What's up with this picture? It's a view from the fire escape at Underscore Central Command. Upon visiting the roof, I found that a great number of buildings in the area still maintain wooden water tanks on their own roofs.
Several people have asked me about them and their purpose. While I have some plumbing experience, I've not had to use those skills since I moved to Manhattan several years ago (unless you count the sinks I tore out in my apartment and replaced). However, I know a bit about these tanks.
Essentially, they're an ingenious yet simple way of dealing with the problem of maintaining water pressure in tall buildings during peak water usage times. Street pressure isn't enough to push water up to your bathroom in your 33rd floor apartment, so modern skyscrapers employ booster pumps throughout the building to help boost the pressure. Older buildings, however, use tanks like the ones pictured here.
Rather than house giant pumps in the basement of the building in an attempt to keep water pressure constant, smaller pumps fill the water tanks during off-peak hours (at night) when demand for water pressure isn't great. Once in the tank on the roof, gravity lends a helping hand when demand picks up. So when everyone in your building is taking a shower at 7:30 AM, you're less likely to experience a drop in pressure. The force of gravity pulling the water down helps keep the pressure up.
Now you know. (And knowing is half the battle.)