Overhead Water Tank (OHT)

An Overhead Water Tank (OHT) is a water storage tank elevated above ground on a staging (column-and-beam structure or shear-wall tower) to provide gravity head for distribution. The elevation creates pressure at the supplying fixtures sufficient for normal use without pumping. The Indian Standards: IS 11682 (RCC) and IS 805 (steel) for elevated tanks; IS 3370 Part 1, 2, 3 for water-retaining concrete; IS 1893 Part 2 for seismic design of liquid-retaining structures.

Design capacity per CPHEEO: residential building tank holds 1-day demand at design LPCD (typically 200 LPCD per person for water-supply-deprived areas, 135 LPCD for normal areas). For an 8-person family: 200 × 8 = 1600 L/day, with 2-day storage = 3200 L. Typical capacities: 500-1000 L for very small buildings; 2000-5000 L for residential; 10,000-50,000 L for commercial; 100,000+ L for industrial / municipal. The staging height must provide 1.5-2.0 m static head from tank bottom to the most-distant fixture. Standard staging heights: 4-8 m for residential; 12-25 m for commercial mid-rise.

Materials: RCC tanks (IS 3370) are most common in India for tanks above 5,000 L — durable, fire-resistant, low maintenance. Steel pressed tanks (IS 805) are common for prefabricated commercial tanks. Polyethylene (HDPE) tanks are common for smaller residential applications — Sintex, Reliance Sintex, Tata Tiscon, Plasto. RCC tanks per IS 3370 use M25-M30 concrete with strict crack-width control (≤ 0.2 mm per Cl. 4.4). The most-violated rule: many Indian residential RCC tanks are water-tight only after extensive plaster + waterproofing repair due to early-age cracking; proper IS 3370 Part 2 design (Working Stress Method with crack-control) prevents this. Site engineers should refuse RCC tank designs that don't include explicit crack-width verification.