 10,057 |
| 50,000 | 100% | 6,626 |
Going from 1,000 to 5,000 litres gains only about 4 points, because the extra storage just carries water from September into October. To cover the whole dry season you would need around 50,000 litres, which is a large underground sump costing far more than the water it saves. For most Kathmandu houses a 2,000 to 5,000 litre tank, plus a recharge pit for the 45,000 litres of overflow, is the sensible choice. This matches the Hydro Nepal advice of a 3,000 to 4,000 litre tank where groundwater can be recharged, as in Kathmandu.
First flush, downpipes and filter for this roof
- First flush: research quoted in the Hydro Nepal guideline puts the dirty first wash at about 0.5 mm to 2 mm of rain, which is 0.5 to 2 litres per m² of roof. For 87.53 m² that is 44 to 175 litres. Flat roofs need the higher end. A 110 mm PVC pipe holds less than 10 litres per metre, so a standpipe alone is too small for this roof; use a small drum of about 150 to 200 litres with a slow drain tap at the bottom.
- Downpipe: the same guideline gives 63 mm for about 46 to 66 m² of roof and 75 mm for up to about 128 m². One 75 mm pipe or two 63 mm pipes (one per half of the roof) suit 87.53 m².
- Filter: a mesh at the roof outlet stops leaves; a sand and gravel or cartridge filter before the tank catches fine dirt. No simple filter makes rainwater safe to drink, so boil or chlorinate if anyone will drink it.
Worked example 2: CGI roof house in Pokhara
A single-storey house near Pokhara has a 60 m² CGI roof (C = 0.85) and five people who will use rainwater for flushing: 5 × 45 = 225 litres per day.
- Yearly supply = 3,868.3 × 60 × 0.85 = 1,97,283 litres, more than double the demand of 82,125 litres.
- April to October each give 7,000 to 48,000 litres, so the toilets run fully on rainwater for seven months even with a small tank.
| Tank size (L) | Share of yearly flushing demand met |
|---|---|
| 1,000 | 69.1% |
| 2,000 | 70.3% |
| 3,000 | 71.5% |
| 6,500 | 75.8% |
| 10,000 | 80.1% |
Pokhara has more than 2.5 times Kathmandu's rain, yet the share of demand met only rises from about 60% to about 75%. The limit is the dry season, not the roof. Wetter towns need a bigger overflow and recharge arrangement, not a bigger tank. The 6,500 litre ferrocement tank that the Hydro Nepal guideline mentions as common in rural Nepal makes sense where rainwater is the main source and people ration it in winter, not as a flushing tank in a town house.
Recharge pit for the overflow
In the Kathmandu example about 45,000 litres a year overflow the tank. Sending it to the street drain adds to flooding in the lanes; sending it into the ground helps the wells and boreholes that many valley houses depend on. Guidance from India's National Water Mission gives a recharge pit for about 100 m² of roof as 1 to 2 m wide, 2 to 3 m long and 2 to 3 m deep, filled from the bottom with boulders (5 to 20 cm), then gravel (5 to 10 mm), then coarse sand (1.5 to 2 mm), with a mesh or settling chamber in front and the top sand layer cleaned from time to time.
Example for the 87.53 m² roof: pit 1.5 m × 1.5 m × 2.5 m deep = 5.63 m³ of digging (the excavation calculator gives trips and cost). With an example split of 1.2 m boulders, 0.5 m gravel, 0.3 m coarse sand and 0.5 m clear at the top, the materials are 2.70 m³ boulders, 1.13 m³ gravel and 0.68 m³ sand (about 95, 40 and 24 cft).
Keep the pit well away from the septic tank and soak pit, and do not dig it against a footing. A pit only works where the soil can take water; in black clay or where the water table is near the surface, ask an engineer first. Our septic tank guide covers the soak pit side of the plot.
Is a recharge pit compulsory?
It depends on your municipality, and the rules are changing. In June 2026 Lalitpur Metropolitan City announced that completion certificates for new buildings would be issued only after septic tanks, soak pits and ground recharge pits are in place, along with two trees and fire safety equipment (Ratopati report). Other municipalities have their own building bylaws, so check the drawing requirements with your ward office or the engineering section before the naksa pass, and show the tank and pit on the plumbing drawing.
Common mistakes on Nepali sites
- Using the yearly total to size the tank. 1,05,000 litres a year does not mean a 1,05,000 litre tank, or that a small tank is enough all year. Always look month by month.
- No first flush. The first rain after weeks of dry weather carries dust, bird droppings and leaves straight into the tank. A first-flush drum is cheap; cleaning a smelly tank is not.
- Overflow piped to the neighbour's wall or the road. Plan where the extra water goes before the plaster work, not after the first storm.
- Mixing rainwater into the drinking line. Keep harvested water on a separate line for flushing and washing unless it is treated.
- Open or poorly covered tanks. The Hydro Nepal guideline stresses a good cover and screened openings to keep out mosquitoes and dirt.
- Roof drains too small. A single 50 mm pipe on a 90 m² flat roof backs up in a heavy July storm and water stands on the slab.
- Recharge pit with fine soil on top. Silt clogs the sand layer in one season. Put a settling chamber or a mesh before the pit and clean the top layer each year.
Frequently asked questions
How much water does 1 mm of rain give from my roof?
1 litre per m² of roof before losses. A 90 m² roof gets 90 litres from 1 mm, or about 72 litres after taking C = 0.80.
Can I drink harvested rainwater?
Only after proper treatment. Roof water picks up dust and bird droppings, and most filters do not remove germs. Boil or chlorinate it, and test it if it will be the main drinking source.
Can I store rainwater in my underground sump?
Yes, if the sump is sealed, cleaned and has a first flush ahead of it. Many owners keep a separate tank so that the municipal or tanker water in the main sump stays clean. For sizing that sump, see our water tank size guide and the water tank calculator.
Does a sloping roof collect more water than a flat roof of the same plan size?
No. Rain falls vertically onto the plan area, so a sloping roof collects by its plan area. A CGI roof does lose a little less water than a flat concrete one, which is why its runoff coefficient is higher.
Where can I practise questions on water supply and hydrology?
Try the engineering MCQ practice page, and find all our calculators on the free engineering tools page.
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