Rainwater Harvesting for a Nepal House: Roof Yield & Tank Size
Estimate how much rainwater a Nepal roof can collect in a year from catchment, rainfall and runoff coefficient—then size a harvesting tank.
GharNaksa Team
Construction tracking for Nepal
Kathmandu-valley houses sit under a monsoon that can dump a metre of rain on a roof, then a dry season that empties municipal taps. Rainwater harvesting (pani sangalan) is the obvious idea. The useful calculation is not “is rain free?”—it is how many litres your roof actually yields, and how large a tank you can place without cracking a slab.
Run the rainwater harvesting calculator with catchment area, local annual rainfall and a runoff coefficient, then size a tank against dry-spell demand. Household drinking storage stays in the water tank size calculator. Roof type affects runoff: a tight CGI roof sheds more than a rough tile roof.
The yield formula
Litres per year ≈ area (m²) × rainfall (mm) × coefficient. A coefficient of 0.7–0.9 is typical: metal high, concrete medium, older tiles lower. Use plan area of the roof that actually drains to your gutters—not the whole plot, and not a 3D surface area unless you are being unusually precise.
- Valley floors and the Terai have different annual millimetres—use a local figure, not a national average.
- Most yield arrives in 3–4 months. Annual litres ÷ 12 is a bad tank-sizing method.
- Gutters, downpipes and a leaf screen are part of catchment, not decoration.
- Overflow must go somewhere safe—not into the septic tank.
Tank size vs roof yield
- 1Decide uses: flushing and cleaning first; drinking only with treatment.
- 2Estimate daily non-potable demand for those uses.
- 3Multiply by the dry days you want to bridge (often 7–21 in a city house).
- 4Cap the tank by structure and space—a full litre weighs a kilogram; roofs need design. See water tank planning.
- 5Add first flush, inlet filter, overflow, drain and a way to clean the tank.
Coordinate the terrace: waterproofing, parapet outlets, solar frames and tank pads fight for the same square metres. Price the coating with waterproofing cost and leave clear roof for solar if both are planned.
Frequently asked questions
Quick answers
How much rainwater can I collect from my roof in Nepal?
Annual yield ≈ roof plan area × annual rainfall × runoff coefficient. A 100 m² roof in a 1,400 mm rainfall belt with a 0.8 coefficient yields about 112,000 litres a year—most of it in a few monsoon months. That is why tank size is about bridging dry weeks, not storing the whole year.
What size rainwater tank do I need?
Size for the dry-spell days you want to cover (or the days between useful rains), using household non-potable demand—flushing, cleaning, garden—not necessarily drinking. A 2,000–5,000 litre tank is common on city houses; larger tanks need structure and a first-flush diverter that actually gets maintained.
Can I drink harvested rainwater?
Only with a designed treatment chain (first flush, filtration, disinfection) and a clean catchment. Most residential systems in Nepal are for non-potable use: toilets, cleaning and garden. Keep drinking water on its own tank—see the household water tank guide.
