OHSR Water Tank BOQ — Worked Example for a 25 kL JJM Village Reservoir
The OHSR (Overhead Service Reservoir) is the universal water-supply structure in Indian villages and urban housing colonies. Every Jal Jeevan Mission (JJM) village scheme builds one. Every government housing project includes one. The IS 3370 + IS 11682 + CPHEEO Manual standard configuration: cylindrical RCC tank with conical or flat bottom, dome roof, raised on 4-6 RCC staging columns founded on a raft slab. This article walks through a complete BOQ for the most common JJM size — 25 kL on 12 m staging — item by item.
Project Scenario
You're estimating an OHSR for a JJM village scheme serving ~350 households (1,750 persons at 5 per HH). At 135 lpcd × 8-hour gravity storage per CPHEEO Manual, you need ~25 kL. Standard 12 m staging height delivers ~1.2 bar gravity pressure at the tail-end of the distribution network.
Design specification at a glance
- Capacity: 25 kL (25,000 litres)
- Staging height: 12 m (ground to tank-bottom)
- Staging columns: 4 nos, 450 × 450 mm RCC M20
- Tank diameter: ≈ 3.43 m (derived from 25 kL / 2.7 m liquid depth)
- Tank wall: 200 mm thick RCC M20
- Tank base + dome roof: 200 mm + 125 mm RCC M20
- Internal waterproofing: per IS 3370 (cement-based crystalline)
- Steel ratio: 130 kg/m³ (water-retaining structure)
The Complete BOQ — 15 Items in CPWD DSR 2023
An OHSR has many more line items than a wall or drain because it has multiple distinct RCC components, each with its own shuttering line. The dome roof is particularly distinctive — DSR has a dedicated rate (5.5) for it.
| # | DSR Code | Item Description | Unit | Quantity |
|---|---|---|---|---|
| 1 | 2.8.1 | Earthwork in foundation pit, all-kinds-of-soil | m³ | 43.78 |
| 2 | 4.1.8 | PCC 1:4:8 below raft, 100 mm thick | m³ | 2.58 |
| 3 | 5.1.2 | RCC M20 in raft foundation, 500 mm thick | m³ | 12.15 |
| 4 | 5.9.1 | Centering & shuttering for raft (under-side) | m² | 24.30 |
| 5 | 5.2.2 | RCC M20 in 4 staging columns (450 × 450 × 12 m) | m³ | 9.72 |
| 6 | 5.9.6 | Centering & shuttering for square columns | m² | 86.40 |
| 7 | 5.3 | RCC M20 in tank base slab, 200 mm thick | m³ | 2.30 |
| 8 | 5.9.3 | Centering & shuttering for tank base (suspended slab) | m² | 11.52 |
| 9 | 5.2.2 | RCC M20 in cylindrical tank walls (200 mm × 3 m height) | m³ | 6.46 |
| 10 | 5.9.2 | Centering & shuttering for cylindrical walls (× 1.20 for curve) | m² | 77.56 |
| 11 | 5.5 | RCC M20 in dome roof, 125 mm thick | m³ | 1.59 |
| 12 | 5.9.9 | Centering & shuttering for dome roof | m² | 12.67 |
| 13 | 5.22.6 | TMT Fe-500D reinforcement (cut, bent, placed) | kg | 4,188.60 |
| 14 | 22.5 | Internal cement-based crystalline waterproofing per IS 3370 | m² | 41.56 |
| 15 | 19.6.2 | GI outlet pipe with isolation valve (staging height + tank height + 2 m) | m | 17.00 |
Total RCC volume = 32.22 m³ across 5 distinct components (raft + columns + tank base + tank walls + dome roof). At 130 kg/m³ that's ~4.2 tonnes of steel — significantly higher per-cubic-metre ratio than non-water RCC because IS 3370 demands 0.24 % minimum surface steel + crack-control bars on every face.
How Each Quantity Was Computed
Tank diameter (derived from capacity)
For a cylindrical tank of capacity V with liquid depth h, the diameter is D = 2√(V/πh). The BOQ Builder picks liquid depth from the capacity bracket — for 25 kL, h = 2.7 m:
D = 2 × √(25 / (π × 2.7)) = 2 × √(2.948) = 3.43 m
Tank total height = liquid depth + freeboard = 2.7 + 0.30 = 3.0 m
1 + 2. Excavation + PCC bed
Raft side = D + 1.5 m projection = 3.43 + 1.5 = 4.93 m (square)
Excavation pit = (raft side + 0.30) × (raft side + 0.30) × 1.60 m depth
= 5.23² × 1.60 = 43.78 m³
PCC width = raft side + 0.15 = 5.08 m
PCC volume = 5.08² × 0.10 = 2.58 m³
3 + 4. RCC raft foundation
500 mm thick raft, sized 4.93 × 4.93 m to spread column loads onto the soil:
Raft volume = 4.93² × 0.50 = 12.15 m³
Shutter area (under-side only) = 4.93² = 24.30 m²
5 + 6. Staging columns
4 columns at 450 × 450 mm × full 12 m staging height:
Col volume each = 0.450 × 0.450 × 12 = 2.43 m³
Total column volume = 4 × 2.43 = 9.72 m³
Shutter area = 4 cols × 4 faces × 0.45 × 12 = 86.4 m²
7 + 8. Tank base slab
Circular slab at top of staging, sized to tank diameter + 200 mm wall thickness on each side. This is a SUSPENDED slab between the four columns, so it needs full-area under-side shuttering:
Tank base radius = D/2 + wall_thk = 1.715 + 0.20 = 1.915 m
Tank base volume = π × 1.915² × 0.20 = 2.30 m³
Tank base shutter = π × 1.915² = 11.52 m²
9 + 10. Cylindrical tank walls
Circumference × wall thickness × height:
Wall volume = π × D × wall_thk × tank_height
= π × 3.43 × 0.20 × 3.00 = 6.46 m³
Shutter area = 2 × π × D × tank_height × 1.20 (× 1.20 for cylindrical-shuttering premium per DSR 5.9.14)
= 2 × π × 3.43 × 3.00 × 1.20 = 77.56 m²
11 + 12. Dome roof
125 mm thick spherical-cap RCC, sized to overall tank radius + 10 % overhang:
Dome volume = π × 1.915² × 0.125 × 1.10 = 1.59 m³
Dome shutter = π × 1.915² × 1.10 = 12.67 m²
13. Reinforcement
130 kg per m³ — typical for water-retaining structures per IS 3370 (1.6-1.8 % by volume, vs 1.2 % for normal slabs):
Total RCC = 12.15 + 9.72 + 2.30 + 6.46 + 1.59 = 32.22 m³
Steel = 32.22 × 130 = 4,188.60 kg (~4.2 tonnes)
14. Internal waterproofing
Cylinder wall surface + tank base — per IS 3370:
Wall area = π × D × tank_height = π × 3.43 × 3.00 = 32.32 m²
Base area = π × (D/2)² = π × 1.715² = 9.24 m²
Total = 41.56 m²
15. Outlet pipe
Runs from tank bottom down through staging to the distribution network at ground level + 2 m extra for distribution-tee fitting:
Outlet pipe length = 12 + 3.0 + 2 = 17 m
What This BOQ Excludes
- Stairs / cat-ladder to tank top — typically ₹15-25k MS angle iron
- RCC column ties / horizontal bracing between staging columns every 3 m vertical (~10 % of column volume)
- Roof-top access manhole + ventilator
- Level indicator + electrode-based water-level sensor
- Ladder + safety cage for tank-top access
- Lightning protection rod (LPS)
- Inlet pipe + sluice valve + non-return valve + bypass arrangement
- Chlorination dosing chamber (typically at WTP, not at OHSR)
- External painting / weather protective finish
- Submersible pump in the source borewell (separate JJM line item)
Sizing Notes for Different Capacities
| Capacity | Population served | Tank Ø (m) | Liquid depth (m) | Wall thk (mm) | Suggested staging |
|---|---|---|---|---|---|
| 5 kL | ~70 HH | 1.78 | 2.0 | 150 | 4 cols 300 × 300 |
| 10 kL | ~140 HH | 2.52 | 2.0 | 150 | 4 cols 380 × 380 |
| 25 kL | ~350 HH (JJM default) | 3.43 | 2.7 | 200 | 4 cols 450 × 450 |
| 50 kL | ~700 HH | 4.85 | 2.7 | 200 | 6 cols 450 × 450 |
| 100 kL | ~1400 HH | 6.02 | 3.5 | 250 | 6-8 cols 600 × 600 |
Common Estimation Mistakes
- Using 100 kg/m³ steel as "RCC standard" — water-retaining structures need 130-160 kg/m³ for crack control (IS 3370 minimum 0.24 % each face). Under-steeling causes leakage and 4-6× cost in retrofitting.
- Forgetting the 1.20× cylindrical shuttering premium — straight wall shuttering is cheaper than curved. DSR 5.9.14 explicitly adds 20 % for cylindrical work. Missing this under-estimates by ~₹15-20k on a 25 kL tank.
- Skipping internal waterproofing — "the M20 concrete will be dense enough" is wrong. IS 3370 mandates a waterproofing membrane regardless of concrete grade. Tank life without it: 5-8 years vs 30+ with.
- Treating the dome roof as a flat slab — wrong DSR code (use 5.5 not 5.3), wrong shutter rate, wrong steel pattern (radial + circumferential, not orthogonal). The cost difference is ~25 % per m³.
- Under-sizing the raft — column load × 4 = 4 × (12 m × column weight + tank dead + water + dome) ≈ 70-90 tonnes total at the raft level on poor soil. The raft must be at least 1.5 m larger than tank diameter on each side. Smaller rafts crack within the first year.
- Forgetting bracing between staging columns — every 3 m vertical interval needs a horizontal RCC tie beam to prevent column buckling under seismic / wind loads. This is ~10 % of column volume and routinely omitted.
What Changes for a Different Tank
- Smaller tank (5-10 kL) — 4 columns 300-380 mm, 150 mm walls, lighter raft (300 mm). Steel drops to ~100 kg/m³ for the lighter structure.
- Larger tank (50-100 kL) — 6-8 staging columns, 250 mm walls, possibly seismic-resistant base isolators in Zone IV/V.
- Taller staging (15-18 m) — pressure head increases (good for serving higher elevations), but seismic + wind loads on the column scale with height². Heavier columns (600 × 600 mm) and full bracing system required.
- Below-water-table foundation — add dewatering during construction + waterproof admixture in raft.
- State-PWD SOR — switch from CPWD DSR to your state's water-supply SOR via the Builder. PHED state SORs are sometimes more current for JJM-specific work.
Get Your OHSR Cost in 30 Seconds
Change capacity (5-100 kL), staging height, column count + section, wall thickness, waterproofing — the Builder regenerates the BOQ + downloads Excel with formulas embedded. Customize for Your Project →
References & Companion Reading
- IS 3370 Part 1:2009 — Concrete (Reinforced) for Storage of Liquids — General Requirements
- IS 3370 Part 2:2009 — Reinforced Concrete Structures (design + crack control)
- IS 11682:1985 — Criteria for Design of RCC Staging for Overhead Water Tanks
- IS 456:2000 — Plain and Reinforced Concrete (concrete grades, cover, mix proportions)
- IS 1786:2008 — TMT Fe-500D reinforcement
- CPHEEO Manual on Water Supply — Chapter 1 (sizing methodology, lpcd standards)
- Sister tool: UGSR (Underground) Water Tank BOQ Builder — for ground-level reservoirs
- CPWD DSR 2023 — full SOR with rates by item