coshotgun
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Hoop House Foundation Question and General Opinions

For anyone with greenhouse construction knowledge, PLEASE give me your opinions:

A little background... We have been planning a greenhouse for about 2.5 months now. I went through all sorts of research and finalized (we thought) on a Connely 18 x 24 aluminum framed commercial type greenhouse. Original quote was 6 grand for the kit which was based on what their website stated as standard engineering. Well when I started reading, the quote was not to the published specs, and so I had them give me a quote for the wind and snow load that was stated on their website. Turns out that added about 2 grand to the kit and shipping was now over 2 grand as well making the price over 10 grand and we have not even started with site prep, foundation etc. Finished greenhouse would have been in the 15 grand range easy.

So back to the drawing boards as this is supposed to be a back yard fun greenhouse, not a bank breaking commercial operation. I am not convinced that any of the Chineese knockoff type greenhouses are worth the money and or will hold up with the larger size we are looking for. So lot's more research and we decided to build a hoop style greenhouse to conserve on the finances but still something very nice and built to withstand Central Oregon weather including winds and snow.

I bought a pipe bender for making all the hoops and any and all bracing that will need to be bent to match the curve of the house. The hoops will be 3 pieces of 1 3/8 inch 17 ga galvanized pipe secured together with screws and clamps.

The structure: 20 x 28 foot high tunnel hoop house. The specifics:

12x12 concrete footings with anchor tie bolts every 2 feet. 2 PT 2x6's bolted to the concrete for the sill. 1 3/8 galvanized hoops spaced every 2 feet and secured to the sill on both sides with a 6 inch galvanized sleeve srewed into the sill with 4 large screws. Hoops will have a truss pattern in the middle top for extra strength.

7 runs of purlins clamped at every intersection of the hoops, with cross bracing in all directions. 7 anchor eye bolts in the middle of the walls and end s for 3/8 inch steel aircraft cable to tie in a cross brace fashion to the concrete. End walls will be framed with wood 2x4's, held securely to the end hoops with special clamps and the base screwed into the sill plate.

Added strength will be provided by 3 concrete piers in the middle with 4x4 posts and 2x8 ridge under the hoops from end to end in the middle to support weight if the snow does not melt or fall off in the winter.

Covering will be a double layer 6 mil UV protected thermal anti-condensate poly with 3 pressure fans secured to the sill all the way around with the wiggle wire system.

Cooling and fresh air provided in a 2 stage system, first stage is a 12" exhaust fan located on the north peak end wall with two motorized 24" intake louver vents on the north wall as well. Second stage ties into the same motorized intake vents with a 24" fan on the south end wall peak. When both fans are running it will move about 5,200 cfm which replaces all the air in the greenhouse in less than 60 seconds.

Main door will be a full split glass storm door with half to be opened when needed for extra air intake.

We will make 12" raised beds filled with potting soil and in between the beds will be 6 inches of washed gravel for the walkways. Gravel to provide a moisture cooling effect in the summer when sprayed with water. Plant water to be proved in the rear by our irrigation water pond, endless supply.

My big question is will a 12" x 12" concrete footing big enough for this structure.

Sound like it might work in a windy and snowy environment??? Any and all input greatly appreciated!!!

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rainbowgardener
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Wow!! sounds like you know way more about it than I do, can't help with your question.

But still sounds like a mammoth undertaking. Have you used greenhouses before? When I lived in zone 6, I had one for awhile, more like 6'x12'. But my experience even in a cold climate like that was that I used the greenhouse for a few weeks in the early spring and a few weeks in the late fall to extend the season. Since I wasn't about to spend the energy/money to heat it, I couldn't use it in winter and during the main growing season I didn't need it. I eventually got rid of it.

But I looked up climate data. Neither Bend, nor Burns sound that harsh -- average Jan low in Burns 17, 30" of snow, average July high 85. But then I looked at frost dates.... Wow. Burns has an average 3 months frost free season, mid June to mid Sept. Bend has an average ONE month frost free season, mid July to mid Aug. !! Why is that? elevation?

So that made it make sense to me why you would really want to have a greenhouse. With a lot of months where the lows are not that fierce, but frost is always possible, a greenhouse would make a big difference to you. Shows you how little hardiness zones tell you. Bend is 6b, Burns is 5b. I was in 6a in Cincinnati, but I had a frost free growing season of six months from mid April to mid October.

So best wishes! Hopefully someone will come by who can answer your questions.

coshotgun
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Elevation here in Bend is 3500 feet, and w have been known to have a little snow on the 4th of July, not often, but it can happen. Frost and freezing a night is the major concern. Last year we made a little 10x20 hoop house out of PVC by only using rebar in the soft ground to hold the hoops. That proved to be of little resistance to the elements here. By beginning of November it was on the ground. This year we are going for bear as the saying goes, I think it will hold up.

We have had a nice greenhouse years ago, a 10x16 lean too on the back side of our house. It was a glass house so we had to paint the glass with the whitewash sun shade every year in the summer. It was small compared to this one, but that was in western Washington where we had a great growing season and the only thing that wintered over was an orchid collection. It had a passive heating systom of water buckets up against the house, and it only took a little 1500w heater to keep the orchids happy all winter. That is where we cut our teeth on the greenhouse antics. The gravel was a great help in the summer as it cools the house down considerably and adds to the humidity that helps as well. We are going to do it again this time as it worked very nicely in that house.

I have made the decision to add a 2x4 to the top of the 2x12 forms for the footing making it 12 wide and 15" tall. That should gt us below the frost line and also add a little more strength. We will also run 2 @ 3/8 rebar in it as that only adds about 45 dollars to the total. I am much more comfortable with the footing now, and I think we should be great. The rest of the structure should hold up to both the snow load and wind ratings. I am still interested in what others have to say about the structure.

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Gary350
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Location: TN. 50 years of gardening experience.

coshotgun wrote:
So lot's more research and we decided to build a hoop style greenhouse to conserve on the finances but still something very nice and built to withstand Central Oregon weather including winds and snow.

Any and all input greatly appreciated!!!
I have built a few small green houses in the past but never had much money to do a good job. I have been working on ideas for my own 9'x14' green house this is one you might like.

Look at drawing on the right side. 8' long 2x4 boards with 6" blocks screwed the the ends keep them from pulling out of the ground. Compact the soil after grass roots grow there again it gets stronger. Years ago I tested this it takes about 500 lbs to pull each post out of the ground. 3 ft spacing on the 24 ft side you need 9 posts x 500 lbs = 4500 lbs not counting the end walls that have post too. I drew my roof at 30 degree angle you might like to make your root at 45 degrees for snow to slide off easier then use 12 ft long boards on the roof instead of 2x10 ft long boards like I did.

Look at the numbers it only takes $251 for pressure treated lumber to build the frame work. Another $117 for 2x4 board to cut down the center to make 2x2 boards to hold the polyethylene to the 2x4 frame with 2.5" long dry wall screws. I learned from experience, staples, roofing nails, duct tape, will not hold the plastic on wind blows it off we have lots of wind 55 to 60 mph. I was going to do post 3' apart but you can do 2' if that works better for you.

Only exact price I don't have is the plastic and dry wall screws. I looked at plastic a few months ago it comes in a roll 50'x100' for about $80 at Lowe's. 2.5" and 3" drywall screws are $3 lb at the hardware store. Looks to me like you can build your 18'x24' green house for about $700. not counting the door on each end.

Nice thing about screws, if you make a mistake or need to do repairs some day it is easy to fix. Also screws are stronger than nails.
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