So here’s a question about rooftop farming

How much does it cost to retrofit a roof to be able to do it?

It’s a warm afternoon in late spring and before us rows of strawberry plants rustle in the breeze as the scent of fragrant herbs wafts across the air. Nearby, a bee buzzes lazily past. Contrary to appearances, however, we are not in an idyllic corner of the countryside but standing on the top of a six-storey building in the heart of the French capital.

Welcome to the future of farming in Paris – where a whole host of rooftop plantations, such as this one on the edge of the Marais, have been springing up of late. Yet this thriving operation is just a drop in the ocean compared to its new sister site. When that one opens, in the spring of 2020, it will be the largest rooftop farm in the world.

Currently under construction in the south-west of the city, this urban oasis will span approximately 14,000 sq metres (150695 sq feet) – also making it the largest urban farm in Europe. With the plan to grow more than 30 different plant species, the site will produce around 1,000kg of fruit and vegetables every day in high season. Tended by around 20 gardeners, they will also be using entirely organic methods.

The thing being that all that earth weighs something. Add in rain that will fall upon is and stay there – you know, soaked up, not just running off – and we’ve a significant addition to whatever the original design tolerance was.

OK, maybe the building was originally designed with this in mind. Super – so, how much did that extra weight tolerance cost?

The real question here being, well…..does it cost more to create a hectare of urban farm than it costs to go buy a hectare of rural farm? If so, why in buggery is it being done?

43 comments on “So here’s a question about rooftop farming

  1. “The real question here being, well…..does it cost more to create a hectare of urban farm than it costs to go buy a hectare of rural farm? If so, why in buggery is it being done?”
    There could be other benefits like soaking excess water when it rains, being cooler in summer, warmer in winter.
    Hopefully someone will calculate the cost benefit

  2. Do we need to worry about it though? Surely the price mechanism will answer the question while we get on with something useful? Unless Bloke in Germany’s suspicion is correct. So another question is “does the CAP apply to rooftop farms, or does one need agricultural land to qualify?”

  3. Forget about the cost, what about the air quality? You wouldn’t want to eat this stuff, would you?

  4. Serious question?: What does this mean in terms of building insurance with the additional weight load on the roof? Will firefighters enter a building knowing that there is a farm on the roof that could come crashing down during a fire?

  5. In 2013 the roof of a supermarket in Riga, Latvia caved in killing a bunch of people. Turned out there was a “winter garden” being constructed on the roof at the time, which didn’t appear to be part of the original design. The collapse followed weeks of torrential rain.

  6. I think the answer is in this bit:

    Not only will it be the largest rooftop farm in the world but they will also be pioneering their own technique in aeroponic ‘vertical’ farming.

    Aeroponics doesn’t use huge beds of soil, it uses a small amount (if any) of growing medium around the roots and mists the roots with water with the right mix of nutrients in.

    It also mentions the possibility of locals having small tubs of soil for their own vegetables, but that doesn’t sound much different from say Merrill Lynch’s rooftop flower beds at their London office. Most vertical agriculture uses minimal soil or avoids it all together so I’d guess any strengthening over the original will be minor.

    Does anyone know about the construction of Kensington Roof Gardens (aka Derry and Toms in the old days) or Fen Court? They’re roof gardens constructed in 1930’s and the last couple of years.

  7. The Sage has it right. This is not an economic decision or a ‘green’ one. It is a virtue signalling one that is going to cost its finance providers a lot of money. In fact it’s advertising isn’t it?

  8. The Mayor of Paris intends to ban diesel vehicles entirely from 2024. Since that basically means no more food delivery trucks, these vertical farmers will become the only source of fresh food in the city and thus will be able to charge sky-high prices. It’s just good financial sense.

  9. Basically its a few tons of weight at most across entire roof. Quite likely the roof and building can handle this.

    The extra weight per cubic metre for rooftop farming is tiny. Now if you want to add significantly to a weight issue, use vertical farming. That farms in cubic, not square. Can be dozens of trays of plants per square metre, not just one.

    How fresh do you want your fruit & veg? An hour from picking it can be in your shopping basket.

  10. The issue is that–outside eco-freak fantasy and lies–we ie humanity– don’t need no stinking roof gardens. Anti-ecos used to say that 10% of NYs roofs (rooves?)–roofspace could grow enough food to feed NY. That was to debunk eco-cockrot about “however can we feed 10 billion people” etc. It wasn’t–I hope–ever proposed as a serious plan.

    The only NY roof garden that stands out in my mind is the one in “The Ultimate Warrior” film . Yul Brynner kills about 200 men using only a small dagger ( and in fairness is quite convincing despite having no martial skills) while Max von Sydow leads a post-civilisation commune in a NY brownstone. With a roof garden. Not a good advert for high air horticulture.

  11. 20 gardeners for 3 1/2 acres (~1.4 napoleons) for one ton a day of goods at its peak… the economics of that don’t look promising.

    Tom @ 9.07am in the UK you need the holding number of your ag land to fill in the form for subsidies.

  12. “How fresh do you want your fruit & veg? An hour from picking it can be in your shopping basket.”

    Only if it’s in season. Rest of the year it’s got to go into storage first, like food from anywhere else. Though I suspect given the scale being insufficient to feed a city, it will all be for fresh consumption and the city just needs to import more food (from the countryside, from storage or from furrin) the rest of the year.

  13. @ Martin
    Bollocks. Roofs are constructed to do what they do. There’s a kg/m3 imposed load limit would normally include snow load. Presumably we’re talking flat roofs & no-one’s trying to hang terracing on pitches a roof is designed to carry light access use. Nobody spends the extra money to raise the load carrying capacity of a roof without good reasons. If they’ve done so, the roofs are already being used for the purpose this was done.
    But it doesn’t stop there. The structure of a building is designed to support the mass of the roof as specified & tranfer it to the ground.. Again, you don’t build walls & structural members to specs higher than necessary. Introduced to this concept as a kid. Uncle’s financial company owned one of the old, originally residential buildings in the City. Used the top floor for files. As the enterprise grew it acquired more files in more filing cabinets. One night, the whole lot dropped through three floors into the basement.
    Lived in Paris for a while. The Haussmann part. Those buildings were well constructed, for their time, but they tend to have the same problem late-Victorian/Edwardian London has. A massive amount of construction over a relatively short period. Where do you find the skilled labour to do it, from a standing start? It takes ten years to become a skilled tradesman. Answer is, you don’t. You use your skilled labour on the bits that show, semi & unskilled to sling up the bits that don’t. Those buildings are now headed for 200 years old. It shows. Rest of Paris isn’t even built as well. Stuff that Haussmann didn’t pull down. Infill from then until now. Some to poor standards.
    Retrofitting Parisian buildings to take roof farms would be amusing. Be worth me starting a business doing it. But I’d need to build something pretty substantial for myself, to keep the money in.
    Disaster waiting to happen.

  14. I think it is not a bad idea actually, it will improve the insulation of the building and I doubt they’ll be piling the muck 6 foot thick, there’ll be no tractors and so on, and so my gut feeling is that the additional weight shouldn’t be a major problem for a well designed and constructed modern building.

    Obviously it won’t be as economic as proper farming, but there is something to be said for the proximity (presumed) to the eventual market.

  15. Obviously, once the roof collapses the proximity of the stuff to the market improves even further:-)

  16. The above prompted memories of a flat we had at the bottom of Montmarte. That had a flat roof we used to sprawl on summer nights & smoke dope. And do other things, more energetic. Ah! Those were the days to be young & in Paris.
    Remember having to saw 2″ off the legs on one side of the bed so we didn’t roll out in the night. The entire street seemed intent on moving to Pigalle

  17. I thought urban farmers specialised in mushrooms and weed? All done under the roof. The danger being from fire, and being caught.

  18. The next step would be to keep cattle or sheep on a roof. They’d need decent fencing, imagining Buttercup or Daisy falling 20 storeys onto some unsuspecting passer-by…

  19. Zactly, Gurzel. Extremely labor intensive.

    ‘they will also be using entirely organic methods’

    So they aren’t really serious.

  20. the site will produce around 1,000kg of fruit and vegetables every day in high season

    I’m no expert on market gardening, but that sounds like a lot.

    I did a quick google and people with market gardens in the US seem to reckon it takes one person to look after an acre and that acre might generate $30-40k a year.

    So this market garden doesn’t seem to add up.

  21. IIRC French building regs require the roof to be able to support 1.5m of snow. (More for alpine areas, of course.)
    So a few inches of soil should pose no structural problem.
    Unless it snows, of course.

  22. Surely they can plant vines on the more steeply angled roofs?

    Also, why isn’t the plural of roof rooves, as hoof is hooves?

  23. 20 gardeners on what?-say 10 yo-yoes an hour? 35 hour week in Grenouille Land-say seven grand a week for wages. That’s before you start with equipment, stock blah blah blah.
    Hmmm.

  24. Bloke in spain – we are talking a bit of liquid, the weight of the plant itself and less than 2 dozen people. Don’t think I’ve ever heard of a building with a flat roof presumably able to do other things on it being unable to handle that kind of load.
    The building itself will be able to. We don’t usually engineer buildings to take the minimum possible conditions before falling.

    The people will most likely be the heaviest part of the whole thing. Know many buildings with flat roofs that either cannot take or cannot be made to take the weight of 20 people spread out?

    Wonko – this urban farmer does not specialise but I do like strawberries all year round and wife wants a banana tree – which will take considerable time I think to keep trimmed low. Lettuce, herbs and spices, carrots etc are common. LED light, nutrients, sorted.

  25. I just got around to reading the article. For a start it’s bollocks. It’s not “…standing on the top of a six-storey building in the heart of the French capital.” It’s on a single storey building – judging by the view of the Tour Eiffel – way outside the Peripherique in the outer suburbs. They might as well have driven 5km down the road & bought a farm.
    And the building depicted, whatever it’s supposed to be for, is the sort of thing would normally have a lightweight steel frame with cladding panels covering it. Over that sort of area, a number of low pitched roofs with valleys in between. You just wouldn’t build a load bearing flat roof over a thing like that. The feature of these sort of buildings is minimum structural supports giving wide areas of uninterrupted floor area. Think factory or warehousing or retail space. Can’t imagine what you’d use this one, with it’s forest of load bearing piers, for. Single storey covered carpark?
    Whatever, it must’ve cost a fvckin’ fortune to build, compared with a more conventional structure. Or to retrofit.

    I’ve been trying to look up recommended loadbearing requirements for ocassional access flat roofs. Sort of thing you put on when you don’t intend a roof terrace. Current building regs doesn’t seem to give a figure but the guidance number I recall from my old paper copy was 25kg/m2 for normal UK climate (snow loads need to be higher in the north)
    That’s what the requirement was for a new roof in the 90s & will have a great deal of belt & braces. Legacy roofs? Who knows? Standards were more relaxed, especially in France. Some of the stuff goes up in France you wouldn’t have got past UK building control 50 years ago. And flat roofs degenerate. Ours are only built with a 40 year design life.
    Looking at the article’s figures, 1000kg of produce coming off of 14,000m2 daily.. 70gm/m2 doesn’t sound a lot but you’d have a helluva lot of vegetation up there at any one time to get that yield. A high proportion of it’s waste not actual crop.Plus the substrate, irrigation water & your farming equipment & bods doing the farming. All being superimposed on what a structure’s required to take anyway.
    Sounds like lunacy.

  26. But I do have an experiment in progress. I’m sort of presuming the terrace on my apartment here has the same load bearing as the floors. But this is Spain, so who knows? I’ve a 90cm dia flowerpot & I planted a 10ft Leylanii in it. At the moment i can just about lift one side of the pot, so let’s say the whole thing weighs 3-400 kg. Leylanii should grow what? 10 -15% p/a? So keep your eyes on this space & I’ll let you know when it lands in downstair’s living room.

  27. It never ceases to amaze me how little people understand what buildings are designed to do, Martin. Why do you think a roof intended to keep the rain off should support 20 people walking about on it? Let alone gardening. Why would anyone in their right mind build such a thing?

  28. Vertical gardening is actually employed for mushrooms. Not in cities though – where the land/warehouses are cheap, like deepest China. The productivity can be nuts, cropping every few days/weeks. I guess the vertical racks make sense for the regular cropping. Impressive when you think it’s a protein source too.

    On a related topic – thinking about starting a small veg patch. What do people reckon gives the best return in terms of value and/or taste here in the UK?

  29. @The Sage August 13, 2019 at 8:53 am

    It’s being done because someone has money to throw around for virtue signalling.

    +1 Also, everything must be hand harvested – combine harvester on roof not going to happen.

    Maintenance? If a leak happens it’s going to be horribly expensive to fix.

    If roof is /\ how does it work?

    @bloke in spain August 13, 2019 at 10:36 am

    +1 When at Uni a first floor library (concrete floor) had signs all over the place saying “Max n kg per square metre”

  30. If so, why in buggery is it being done?

    Because it makes those who actually get access to the produce feel good – even better when they know that they’re getting access to it when tons of other people were forced to pay for it but won’t.

    I have a 144m2 house – that might be enough surface area to grow enough to make up a noticeable portion of a small family’s food budget. Put in on top of an apartment building – now there’s 4-8 small families trying to share that. What are they going to get out of it except some fresh herbs.

    Or on a high-rise where there will be 16+ couples sharing that space.

    Vertical farming helps but on a cost to benefit ratio it’d still be better to build another story, rent it out, and then use the proceeds to buy cheap land outside the city the renters could build their vertical farm on.

  31. On a related topic – thinking about starting a small veg patch. What do people reckon gives the best return in terms of value and/or taste here in the UK?

    Mrs BiND has some raised beds at the bottom of our garden in a sheltered south facing position.

    I’ve come to hate the damned things but courgettes are always in her meals and lying around the kitchen. Tomatoes usually do well, they were free seeds from the Telegraph a few years ago and she dries out a couple of tomatoes worth of seeds at the end of the year for replanting.

    Green beans, mangetout, lettuce, leeks, spring onions parsley and herbs all do well although she doesn’t plant a lot of them. She occasionally plants potatoes but they need a bit of work and her carrots taste OK but again need a bit of work and aren’t very productive, but that’s probably the soil.

    She’s also planted a blackberry against a shed wall which has gone berserk and we’ve cupboards of jams, you can’t even give seasonal stuff away in the country. Also kicking around the garden are redcurrants and gooseberry plants, but the birds generally get to those first and she’s stopped using a net because too many were getting trapped.

    She has a couple of compost bins and nothing gets slung that can’t go on them and its my job to dig them out early spring for feed and uses something on the tomatoes. Other than that they need a lot of watering which she does every night if it doesn’t rain, but then she’s got lots of hanging baskets as well so is watering anyway.

  32. @BiND
    Yes, if you want a mentally relaxing, but physically exhausting, hobby – grow vegetables, either in your own garden or an allotment (£25 a year here in the Chilterns, half-plots also available). Just don’t try to pretend the produce is ‘free’, once your time has been costed at minimum wage 🙂

  33. For 30 years I had nothing to worry about but two window boxes, now I have a quarter-acre. With six very old apple trees I will be living on apple cake and pies until next spring. And spending an hour or two each week clearing up winfalls. Potatoes did well last year, beans this year, peas are nice but not much of a yield.
    To be honest it is probably more trouble growing vegetables than it is worth: time and money spent at the garden centre . It is a nice hobby now I am retired but if I didn’t have a patch of ground that has to be kept from growing out of control I would quite happily buy what I needed when I needed it.

  34. imo if growing own, grow expensive stuff – not pots, carrots etc

    Grow Sugar Snap, Berries, Russet Apples, Plums etc and some salad veg, perpetual spinach etc Some colour: edible flowers & leaves eg Nasturtiums

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