Here is a site that may give you some ideas. http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#Greenhouse%20Plans

If you build your own I would suggest either aluminum or galvanized steel for long life. This is a link to one of the major suppliers of galvanized steel to the industry. http://www.atc-mechanical.com/applications/greenhouse-structures/

Many of us on here have built our own greenhouses. It may take some digging on here but it is well worth your time for the ideas you can get. Your heat source is also something to consider from the beginning. I have radiant heat in my concrete floor which is heated by circulating hot water through Pex tubing in the floor (you would need a circulating fan to make this effective). The heat source is an on demand hot water heater using propane fuel. You might consider some version of a Modine heater fired by natural gas or propane. With a Modine the heat would be circulated with a fan that is part of the unit. Some people use wood stoves or pellet stoves.

Good luck with your project !

Hi, If you want it certain size they don't sell and have a look to it, do it yourself it's fun. I would suggest a hoop house look. Are your plants for growing zones 9 or 10? Heating will get ya with something that size in z6. It's just harder to regulate a winter temp. It can be very common that things go into dormancy through the dead of winter.

Thank you all for the tips and advice. For a Hoop house, what is a temperature that can be achieved in the winter. I have mainly tropicals like bananas (musa manzano, basjo, and dwarf cavendish), White bird of paradise, palms (chinese windmill, pygmy date palm, several mexican fan palm seedlings), papaya plants, avocado, mango and lemons.

Is there a way to keep those alive and growing in a hoop house?

I had looked at one time at wood for the frame for a traditional greenhouse (since its the cheapest), but with the humidity, I know that this is not a long term solution. I don't know how to work with aluminum or galvanized steel enough. Especially connecting everything and attaching covering like multi-walled polycarbonate.

Here is a pic from early March last year (2017)

If you used treated wood I wouldn't see a problem. The last GH had a wood frame 7 or 8 feet up. The roof was the only issue so I made this one..

A decent winter temp from Dec-Feb could be anywhere from 52-68F. Lows of no less then high 30's low 40'sF. A lot of things go dormant like palms, bananas, figs avocado, papaya did alright until Dec but died when we had the extreme cold. Things like cordylines, yuccas some passion vine, spider plants and most citrus do fine in that weather and continue to grow all winter. Spring is as early as Feb by March it can get pretty warm. The only thing I can say is just experiment. You'll get a feel for the climate you create and is very rewarding.

As you are not comfortable working with metal from scratch you might consider a carport frame built to your specs as far as height, width and depth. Versa Tube I think is one that come prefabricated . Then you could cover it with the material of your choice. There are You Tube videos showing their construction.

I just looked at the Versa Tube products and I might try this. Going to crunch some numbers and going to see what else I would need. I will have to get a quote on the cost to run a Natural Gas line from the house to supply the heating for this. Even with electric I would need to have someone come out to do that work since most of those are 220V and need to be hooked up by a pro.

So if things go by plan I will break ground the first week of Spring.

A good calculator for greenhouse heater requirements is found at :
greenhouse heater calculator

Using your zone 6b, the hardiness zone map shows -5 to 0 F winter lows. Assuming 20 X 16 X 12 single poly hoop-house, and maintaining 32 F. inside, it would take at least a 40,000 btu/hr heater. With no added heat, a single layer poly hoop-house will be the same temperature as outside by early morning in the winter. A summer ventilation system would also be needed, since the temps. inside will be very high by mid-day without adequate ventilation and some sort of mist/evaporation cooling system.

If you are going to go with polyfilm, why not go with a double insulated film covering with a blower? This will give you roughly the same R value as a 4mm twinwall polycarbonate. It will cost you less to heat than a single polyfilm greenhouse.

Poly Inflation Fan Kit