How long to crumble?

Question

The story begins a few hundred years after a plague and limited nuclear war decimated most of the planet. I don't want much knowledge to have remained, just myths and legends of a glorious past.

The question is - how long after a limited nuclear war (mostly targeting weapons sites, I guess) and a massive plague that emptied the cities would it take for the cities to be overgrown and crumbled to where there is essentially nothing left? Concrete has to crumble, metal to disintegrate. How many years?

Answer 1

A great real-world example is Pripyat, Ukraine, where the chernobyl nuclear accident forced an evacuation in 1986.

It's been 30 years, and many of the buildings, while still mostly upright, have become overgrown.

Of course, how overgrown an abandoned area becomes is completely dependent on the local environment. In the high heat, humidity, and growth of the rainforest, it would take less than a decade for buildings to become completely overgrown. Things like saltwater, high wind, and large trees will accelerate decomposition of buildings. A building in the arctic circles could feasibly stay completely intact for centuries.

Of course, things like plastics, stainless steel, and chemical contamination will stick around for a very long time.

I'd say a decent rule of thumb is probably around 100 years, though denser cities (New York, London) will probably take much longer than that to disappear. Add more time if in cool, dry environments, and less time in humid, salty, and disaster-prone areas.

Answer 2

The best way to answer this is by looking at real-world examples. This is a relatively simple question with a more complex answer, but nevertheless:

In 1986, the Chernobyl nuclear disaster created the closest scenario to yours in the city of Pripyat. It's been thirty years since the disaster and its already beginning to fall apart.

On average, concrete begins to crumble between 60 and 250 years; but that's if they're properly maintained. With storms, radiation, and other natural weathering, buildings would start to collapse relatively quickly.

On the other hand, the massive constructs that are skyscrapers are designed to last between 500 and 1000 years, and could survive slightly longer.

But as for plants to take it over, that would take between 5 and 60 years. Of course, this may not be totally plant-ridden, but its definitely a quick start for a true "overrun" look.

So in other words, your "few hundred years" is definitely a good start for most of modern civilization to be lost, save some powerful monuments and well-built skyscrapers.