A monsoon storm in Mumbai delivers 80-120 mm/hr peak intensity. Chennai’s North-East monsoon: 100-150 mm/hr peak. A roof drainage system designed for “average” rainfall ponds, overflows, and floods occupied space at peak. IS 15797:2008 is the Indian code for sizing rainwater drainage; it specifies design storm intensity by region and translates roof catchment area to downpipe size.
This guide walks IS 15797 from rainfall intensity through pipe diameter selection.
Step 1: Design storm intensity for your site
IS 15797:2008 Annex A provides design storm intensity (mm/hr) for one-in-five-year and one-in-ten-year return periods, by city. The 5-minute peak intensity is what drives drainage sizing.
| Region / city | 5-min peak intensity (1-in-10-year, mm/hr) |
|---|---|
| Mumbai, Konkan coast | 100-120 |
| Chennai, Coromandel coast | 100-150 |
| Kolkata, Gangetic plain | 80-100 |
| Delhi, NCR | 80-100 |
| Bangalore, Mysore | 60-80 |
| Pune, Hyderabad | 60-90 |
| Ahmedabad, Surat | 60-80 |
| Cochin, Trivandrum | 80-100 |
| Northeast (Cherrapunji, Guwahati) | 150-200 |
For commercial buildings, design to 1-in-10-year return; for critical infrastructure, 1-in-25-year. For rooftop drainage of normal commercial: 100 mm/hr is the practical Indian baseline.
Step 2: Catchment area calculation
Effective catchment for rainwater drainage:
A_eff = A_horizontal + (A_vertical × cos θ)
Where:
- A_horizontal = horizontal projected roof area
- A_vertical = adjacent vertical wall surface that drains onto the roof catchment
- θ = inclination angle (for sloping rooves, the projected horizontal is what matters)
For a flat roof with no adjacent vertical walls draining onto it: A_eff = A_horizontal. For a sloping roof: A_eff = projected horizontal area (NOT slant area).
For typical Indian commercial:
- 100 m × 50 m flat roof = 5,000 m² catchment
- With 4 m parapet wall around perimeter draining into the roof: add (100 × 4 + 50 × 4) × 2 × 0.5 = 600 m² (factor of 0.5 because rain hits the wall at angles)
- Total A_eff ≈ 5,600 m²
Step 3: Design flow rate
Q_design = (I × A_eff) / 3,600 (m³/s)
For 100 mm/hr rainfall on 5,600 m²:
Q = (100 × 5,600) / 3,600 / 1,000 = 0.156 m³/s = 156 L/s = 562 m³/h
This is the total flow that the rainwater system must remove during peak storm.
Step 4: Downpipe sizing
IS 15797 Annex C provides catchment area per downpipe diameter for a 100 mm/hr design intensity:
| Downpipe diameter | Catchment area (m²) |
|---|---|
| 75 mm (3″) | 25 |
| 100 mm (4″) | 60 |
| 125 mm (5″) | 100 |
| 150 mm (6″) | 165 |
| 200 mm (8″) | 350 |
| 250 mm (10″) | 600 |
For our 5,600 m² roof at 100 mm/hr:
- Total downpipe area required: 5,600 / (max catchment per pipe size)
- Single 250 mm pipe handles 600 m² catchment → need 5,600/600 = 9.3 → minimum 10 downpipes of 250 mm
- More commonly: 28 downpipes of 150 mm (165 m² each) → total catchment 4,620; need 32 downpipes
- Or distribute as multiple sizes: combination
For typical Indian commercial — 6 distribution points each handling 1,000 m² — use 250 mm downpipes at each.
Step 5: Horizontal carrier sizing
Downpipes connect to a horizontal carrier (or directly to the storm sewer). IS 15797 Annex E provides carrier flow capacity at standard slope (1:100):
| Carrier diameter | Flow capacity at 1:100 (L/s) | Equivalent catchment at 100 mm/hr (m²) |
|---|---|---|
| 100 mm | 8 | 290 |
| 150 mm | 25 | 900 |
| 200 mm | 50 | 1,800 |
| 250 mm | 90 | 3,240 |
| 300 mm | 145 | 5,220 |
| 400 mm | 280 | 10,080 |
For 5,600 m² catchment: 300 mm carrier handles 5,220 m² → at our 5,600 m² we need either:
- One 400 mm carrier (handles all)
- Or two 300 mm carriers (one collecting half the roof, one the other half)
- Or three 250 mm carriers
Step 6: Slope and clean-out
IS 15797 minimum slope for horizontal carriers: 1:100 (10 mm/m). Maximum slope: 1:25 to prevent excessive velocity that causes turbulence and noise.
Clean-out access: every 30 m of horizontal run, plus at every change of direction > 45°. Clean-out cap or rodding eye, located accessibly.
Worked example: 25-storey commercial tower, 80 m × 60 m typical floor
Roof: 80 × 60 = 4,800 m² (conservative — assume 4,800 m² with no adjacent walls draining)
Rainfall design: 100 mm/hr (Mumbai location, 1-in-10-year)
Total design flow: (100 × 4,800) / 3,600 = 133 L/s
Downpipe distribution: 8 downpipes around perimeter, each draining 600 m² → 8 × 250 mm downpipes
Horizontal carrier: 4 carriers (each handling 1,200 m² = 33 L/s) → 4 × 200 mm at 1:100 slope, all discharging to a central rainwater tank or storm sewer
Slope: 1:100 to a central junction box; junction box pipe ≥ 300 mm to handle the combined flow.
Clean-out at every 30 m of horizontal run plus at the junction box.
Roof drain at each downpipe: dome strainer, anti-vortex design (for high-flow), with double-grating to prevent ingress of leaves/debris.
Five common rainwater design mistakes
1. Using average annual rainfall instead of peak 5-min intensity. Average is 5-10 mm/hr in many regions; peak 5-min is 100+ mm/hr. The drainage must handle peak.
2. Forgetting parapet wall contribution. Adjacent walls drain onto roof; if not accounted, roof ponds.
3. Combining rainwater drain with sewage. Storm water and sewage must remain separated (NBC 2016 Pt 9). Combined sewer = storm overflow contaminates ground.
4. Slope too aggressive (1:25 or steeper). Velocity too high = noise + erosion. 1:50 to 1:100 ideal.
5. No anti-vortex roof drain. A dome strainer collapses under heavy rainfall — anti-vortex design prevents this.
Quick checklist
- [ ] Design storm intensity (1-in-10-year, 5-min peak) confirmed for site
- [ ] Roof catchment area calculated (horizontal projected + adjacent vertical wall contribution)
- [ ] Total design flow = I × A_eff / 3,600
- [ ] Downpipes distributed so each ≤ table catchment for size
- [ ] Horizontal carrier slope 1:100 to 1:50
- [ ] Carrier diameter from flow table at design slope
- [ ] Clean-out every 30 m + at every direction change
- [ ] Anti-vortex roof drains; dome strainer with double grating
- [ ] Discharge to storm sewer or rainwater harvesting tank (separated from sewage)
References: IS 15797:2008 Indian Code for Roof Drainage; NBC 2016 Pt 9 §3; IMD Indian Storm Intensity Maps; CPHEEO Manual on Sewerage and Sewage Treatment.
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