I think you are wasting energy considering lifting the nutes so much they should be as high as you can get them and still drain to avoid wasting all that lift. the operating costs will be labor minerals water and electricity. All of them are subject to unpredictable increase and energy is quite likely to go up so desighn for minimul waste is important. labor to tend these units will be a problem as rooftop foot traffic is typicly restricted as much as possible so you may have to make desighn changes that accomodate rooftop use such as deck or patio type retrofit of the buildings that could be extremely costly even in the original desighn. most rooftops are not meant for regular foot traffic or carts. Unless land price is extremely hi, which is unlikely with 2 story construction being common, i think a ground sytem would have a lower initial cost so compare the 2 before pursuing the idea too far. Also consider roof repair and how it would be accomodated. In the end i would think you could buy an acre of land cheaper than retrofit an existing roof of 1/4 that size or allow for increased roof repair budget to remove or work around the units. I would not let you put one on my building. Don't expect any building owners or managers to be very receptive to any idea that may get in the way of their primary function of the building, increase repair expense,or create any other type problems or annoyances. Also consider they will want to be payed quite well to over come unseen issues.

I saw your post & it sounds like you are putting your nutrient on the ground with a 3lift. That sounds appropriate to me  you need to get a certain amount of height to get the right angle of drainage for your system.
I have an NFT system; my channel is simply a descending gutter. I would not recommend the gutter braces for holding a system up  a strong wind would knock it down. They need to be screwed into something secure.
I have a NFT system with a 6Â lift. The pump needed for such a lift cost me $20.00. The only problem is it is noisy so I am thinking of converting to a NFT/wick hybrid system. The money I save in produce outweighs the cost I spent on the pump.
DonÂt be discouraged by those who disagree with your idea. I have worked in placed where they had outside gardens on the roof in raised concrete beds. It depends on the building.

In the Districto Federal (Mexico City), people garden on almost every roof I have seen. They either raise turkeys or vegetables.

It's an excellent use of space.

20 feet of pump head at the low volumes used in hydro is cheap, either for the pump purchase or operating costs.

It snows in Ct which is a thing you should consider for structural loading of hydro stuff.

Thanks everyone for your replies! I'm not worried about the hydro setup, i'm worried about getting the structure to stay on the roof and getting approval.

Willard3, do you know if they (in Mexico City) grow undercover, if so what are they using?

Thanks again,

Joe

PS- please excuse the fact that i double posted in other forums, I did not read the rules and for that I'm sorry and it wont happen again.

In Mex City, they grow right on the roof surface with nothing overhead.

Rarely rains......
Never snows.......

sorry i misunderstood sounded like you were planning a 30 foot lift every cycle. the type of buildings will have a good deal of influence on practicality. mexico is mostly concrete structures. USA is mostly tin over bar joists styrene and rubber over that. Have you priced renting bare ground? Not trying to be real negative but i can't imagine anyplace but downtown tokyo having valuble enough land to justify the added expenses of making a rooftop available for that type use. You may get universities or other experimental type buildings to give it a go when the project has no need for financial viability.just for referance in my locale a 20x20space in an office building will probably start at 3-4oo a month. A year of rent will buy you a bare acre suburban lot with electric available. i would expect the building manager to want at least that and likely much more to grant use and access to his roof top. If you do find a building that will allow you to experiment what will you grow and have you developed a market already? when you present a new idea it is always better if you can show at least some production and sale history to help sell the idea. Also if you have spent time on roofs you already know light heat and wind can be more intensified on rooftops than on the ground. Some roofs such as theaters or clubs get bottles thrown on them a lot. It is not that Iit is not a good idea or use of space but i just don't think land values are any where near high enough yet to make it financialy viable. Another concern is that even in areas of higher land values building values also tend to be rather high so it may still cost too much. when you search for possible places to place you may want to be prepared to show how many years experience you have producing and selling what plants as well as know exactly how much per square foot or unit it is going to cost to install and produce one season. will you be planning on daily tending several times or will you use expensive sophisticated equipment that regulates its own temp and ventilation? when you harvest and clean up how will you get up and down through the typical ladder and roof hatch? drop the produce by rope and bucket? I have been on a lot of rooftops servicing ac units and most of thm you either lean up a ladder or climb a permanet ladder through a hatch we use ropes and buckets to get our material tools and parts up and down seems rather awkward to handle any plant matter that way. Anything that weighs over 100 lbs or so such as a new unit we call a crane for. Most roof hatches are 2ft x 2ft aprox so will your materials fit. If you drag them up over the side you have to make sure you don't skin up the walls or break anything. Have you ever been on rooftops? some you have to walk only on the rows of screws and some you have to be careful not to step on air bubbles. Some have rubber mats layed out you have to stay on. some are slick when cold and you have to make sure you don't slide off and fall. some buildings like factories are so concerned with liability they require fall protection(harness and lanyard) to be on their roof even a flat roof with 3 ft parapet wall all around. I have never been on a rooftop the owner or manager was not extremely concerned over roof damge. Roof repair is quite costly as is roof damage and rain leak damage. In usa i would say your only chance would be some type of experimental or plant related type building a botanical gardens buiding a university a science center something like that and it will not be financialy viable but done as an exhibit.

Your issues are going to be:
- Weight - yes, NFT is the way to go but then you need to worry about backup power water, etc. for when, not if you have electric or pump or water supply failures.

- Traffic - as Mark indicated most US rooftops are not built for added weight or traffic and flat roofs with tar or rubber membranes are very delicate

- Heat- how do you plan to heat the hoops?

Access - most roofs do not have convenient stairs for regular traffic

- Drainage - these type of systems inevitably have leaks or runoff.

There are projects of this type in Canada. I've wanted to do this for years but my architect husband has cautioned me about all of the above. Yes, it can be done but it takes a special structure which is the exception rather than a rule. If your objective is to demonstrate urban growing you might look into the alternative of using "brownfields" instead, using contaminated lots which cannot be built on until they have been remediated. IN the meantime a non-soil disturbing activity like surface growing beds would be a good use if you can get owners and bureaucrates to agree.

Roof top gardening is very common in Japan. In fact, trees are grown sometimes. Of course the structure I am sure can take the weight of the garden. It's common in Manila as well. In fact many high rise buildings have their water tanks on the roof top. The engineers have taken into consideration the fact that Tokyo and Manila are earthquake prone. In any case best to check if your building can take the load. Check also the legal as well as the social implications of your venture.

Featured on NBC TV Asia was a roof top hydroponics system in Singapore. The set up is on the roof top of the Changi Hospital. The system consisting of large PVC tubes, supplies the vegetable needs of the hospital.

Here's an interesting article I came across on the Urban Vertical Farm concept. I thought you'd be interested....