---
title: "Components of rainwater harvesting"
subject: "Building Technology"
chapter: 12
key: "semester_6/building_technology:rwh-components"
source: https://www.bctengineer.com/q/semester_6/building_technology/rwh-components
site: BCT Engineering
---

# Components of rainwater harvesting

Building Technology · Chapter 12: Other services in building

- Asked in: 2077 Chaitra, 3 marks; 2069 Bhadra, 8 marks (asked 2 times)
- Video: [4. RRWH system and its components _ WIN Foundation](https://www.youtube.com/watch?v=G5RekUT_Y88) — WIN Foundation
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/rwh-components
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12

## Question

Illustrate (discuss) the components of the rooftop rain water harvesting system.

## Short answer

A rooftop rainwater harvesting system includes a catchment roof, gutters with mesh, down pipes, a first-flush diverter, a gravel/sand/charcoal filter, a storage tank with overflow, and optional disinfection. Sizing: annual harvestable volume = rainfall × area × runoff coefficient (0.8-0.9).

## Answer

**Rainwater harvesting (RWH)** is the collection and storage of rain falling on roofs or the ground for later use, or for recharging groundwater, instead of letting it run off.

#### Components of rooftop RWH
1. **Catchment:** the roof (RCC, tiles, GI sheet) that receives the rain; it should be a clean, non-toxic surface.
2. **Gutters:** half-round channels at the roof edge (GI, PVC, aluminium) with a slope of about 1 in 100, leading water to the down pipe; fitted with mesh to stop leaves.
3. **Down-take (conveyance) pipes:** PVC pipes (75-100 mm) taking water from the gutters to storage.
4. **First-flush device:** diverts the first dirty rain (first 10-15 minutes, or about 0.5-1 mm of rain), which carries dust, bird droppings and leaves, to waste.
5. **Filter unit:** layers of coarse sand, gravel and charcoal (or a mesh/sand filter) to remove suspended matter.
6. **Storage tank:** underground or overhead tank (RCC, ferrocement, plastic) sized for demand and rainfall, with inlet, overflow, outlet, vent and provision for cleaning.
7. **Recharge structure (optional):** recharge pit, trench or well for the overflow.
8. **Disinfection / delivery:** chlorination or UV before drinking use; pump and distribution.

```text
   \   roof (catchment)   /
  ===+====================+===
     |gutter              |
     v                    |
   down pipe              |
     |                    |
 [first flush]--> waste   |
     |
 [ filter: gravel/sand/charcoal ]
     |
 [ storage tank ]--overflow--> recharge pit
     |
   pump -> use
```

**Sizing:** annual harvestable volume = annual rainfall (m) x catchment area (m^2) x runoff coefficient (about 0.8-0.9 for roofs). Example: a 100 m^2 roof with 1.5 m of rain and a coefficient of 0.85 gives 127.5 m^3 per year. The tank itself is sized from the daily demand and the length of the dry period.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The catchment of rooftop RWH is:
   A. The roof surface receiving rain
   B. The storage tank
   C. The recharge pit
   D. The filter
   Answer: A. It should be clean and non-toxic. (Key point: Catchment)

2. Down-take pipes are typically:
   A. Steel of 500 mm
   B. PVC of 75-100 mm
   C. Clay of 1 m
   D. Rubber of 10 mm
   Answer: B. They convey water from gutters to storage. (Key point: Down pipes)

3. Which is a storage tank feature?
   A. Completely sealed with no outlet
   B. No overflow
   C. Inlet, overflow, outlet, vent and cleaning provision
   D. Only an inlet
   Answer: C. Sized for demand and rainfall. (Key point: Storage tank)

4. 100 m^2 roof, 1.5 m rain, coefficient 0.85 gives:
   A. 150 m^3
   B. 85 m^3
   C. 12.75 m^3
   D. 127.5 m^3 per year
   Answer: D. 100 x 1.5 x 0.85 = 127.5. (Key point: Sizing example)

## Related in Building Technology

- [Rainwater harvesting and its treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-treatment) — Building Technology, chapter 12, asked 3×
- [Types of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-types) — Building Technology, chapter 12
- [Short note: Rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arainwater-harvesting-note) — Building Technology, chapter 12, asked 5×
- [Benefits of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-benefits) — Building Technology, chapter 12
- [RWH: reasons and basic components](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-reasons-components) — Building Technology, chapter 12
- [Short note: Water supply and distribution system](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-distribution-short) — Building Technology, chapter 12, asked 2×

## Related in other subjects

- [Typical water supply system and components](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atypical-ws-system-components) — Water Supply Engineering, chapter 1, asked 4×
- [Objectives of water supply and components](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aobjectives-ws-rural-diagram) — Water Supply Engineering, chapter 1, asked 3×
- [Water cooling systems and components](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Awater-cooling-systems-components) — Internal Combustion Engines, chapter 7
- [Components of canal irrigation system](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-components-sketch) — Irrigation and Drainage Engineering, chapter 3, asked 7×