Water Tanks on Roofs?

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.

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

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

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………

All the new high-rises have several booster pump zones to serve the higher floors. Mad Dog

The beauty of the tanks on the roof — or on mechanical floors — is that you can get away with really miserably undersized water mains and still provide enough pressure both for domestic use and, perhaps more important, at least some sprinkler fire flow.

On the domestic, suppose a 50 unit apartment building, 4 person occupancy — that's a daily load of around 16,000 gallons . It probably peaks at at least 250 gpm, maybe higher. That's a heck of a load for even a good size water main. But… if you have a nice tank of that capacity or so, you can chug happily along at about 12 gpm and meet the load very handily. That's a reasonably sized tank — though rather heavy.

My time to shine!

City pressure in most of NYC is around 35psi, although there are some notable exceptions, such as Washington Heights. Add an RPZ to the main coming in and there goes another 8-10psi, so you only get 4-6 floors before pressure is insufficient in the average building.

Many newer buildings will use a constant pressure booster system to pressurize the building. Those basically runs pumps non-stop to keep the building at the needed pressure (although if sized and set up well, will often go to sleep for some amount of the time in low usage times). Old school boosters use pressure regulation valves (Cla-Vals) for pressure regulation, new systems use drives to just control the speed of the pump instead. The one downside with a booster type setup is when the pumps stop, so does the pressure as soon as there is usage. Large buildings can drop 100psi in mere seconds when the pumps turn off.

Roof Tanks, while old school, are still relevant also a simple and elegant solution for domestic water. Many domestic tanks also provide fire service water as well, but some are dedicated to just fire service. The tank is filled by tank fill pumps from the city main down in the basement (often referred to as house pumps), and then gravity brings the water down the to units. Conveniently the roof tank acts as a buffer so you can have higher peak usage than what the pump might be able to make up, so pumps can be sized with a fill GPM below potential max usage. The average tank is between 7.5k - 20k gallons. Sometimes buildings with roof tanks don't actually have enough elevation on the tank above the top units and so the top few floors might have insufficient pressure. In this case they will usually add a small booster to add 15-20 extra psi pushing downwards.

Very large buildings will also have what are referred to as Day tanks to act as a buffer between city main and the building. These day tanks are located in the basement mechanical levels and are required if the building has the potential to pull more than a 400 GPM from the city at once to prevent local pressure sag on the city mains.

Very tall buildings will use a combination of Day tanks, Roof tanks, and boosters and will be zoned in into sections of 20-30 floors per section

Here's a photo of what the inside usually looks like and don't worry that this water looks rather green, this specific tank is for fire service only! You can see the fill valve filling dumping water in (ballcock is offscreen to the left) and also the typical float balls for pump run and high/low alarm although those actually aren't in use in this tank. The pump controls here use a submersible transducer for level measurement which is at the bottom of the blue wire that runs down into the water.

Slightly off topic: $$$$ could be saved over long term with separate supplies for potable; irrigation; and fire.

Yes, the water sitting in the pipes is Black, skanky and smelly but it does the job! after the 1st 15 - 30 seconds its clearing but I still wouldn't drink it.

By the time the FD shows up it clean.

FD inside building that required sprinkler activation would most likely be in SCBA air packs already.

even if the cavity you are hacking in to is wet, they will stick to stuff and dry out later.

I've always found it rather amusing that those that pay the most in buildings usually have the worst pressure and are right next to the operational noise of any rooftop equipment, fans.

I'll never have to worry about having those problems though 😂

Wasn't the top floor the low rent space before practical elevators were invented?

Another surprise is fluctuating flows with cheapo (light weight) booster stations. Pressurized reserve is necessary.

I've used that kind of setup in applications where I had a well which had inadequate yield — fill a storage tank (pump the low yield well all day) and use a small booster — or in a couple of schools, several! — to get adequate system pressure. I seem to recall there was a self-contained Jacuzzi unit I used. But that was 50 years ago now! I suspect there are new ones now!

For applications like a hotel with many simultaneous luxurious showers; storage is essential, somewhere.

2 gallons per minute is the maximum flow in new and replacement water appliance fixtures.

there is usually a flow restrictor that can be removed from most shower heads. the federal regulation is 2.5 gpm which is adequate. most shower heads are set up for considerably less at usually 80 psig out of the box so if they aren't adjusted for the actual pressure and maximum flow you get a whole lot less than what is permitted. generally people that understand orifice calculations are not installing shower heads in hotels.

1.8 or 2 gpm shower head flows in California and Colorado.