| Primary value | 80 sqm per 100 mm pipe (75 mm pipe → 50 sqm · 150 mm pipe → 175 sqm) |
| Applies to | Terrace rainwater drainage in residential and commercial buildings · Sloping roof rainwater drainage (pitched roof gutters) · Sites in Indian rainfall belt (50–150 mm/h design intensity) |
| Exceptions | 75 mm dia pipe → ≤ 50 sqm catchment |
| 100 mm dia pipe → ≤ 80 sqm catchment | |
| 150 mm dia pipe → ≤ 175 sqm catchment | |
| Coastal / heavy rainfall (Mumbai, Mangalore) → Reduce by 25% | |
| Design rainfall intensity → 50 mm/h (most India) · 100 mm/h (heavy belts) | |
| Measured as | Maximum roof / terrace area in square metres that can drain through one downpipe of given diameter, assuming standard Indian rainfall intensity and 100% runoff coefficient (paved terrace). Increase pipe count if catchment exceeds the limit. |
| Source | NBC 2016 — Part 9, Section 2, Cl. 5.5 ✓ Verified |
16 related items across IS codes, knowledge articles, design rules, maps and tools
Undersized rainwater pipes are the #1 cause of terrace flooding in monsoon — water backs up at the parapet, finds the slab joint and seeps into the floor below. The 80 sqm rule per 100 mm pipe is the conservative IS / NBC number that handles typical Indian rainfall intensity with one pipe per ~9 m × 9 m grid.
Standard terrace layout in apartments uses four 100 mm corner downpipes for a slab up to ~24 × 13 m (≈ 320 sqm = 80 × 4). Larger terraces add intermediate pipes; coastal projects and heavy-rain belts upsize to 150 mm pipes throughout.