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Setting: Large city full of tall buildings, plus zombies

The survivors have taken to the higher floors of the buildings. The streets are very dangerous, so going down there is only for emergencies.

  1. What would be the supplies needed to survive indefinitely? I guess I'm assuming that they would farm on the roof (and collect rain water), but what would they need to collect?

  2. How many people could feasibly survive together in this scenario?

  3. How would you connect buildings? Could you attempt to build rope bridges across?

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    $\begingroup$ Oh man, there goes another bunch of hours not doing what I was supposed to be doing - curse these zombies! xD $\endgroup$ – mechalynx Oct 12 '14 at 14:17
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    $\begingroup$ I'll try and space my zombie questions out so as not to totally ruin your life ;) $\endgroup$ – MadPink Oct 12 '14 at 14:52
  • $\begingroup$ This is quite dependent on climate - a desert area there won't be enough water, a polar location would need a lot more stuff for clothing/heating etc. $\endgroup$ – Nick Wilde Oct 12 '14 at 15:37
  • $\begingroup$ For my purposes, New York City is the locale I am most interested in. $\endgroup$ – MadPink Oct 12 '14 at 15:41
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    $\begingroup$ Rooftop agriculture takes time to get going, and assumes they have the seeds, soil, etc to begin with. Are you assuming that, somehow, they've got enough food to get to their first harvest, or is that part of the consideration here? $\endgroup$ – Monica Cellio Oct 13 '14 at 0:24
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The Food

Let's see how much energy a person needs, so we can see how many people we can feed with our food, assuming we can grow food.

Daily needs for an adult are about 3500 kcal according to this calculator (it uses large calories) for a male (because there's higher needs), 32 years old, 1.80 m tall and 70 kg in weight - this other calculator confirms this, giving 15.62e3 kJ which is the equivalent of 3731 kcal. Let's average those and round to get our daily needs, per individual are 15e3 kJ or 15 MJ.

Assuming they live off roof-top crops, they'd be essentially living on a vegan diet and possibly on a raw vegan diet as well. Now, I'm going to assume they eat and grow only soy for simplicity - I'm handwaving the dangers of eating only one kind of food, especially if it's soy[5] - variety is necessary. The outlook is pretty bleak though: going by existing rooftop cultivation examples such as this guy's, it seems you need at least 3 months before you get food out of this - you also need enriched soil, many kinds of plants to make sure you've got food all year round and there's that looming nutritional variety problem. Even those that have done this kind of thing, don't replace all their intake needs from these gardens[7][8]. But those problems can be handled narratively - lets see if we can force enough food out of soy for the time being.

According to this chart (it's badly made, the lists are images so you can't search >:( ) 100 g of soy beans contain 545 kJ boiled. That means we need about 2.75 kg of soy beans, per person, per day. The wikipedia article on soybeans gives us an average yield for soy bean crops of 2.5 tonnes per hectare in 2010. I'm assuming that's the total over a year, but I'm no farmer so I can't be sure. Since that's for a professional, well-tended crop with modern technology, lets cut that yield to 1/3, giving us about 0.8 tonnes per hectare. Converted to kg/m^2, that's 0.08 kg/m^2. For a single person, we need 979 kg of soy beans per year, which requires an area of 12.24e3 m^2, or a square 110 m on each side. Going by this paper, the total flat roof area in NYC is 21,249 acres divided over 144,832 roofs. That's about 342 m^2 per roof, which means one person, over the course of a year, needs almost 36 roofs to feed themselves, just by roof yield, so that doesn't work. It can still be a good supplement, but you'd probably want to find some way to grow food within the buildings themselves, like this:

vertical garden

which is known as vertical farming. If, however, you can find other sources of food, such as scavenged food or make a cat farm, you'd need about 0.62 kg of food per adult, per day (if you're an astronaut at least). That amounts to 220 kg for an adult over the course of a year, excluding water. You should be able to get this much if dietary intake is varied, especially if it contains meats and dairy products (since there is dried meat, jerky etc. left over in supermarkets, we can assume they get some of this, even while scavenging in the city). The less varied the food is, the more you need.

The people

How many people can survive this way would vary largely. It depends on what food you have available, how many buildings are close-by, how much food is scavengable and in what area. It's hard to make an estimate, but I think the biggest factor by far would be scavengable food by area. As far as I can tell, there just isn't enough yield on these crops and too small an area to get enough to feed an entire group of 4-6 people. It would take tons of work for even one person to manage alone, with just their crops - not to mention farming is a huge amount of work, even on a rooftop.

Bridges

Rope bridges are fairly straightforward, but you'd want someone on the other side, or it can be very tricky to get one to work. I don't know much about how to make them and I can't find much information on it, but as far as I understand, you'd need enough rope to cross the gap one time (with a noose), then add 2 gap lengths if you want to be balancing on a single rope, with 2 side-ropes for handling. You'd probably need 4+ gap-length's of rope to get the simplest of board bridges, possibly double that length.

The distance between buildings can be estimated based on street distance.Going by this NYC street design guide, it seems we'd probably have a minimum of 20 ft and probably a maximum over 72 ft for large streets. If you want to connect buildings close by, with just a side path in between, we're probably talking about as little as 16 ft of distance. Of course, some buildings may be so close you don't need anything besides your legs to cross over.

If we want to create a suspension rope bridge, we'd probably need at least 64 ft of rope and some boards. That's 19.5 m. You would also need some powerful anchors, but that might be easy to find if there's pipes, chimneys etc. - it would depend on what's available.

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  • $\begingroup$ That's a lot of calories for someone who isn't moving around much (due to not leaving the building...) $\endgroup$ – Tim B Oct 12 '14 at 18:29
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    $\begingroup$ @TimB what about someone who's farming, or possibly having to carry supplies through a zombie horde? I did specify large amounts of activity to the calculators. $\endgroup$ – mechalynx Oct 12 '14 at 18:30
  • $\begingroup$ True, it depends on just what they are doing though. $\endgroup$ – Tim B Oct 12 '14 at 18:33
  • $\begingroup$ I disagree with two assumptions here: 1) The 3,500kcal is for someone who is very active and such a survivor will not be. 2) You cut yields for inexperience but don't add for the fact that they can pack more tightly than farm crops because it's not going to be combine-harvested. Overall, though, you're right, it doesn't work. $\endgroup$ – Loren Pechtel Oct 14 '14 at 4:04
  • $\begingroup$ @LorenPechtel I bias my estimations towards giving an upper margin, since being absolutely realistic with so little information (and without experiment) is next to impossible. This way, if it works out in the numbers, it's almost certainly possible, but if it doesn't you have an idea of the scale of the difficulty and it just takes a bit of number fudging towards making it actually work in a story. Hence my overestimation of energy and underestimation of yield - considering that I know very little about how this would pan out, I feel this is more useful. $\endgroup$ – mechalynx Oct 14 '14 at 6:36
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Disclaimer: You're going to see a lot of Fermi estimation here. A lot. So don't use my numbers in a scientific study - I certainly wouldn't.

Food

Rooftop farming/urban agriculture is the only way these people are going to survive. Fortunately for the vegans, there's no way animals can be raised here. Open fields, spacious pens - all things animals need, and all things New York City lacks (I'm assuming the zombies have commandeered Central Park). So the inhabitants of what remains of the city will be forced to turn to fruits, vegetables, tubers, and anything else they can grow.

Urban agriculture relies on the premise of growing a lot of food in a small, crowded, artificial, and generally unpleasant space. The best places to put "farms" would be on the tops of buildings (and yes, the zombies have taken over the High Line. Darn.). So How much space do we have? I'll stick to Manhattan for now. It has an area of about 59.5 square kilometers. Taking away Central Park's 3.41 square kilometers, we have roughly 56.09 square kilometers. I'll estimate that a good 33% of that is roads, so we have about 37.39 square kilometers available. But not all of the roofs are flat! However, those that have slanted ones could be used as hanging gardens (which I'll get to later), so it all sorts itself out in the end.

With supplies scarce, the top crops will be those that don't need a lot of resources - food, water, etc. Desert-loving plants are always a good bet. Sunlight will be scarce on some of the other floors (and there won't be enough power to illuminate the plants), so any darkness-loving plants (well, there really aren't any) would be welcome. You also want plants that can grow vertically. To add to the criteria, the plants have to be easy to care for - you simply don't have the time to teach everyone how to harvest rutabagas. There are a lot of options - really because there aren't any crops that fit these criteria - so it all comes down to whatever seeds you have lying around. And in a city, there probably aren't a lot.


People

How many will survive the apocalypse? Well, how many can the farms support? We can also use other floors of the buildings to make makeshift vertical farms. So we'll say that an average of 15 floors per building can be used throughout the city. Now we have 560.9 square kilometers of "land" available for farming. I honestly cannot find good statistics for how much land one person needs to support themselves for a year that would apply here, so I'll say that perhaps one-third of an acre, efficiently designed, could sustain a person. This is about 0.001348952 square kilometers per person. With 560.9 square kilometers available, Manhattan can support 415,804 people - if we push it.

So the other 74.4% of Manhattan's once 1.6 million+ strong population are gone. As the Soup Nazi might say to one, "No food for you!" The city simply can't support them. But the other 416,804 are fine - for the time being.


Getting between buildings

The whole no-ground-level restriction is a problem, because the ground is generally the easiest way to get from point A to point B. The aerial element means you will have to make bridges of some sort. I'm not sure rope bridges would be a good idea. Who has that much rope lying around? The alternative material would be telephone wires, but there aren't a lot of above-ground ones in New York City!

I would just go with more static bridges - really just glorified planks. If you can nail together a bunch of 2-by-4s and cross Broadway, you're set. But a) Don't fall and b) There isn't a lot of material to make bridges, is there? Sure, you could cannibalize some construction sites, but you're going to run out eventually. You could try to play a giant modified game of Jenga - taking out walls in buildings and, in this case, using them to make bridges - but you'll still run out eventually. Besides, most skyscrapers are made of steel and other metals. You can't break them apart as easily as wooden buildings.

But for the sake of this question, let's make some assumptions. Say you can knock down two of the four outer walls of each floor of each building. NYC apparently has 5,937 high-rise buildings, which I'll take to mean buildings ten stories or above. Let's say 75% are in Manhattan. Maybe they have an average of 50 stories apiece, and they are, on average, 50 feet wide and 10 feet per story. So now we have .75 times 5,937 buildings times 50 stories/building times 2 walls/story times 500 square feet/wall = . . . 222637500 square feet of material. Some of it may be glass, but glass is strong enough. For all intents and purposes.

Let's make another guess, and say that the average distance between buildings in Manhattan is 50 feet, and each bridge should be about 10 feet wide (Don't trip!). You can therefore have about 445275 bridges in Manhattan. They'll all have to be at about the second or third floor, in order to avoid high-level wind gusts and low-level zombies, so you can forget about great views, but they'll suffice.

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    $\begingroup$ Oh, good, someone got through my entire answer and didn't want that period of their life back. $\endgroup$ – HDE 226868 Oct 13 '14 at 0:28
  • $\begingroup$ 15 whole roof-areas available per building sounds unrealistic. Looking at the picture in ivy_lynx's answer, I'd guesstimate more like 5-10 roof areas at most, depending on building height and incident sunlight. Probably closer to 5 than 10, but it would depend very much on how "stout" each specific building is (relationship between height and ground surface area). $\endgroup$ – a CVn Jun 1 '15 at 8:54
  • $\begingroup$ Your comments about "high line" and "hanging gardens" get the idea that in the mid- to long-term, they probably should try to get some building material and build strong bridges or spaces between buildings. That expends their vital space, where they could grow more plants. $\endgroup$ – bilbo_pingouin Jun 1 '15 at 10:30
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    $\begingroup$ Animals - guinea pigs, rabbits and chickens can all be raised in confined spaces and produce usable meat. $\endgroup$ – WhatRoughBeast Jun 1 '15 at 14:26
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Some things to consider:

  • Water: You have 21,249 acres of roof area (as reported by @ivy_lynx), which is ~86 million square meters. New York annually has 700-1500 mm of rain, which gives you ~86 million cubic meters of water annually. That means, assuming people use on average 2 liters per day, rainfall can sustain ~120000 people. And that's assuming you use all the roof are for collecting rain (on the other hand it should be quite easy to expand that with some glass, etc.).
  • Bridges: More than just bridges, you need an effective communication (i.e. cargo transport). I think that pairs of directional ziplines might become common. I believe that there should be some accessible supply of ropes that people use for construction work.
  • Light: either electricity (i.e. fuel for generators) or fire (e.g. wood). There are number of tasks that require light at night, and that might become some really important resource.
  • Entertainment: Books (easiest), story tellers, musicians, etc. That won't be necessary at first, but depending on the length of the stay, it might become an issue (people with too much time on their hands often become troublesome).
  • Child care: There are just so many ways a child could hurt itself in such a setting, so perhaps special child care areas would emerge.

I hope this helps ;-)

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  • $\begingroup$ I definitely agree on ziplines. They would use far less material per connection and since you're fastened to them it would be safer than any simple bridge is going to be. $\endgroup$ – Loren Pechtel Oct 14 '14 at 4:08
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There are a lot of good answers here already, but I'd still like to add my 5 cents.

  1. Organise well. Ensure security against both zombies and renegades and manage your resources efficiently for maximum survival. Cooperation is they keyword, lacking any police conflict will kill you faster that anything else.
  2. In most big cities there are already several rooftop gardens present today. Best start there. Learn and extend. Without elevators only a limited set of floors can be utilized anyway. Acquire books on the subject of (rooftop) cultivation and nutrition.
  3. Cultivate beans, potatoes, unions and veggies. In separate plots so pests will not eat all in one go. The yield per year divided by yearly consumption will determine the number of people you can feed. Anything else is conjecture. Any number problem is easily solved: Scavenge to make up the deficit. The population level will naturally go to the sustainable level. Life as it has become. Life as it has been for almost all of human history.
  4. Keep bees. They will fertilize your crops.
  5. Keep rabbits or rats or even goats as long as you can. High calorie food at high investment, definitely good for morale. Goats can provide milk and cheese.
  6. Produce methane gas from waste to have fuel to cook with.
  7. Dry and store your surplus for winter and sure to come bad times.
  8. BEWARE OF FIRE as there is nowhere to run to and no one to call. Best cook outside, prohibit lights inside. Sundown == bedtime.
  9. Keep your buildings whole. They WILL degrade.
  10. There is no way to safely build bridges without going down. So go in fast and use what is on hand. Steal buses, park in street to connect buildings and possibly create a semi safe zone.
  11. Implement a plan to pacify the environment, then expand. Possibly by luring groups of zombies somewhere, and then drop heavy stuff on their heads as a friendly competition or free time hobby? One day the city will be a zombie-infested death trap of collapsing buildings. One day you will need to move to the countryside. Develop a plan for that.
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