How we supply it: The rainwater harvesting structure is built by a partner plumber or contractor who quotes and bills the work and pays us a referral commission. The filter media are ours: 40 mm and 20 mm jalli and M-Sand from our crusher, best added to a load you are already ordering since our minimum is one tipper load.
In one line: Rainwater harvesting is collecting rain from roofs and paved areas and either storing it for use or letting it soak into the ground through recharge pits and wells to raise the groundwater table.
What the Tamil Nadu rule means for your house
Rainwater harvesting is collecting rain from roofs and paved areas and either storing it for use or letting it soak into the ground through recharge pits and wells to raise the groundwater table.
Tamil Nadu made rainwater harvesting structures compulsory for buildings in 2003, and approval authorities expect them on building plans. Check your local body's current requirement when you apply.
Methods
| Method | Suits | Note |
|---|---|---|
| Recharge pit (percolation pit) | Sandy or mixed soil | Shallow, layered stone and sand |
| Recharge well | Clayey soil, small plots | Deeper, perforated rings with filter media |
| Recharge trench | Long compound edges, driveways | Takes runoff from paved areas |
| Storage in a sump | Clean roofs, separate tank | Needs a filter and first-flush diversion |
Recharge or storage?
Recharge needs no pump and little upkeep beyond cleaning the sand layer. Storage gives usable water but needs a clean roof, a filter, a first-flush diverter and a separate tank.
On clayey soil a shallow pit drains slowly, so a deeper recharge well works better. On sandy soil a pit is usually enough.
How much rain your roof collects
Typical estimate: roof area × annual rainfall × about 0.8 runoff for a concrete roof. Chennai's average annual rainfall is roughly 1,200–1,400 mm, most of it in October to December.
That fill is far below our one-load minimum of about 18 MT. Add it to a load you are already ordering for the house.
- 1,000 sq ft roof (92.9 m²): about 90,000–1,00,000 litres a year
- One 100 mm rainy day on that roof: about 7,400 litres
- A 1.2 × 1.2 × 1.5 m recharge pit holds about 2.2 m³ of fill, roughly 3.4 MT of jalli and sand at our bulk densities
Building a recharge pit
- Bring all roof downpipes into one line with a cleanout.
- Fit a filter chamber or first-flush arrangement so the dirtiest first rain and leaves go to waste.
- Dig the pit at least about 1 m away from foundations, and well away from the septic tank and soak pit.
- Fill in layers: 40 mm jalli at the bottom, 20 mm in the middle, coarse sand on top, under a removable slab cover.
- Lead an overflow to the street drain.
- Clean the top sand layer and the filter before every north-east monsoon.
Checks after building
- Pour a few buckets into the finished pit; the water should soak away, not stand for hours.
- Walk round the house and confirm every downpipe reaches the system.
- Check the cover is secure and safe for children.
- Check the mesh or filter at the inlet.
- Look at the pit after the first heavy rain for overflow and silt.
Upkeep
- Before the monsoon, scrape off and replace the top few centimetres of sand.
- Clear roof outlets and gutters of leaves.
- Keep oil, detergent and sewage out of the system.
- If the pit drains slowly after some years, lift and wash the jalli layers.
Mistakes that make pits useless
- A pit dug right against the foundation.
- Fine sand or soil on top instead of coarse sand, which clogs.
- No overflow, so water backs up onto the terrace.
- Bathroom or kitchen waste connected to the recharge pit.
- A pit built for plan approval and never cleaned again.