How do skyscrapers and tall buildings deal with rain?

Ever looked up at skyscrapers like The Shard or all those tall buildings at Canary Wharf and wondered how they cope with rain?

Where does it go? What stops it pooling on the top of a flat roof like a lake? Or pouring over the side of structures in a massive waterfall?

The answer is roof designs, drainage systems and pipework. All of which play a huge role in high-rise structures in wetter and colder climates than the UK.

Take Canada for example. The skylines of Vancouver, Montreal and Toronto are littered with tall buildings. These have the additional pressure of melting snow and ice to cope with.

Or Singapore. The sixth-wettest country on Earth has over 100 skyscrapers in its central business destination. These building face over 3000mm per year of rain on average. Double the UK average of between 800mm to 1,400mm.

Here are some of the ways in which skyscrapers and tall buildings deal with rainwater.

Sloping roofs

Flat roofs on the top of tall buildings are not totally flat. Instead, they are designed to slope slightly inwards towards a shallow bowl.

This ensures rainwater is not directed over the side of the roof, where it could cascade down and cause major issues below.

At the centre of each of these bowls is a drain. Rain flows down the slope, into the drain and enters a pipe system.

It is the job of said pipe system to take the rainwater safely from the roof, through the building and discharge it into below-ground storm drains.

Pipe systems in skyscrapers to deal with rain

The pipe systems which do this are called rainwater or drainage stacks. Whereas household guttering and downpipes are installed on the outside of buildings, rainwater stacks are contained inside.

They are almost always boxed into wall cavities or hidden between floors. This can make rainwater stack repair difficult. As can the complexity of the pipework system, snaking for hundreds of metres across buildings.

If you ever played with Marble Run as a kid, you will have some idea of what rain drainage systems in skyscrapers look like.

You might be wondering at this point why a rainwater stack cannot just head straight down vertically, like your household downpipe?

The reason is gravity and pressure. Water is heavy, as anyone who has dealt with water hammer can attest to.

Imagine a large amount of rainwater being directed at the same time down a 200-metre-long vertical pipe at a speed of almost 5 metres per second. It would smash into the sewer at the bottom with devastating impact.

Building regulations therefore set a maximum distance of how far a rainwater stack can travel vertically before an elbow must be installed, often followed by a horizontal section.

These elbows and horizontal sections slow the rainwater down, creating a much safer system and helping to protect the pipe itself.

Back and forth the rainwater stack snakes until it reaches the ground floor, where it deposits into the sewer system.

Rainwater harvesting

Rainwater harvesting has been around for thousands of years. The Romans in particular loved the concept. They constructed aqueduct systems to deliver rainwater and spring water into their cities.

The climate crisis has brought rainwater harvesting back into fashion. Where the Romans had their aqueducts, 21st-century humanity has its skyscrapers.

Rainwater harvesting tanks are now being installed on the roofs of tall buildings around the world. What used to be empty space is turned into a collection point for rainwater, which can then be used for everyday functions in the building.

Many tall buildings are now installing rainwater harvesting tanks. They turn existing empty space on top of a skyscraper into a collection point for rain. This water is then used for everyday functions in the building.

British architects Foster + Partners are going a step further. They have designed a brand-new skyscraper for the Chinese city of Shenzhen with rainwater harvesting at its heart.

The proposed building will be the headquarters of China Merchants Bank. It will stand 350 metres tall and have a built-in rainwater harvesting system able to provide up to 70 percent of the water requirements for the tower.

Rooftop gardens

Or how about letting nature deal with rainwater, just as it does in gardens and parks at ground level? Rooftop gardens on high rise buildings are becoming increasingly common. Many in major cities are even open to the public.

Planting flowers, trees and grass on the roof of a tall building creates a unique space for people to hang out. It is good for the environment. And it helps deal with the otherwise complex subject of rain and skyscrapers.