A roof-level tank is often used to supply domestic water for kitchen and bathroom faucets, showers, toilets, dishwashers, and washing machines. The tank also typically supplies water when needed to the building’s closed-loop heating system. (To prevent the possibility of cross-contamination of the potable water supply, a backflow prevention device is usually installed at the domestic connection to the heating system.)

Additionally, many domestic water roof tanks supply the building’s fire standpipe system. Any residential building taller than six stories or 75 feet in height is required by the New York City Building Code to have a fire standpipe system with a minimum 3,500-gallon water reserve. Fire standpipe tanks are usually installed on the roof, either standing alone or alongside the domestic water tank.

At the time your building’s roof tank was removed, some accommodation should have been made to ensure a standing supply of water to the fire standpipe system if one was not already in place.

Galvanized steel pipes tend to corrode on the inside, building up rust and scale.

In a typical roof-tank system, the city’s water main enters the building and is connected to a roof tank “fill” pump in the basement. The fill pump sends water up through a supply riser to one or more wood roof tanks, usually between 5,000- and 10,000-gallon capacity. The fill pump does not operate continually; it engages only when a float gauge in a roof tank signals that water has dropped below a certain level and the tank needs to be replenished.

Wooden roof tanks operate as gravity-based systems. Water travels down from the tank to main pipes called overheads, which run horizontally either above the ceiling on the top floor of the building or across the ceiling in the cellar. Supply risers carry water from the overheads to each floor, where they branch off to feed fixtures in individual apartments.

Some buildings were designed with multiple pressure zones: with overheads located at intermediate floor levels, or in the case of a two-zone system, at both the cellar and the top-floor levels. In a two-zone setup, down-feed risers from overheads on the top floor supply water to the upper zone, i.e., the upper floor apartments. Similarly, up-feed risers from the cellar overhead, pressurized from the city main, supply the lower zone, or lower floor apartments.

Removing the roof tank may have very well caused a drop in available water pressure within your building, especially on the top floors. A common misconception is that the closer an apartment is to a rooftop tank, the better the water pressure in that apartment will be. In fact, the reverse is true. In a gravity system, pressure increases as water travels in a down-feed riser, and decreases as it flows in an up-feed riser. This is why the upper floors in a building tend to suffer from lower water pressure more so than the bottom floors.

Rust and scale cause discolored water and reduce the opening inside the pipes, decreasing water pressure.

Water at city main pressure can typically supply adequate pressure for the lower floor apartments. (Most plumbing fixtures require at least 15 to 25 psi to function properly.) When your building’s roof tank was removed, a water pressure booster pump was probably installed in the cellar to increase pressure to the upper floors. Without a booster pump, the pressure on the higher floors would be even less than it is now.

Reinstalling your building’s roof tank may help restore the lost pressure on the upper floors. The old fill pump and supply riser to the roof tank were probably removed with the old tank, so new ones may need to be installed with the new tank. Even if the old pump and riser were abandoned in place, it’s doubtful they would be reusable for the new system. Furthermore, even with a new tank in place, one or two small booster pumps may be necessary to increase water pressure on the upper floors.