---
title: "Fire safety features; first flush in RWH"
subject: "Building Technology"
chapter: 12
key: "semester_6/building_technology:first-flush-fire-safety"
source: https://www.bctengineer.com/q/semester_6/building_technology/first-flush-fire-safety
site: BCT Engineering
---

# Fire safety features; first flush in RWH

Building Technology · Chapter 12: Other services in building

- Asked in: 2079 Chaitra, 5 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/first-flush-fire-safety
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-12

## Question

What are the features required for a building to have, for safety against fire? What is first flush and how are they used in rainwater harvesting?

## Short answer

Buildings need fire-resisting materials, compartmentation, escape routes, detection/alarm systems, extinguishers/risers/sprinklers, smoke control, and proper setbacks. The first flush is the initial roof wash that carries contaminants; a diverter (e.g., bal-float chamber) discards it and directs clean rainwater to the storage tank.

## Answer

#### Features of a building for fire safety (NBC 107, Provisional Recommendation on Fire Safety)
1. **Fire-resisting materials and structure:** structural members (columns, beams, floors, walls) of non-combustible materials with the specified fire resistance rating (1-4 h), e.g. RCC with sufficient cover.
2. **Compartmentation:** division of the building by fire-resisting walls and floors to limit spread; fire doors on openings.
3. **Means of escape:** enough exits, protected corridors and stairs (fire-resistant, smoke-free), travel distance within limits, exit signs and emergency lighting, and at least two exits for multistorey buildings.
4. **Fire separation and setbacks:** open spaces around the building for fire vehicle access and to avoid spread to neighbours.
5. **Fire detection and alarm:** smoke or heat detectors, manual call points and alarms.
6. **Fire-fighting installations:** extinguishers, hose reels, wet or dry risers, hydrants, sprinklers, and an underground/overhead static fire tank.
7. **Ventilation and smoke control:** openings or exhausts to remove smoke; avoid vertical shafts that spread fire.
8. **Safe electrical wiring and storage of flammables**, and a refuge area or fire lift in tall buildings.
9. **Access for fire engines:** road width, turning space and hydrant connections.

#### First flush
The **first flush** is the first portion of rainfall after a dry spell, which washes the roof and carries dust, bird droppings, leaves and pollutants. A **first-flush diverter** discards this water.

**How it is used:** a vertical pipe (about 100 mm) with a ball float valve, or a diverter chamber, is fitted on the down pipe before the filter. The first 10-15 minutes of rain (about 0.5-1 L per m^2 of roof) fills it; once it is full, the float closes the diversion and clean water flows on to the filter and tank. The collected dirty water drains out through a small slow-release hole. This keeps the stored water cleaner and reduces filter clogging.

```text
   from roof
      |
      +---> to filter/tank (after chamber full)
      |
   [ chamber + ball float ]
      |
   small drain (dirty water out)
```

## Check yourself

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

1. What is required of structural members (columns, beams, floors, walls) for fire safety?
   A. Combustible materials to burn out quickly
   B. Non-combustible materials with a specified fire resistance rating (1-4 h), such as RCC with sufficient cover
   C. Only decorative finishes
   D. Timber frames with thin cover
   Answer: B. Structural members should be non-combustible with a fire resistance rating, e.g. RCC with adequate cover. (Key point: Fire-resisting structure)

2. What is the purpose of compartmentation in fire safety?
   A. To limit the spread of fire using fire-resisting walls, floors and fire doors
   B. To allow smoke to move freely between floors
   C. To reduce the number of exits
   D. To increase the travel distance to exits
   Answer: A. Dividing the building with fire-resisting walls and floors, with fire doors on openings, keeps the fire from spreading. (Key point: Compartmentation and escape)

3. What is the first flush in rainwater harvesting?
   A. The last rain of the season
   B. The first portion of rain after a dry spell that washes the roof and carries dust, droppings and leaves
   C. Water overflowing from the tank
   D. Water leaving the filter after treatment
   Answer: B. The first flush is the first rain after a dry period which carries roof pollutants; it is discarded by the diverter. (Key point: First flush definition)

4. How does a first-flush diverter work?
   A. It chlorinates the incoming water
   B. A chamber with a ball float fills with the first rain; once full the float closes it and clean water flows on to the filter and tank
   C. It sends all water directly to the tank
   D. It opens only after the tank is full
   Answer: B. The first 10-15 minutes of rain fill the chamber; the float then closes the diversion, and the trapped dirty water drains slowly through a small hole. (Key point: Working of first-flush diverter)

## Related in Building Technology

- [Short note: Rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arainwater-harvesting-note) — Building Technology, chapter 12, asked 5×
- [Rainwater harvesting and its treatment](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-treatment) — Building Technology, chapter 12, asked 3×
- [Benefits of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-benefits) — Building Technology, chapter 12
- [Components of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-components) — Building Technology, chapter 12, asked 2×
- [Types of rainwater harvesting](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arwh-types) — Building Technology, chapter 12
- [Short note: Fire protection in building](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ashort-fire-protection) — Building Technology, chapter 12, asked 2×

## Related in other subjects

- [Short note: Fire regulations and site safety](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Afire-regulations-safety-site) — Construction Management, chapter 11
- [Short note: Safety against piping and uplift](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Apiping-uplift-safety-note) — Irrigation and Drainage Engineering, chapter 5
- [Features of end-fire array](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aendfire-array-features) — Propagation and Antenna, chapter 2
- [Cable selection against fire, halogen-free cable](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-selection-fire-halogen-free) — Industrial Power Distribution and Illumination, chapter 3