Water Tanks on Roofs?

Zman

I have always noticed water tanks on the roofs of buildings in Boston and NYC. What purpose do they serve?

mattmia2

the pressure to get the water to the top of the building is more than the main pressure so they pump it to a tank on the roof and feed the building from that.

it can also be done with booster pumps.

Jamie Hall

They may also serve fire protection. Either way, you don't have to size your booster pump and service pipes for the peak load — just big enough to pump the average water use per day up there, with some reserve.

pecmsg

city water pressure won’t lift that high. Pumps are used to lift it up then gravity delivers to the building.

Jamie Hall

I might add that some very tall buildings may have tanks on intermediate floors as well. Others will use pressure reducing valves.

Mad Dog_2

For the last Century and change, they've been assembled on the roof with wood slats like a barrell which are held in place with heavy duty straps. They leaked for a day or two until the wood joints swelled up. They have a large ball Cock much like a toilet tank.

More recently they use Stainless steel. In NYC the two companies that have been doing them forever are Rosenwach and Isseks. Mad Dog

Mad Dog_2

Average NYC Street water main pressure will usually be good up to 4 stories max, thus roof tanks

Steamhead

IIRC many of those tanks had steam coils to keep them from freezing. These ran off the steam boiler that heated the building. Makes things tricky when sizing a replacement boiler………