Question

Cement concrete roofs have a run-off coefficient of:

• A. \(0.60 - 0.70 \)
• B. \(0.80 - 0.95 \)
• C. \(0.50 - 0.75 \)
• D. \(0.70 - 0.85 \)

Hint:

Impermeable surfaces like concrete roofs, asphalt roads, and metal sheeting shed nearly all water, giving them a very high run-off coefficient ($C \approx 0.90$).

Answer 1

The Correct Option is B

Concept: The run-off coefficient ($C$) is a dimensionless parameter used in hydrology and drainage design (such as the Rational Method formula, $Q = C \cdot I \cdot A$). It represents the ratio of the volume of water that runs off a surface to the total volume of precipitation falling on it. A higher coefficient indicates a less porous surface, meaning more water runs off and less is absorbed.

Step 1: Analyzing the surface properties of concrete roofs.
Flat or sloped roofs made of reinforced cement concrete (RCC) are highly impermeable:
• Concrete is a dense, hard masonry material designed to shed water quickly to prevent leaks and structural damage.
• Because a finished concrete roof absorbs minimal water and has no soil or vegetation to retain moisture, nearly all rainfall immediately converts into surface runoff.
• Consequently, smooth, watertight surfaces like concrete or asphalt roofs have high run-off coefficients, typically ranging between 0.80 and 0.95. This means that 80% to 95% of all rain falling on the roof directly enters the stormwater drainage system.

Step 2: Comparing with more permeable surface coefficients.
Lower run-off coefficients correspond to softer, more absorbent surfaces:
• Unpaved soils and lawns: Range from 0.10 to 0.35 because the earth absorbs a large portion of the rainfall through infiltration.
• Gravel walkways: Range from 0.50 to 0.70 due to temporary water storage in the gaps between the stones. Since a solid concrete roof is highly impermeable, it falls into the highest category: 0.80 – 0.95.