I think you are on the right track. Keeping the film up will protect your plants from adverse weather. Last season we had so much rain that all of our outside tomatoes suffered with disease, not to mention a lot of the fruits cracked. The tomatoes in the high tunnel did great.

You shouldn't need to drop the sides at all during the season, just late in the season when cold threatens. It shouldn't get too hot with the ends off and the sides raised. We did have an exhaust type fan stationed at the end of the house for the really hot days, and would occasionally leave that on. I would suggest soaker hoses or drip tape in your beds - the soil will get dry fast.

What sort of film are you thinking of using? Make sure it's UV, preferably reinforced. I'm wondering if you're underestimating the loads your film will have to withstand?

Since you're building your frame out of rebar, you won't be able to secure the film against it. You'll have to run ropes over the top, which will be a pain to remove and readjust if you plan to roll up the plastic, then roll down again. A light wind could rip the whole kit-and-kaboodle free and send it into the next county. Plus, unless you cover the rebar with something soft and durable, you'll have rubbing/wearing issues to contend with. Rebar is rough in texture, unlike the hoops normally used for gh's. Finally, I'm not confident that ordinary film can support snow and ice on two-foot spans.

I think you'd go through the same amount of effort to build a wooden structure, that would avoid some of the problems rebar will give you. You could staple most of the film down, leaving areas you could open for venting purposes if necessary.

One other thing: I'm not sure you could attach the ends of the rebar adequately against the 2x10s to survive the astronomical lateral forces you'll get when the wind blows against the structure. What sort of hardware would you use? Aluminum/steel hoophouses use forged attachment points.

BTW, I'm originally from the Utica area, and know what sort of weather considerations you have.

-Bruce

ekgrows, thanks for the comments. We also had garden damage last year due to extreme weather, so the idea of having a cover in place all the time is very appealing to me. As you point out, the price I'll pay for that coverage is that I'll need to be more generous and diligent with the watering. But my biggest concern was that it might get too hot in there, and your comments indicate you think it will be manageable, which is reassuring.

Bruce, great points. You've hit on some that have crossed my mind before, so let me tell you what I was thinking.

I was going to use 21' 4 year 6 mil CLEAR Greenhouse Film from Growerssolution.com. I was going to be putting the rebar at 2' centers, but I was also going to be running rebars longitudinally under the hoop bars at about 2' centers as well, at least on the upper, more horizontal part when the snow will tend to sit until I can get it off. That means that the film would be supported in both directions on a 2' x 2' grid. It seems to me that the film should readily hold that weight, and I'm confident I can shore up the middle of the greenhouse so that the frame won't collapse.

The roughness of the rebar is an issue that I knew I would have to deal with. I am definitely going to have to cover it with something, and I'm not sure yet what that will be. One thought is the kind of foam wrap that you find used on some packaging. It's usually white and comes in rolls. It will be a bit tedious putting it on and taping it, but I should only have to do it once. I thought of 1/2 pipe insulation, but that seems too thick and expensive. Any suggestions for another material?

As for securing the film, my plan is to secure all the edges with a wood battens. I've seen pictures and details of that being done in book The 12 Month Gardener by Jeff Ashton. The ends will have wood framing, so I can secure them easily with staples and battens. The sides will be a lot more interesting, since I'm planning a way to be able roll up the sides a bit in warm weather. I don't have that fully detailed yet, but it will involve a 20' 2x4 secured to the rebar hoops about 16" up from the bottom, and the film will be secured to that 2x4 with staples and a wood batten. The bottom 16" on each side would be handled with a batten against the 2x10 frame in the cold weather, but perhaps weighted during the warmer weather so that it could be readily put up and down. Jeff also has a detail for that, although I'm sure I'll do something a bit different than his detail. Still thinking...

I don't think I'll need ropes, but maybe you have a strong point because of the open ends. When it's close in the winter, the force of the wind is from the outside blowing against the rebar frame, but in the summer with the open ends, the wind can blow in and push outward on the opposite side. Hmmm, I'll have to think about that one. A few ropes over the top may be needed at a minimum. Good point. Fortunately, although it's plenty windy in our area, my garden location is somewhat sheltered.

That said, I don't think the wind load against the outside frame will be that enormous. I am thinking of using pieces of 1/2" emt conduit to attach the rebars to the frame. I'll drive the conduit a foot of so into the ground on the inside edge of the 2x10 at each rebar location, and anchor it to the frame with emt anchors. Then the rebars will slide down inside the emt. I'm also going to set the 2x10's on the 20' sides at a small angle, like 10 degrees, pointed toward the middle of the greenhouse so that the rebar starts out already heading in the right direction, instead of pointing straight up into the air.

Great points. Does any that make sense to you?

Cover your rebar with old rubber water hose....or cheap new water hose. Thats what i have on my small rebar tunnel. Also keeps the rust down.

Sounds like you've really thought this project out thoroughly! I think I follow what you have planned.

The main criticism I have is with the 6-mil film you plan to use. Having used 6-mil myself for several years before arriving at the product I use now, I can say that it's flimsy and easily prone to tearing. Commercial gh's that use inflated film covering will use 11-mil on the exterior, and, if they're trying to cut costs, will use the 6-mil on the inside because it doesn't require as much durability.

By attaching your film just on the edges of the structure, you'll be asking for trouble. 6-mil stretches, and no matter how tight you pull it, it'll still flap in the wind. Also, with so much potential force focused on a single attachment point along the edge, it'll be prone to pulling free...unless you somehow roll it a couple times around something to evenly-distribute the force.

The problem with the wind blowing over it is the element of "lift." Depending upon the wind direction, even with the ends open, the covering will alternately want to rise, then snap back down against the frame. A few hours of windy conditions is all you could ask of 6-mil.

Even though the cost is higher, I'd highly recommend the 5.2-ounce translucent tarp linked below. It's 12-mil, and has a woven fabric of fibreglas "tapes" inside the plastic for strength. It's like iron, doesn't stretch, and can withstand just about anything you throw at it. Since it's stronger, and heavier, you'll have less of an issue with it tearing free from attachment points. You also may be able to get away with wider spans, saving you on some rebar. Another plus is the light diffusion through it -- no possibility of burning tender plants/seedlings.

I'd have much greater confidence in this material meeting your needs, only attaching it along the edges.

The one weakness as I see it for your application is that the largest width available is twelve feet. FarmTek has two products that would be suitable for this purpose. The first one is their Super Tape Seaming & Repair Tape, which is $22.95 for a 3"x60' roll, or $32.95 for a 6"x60' roll. The second is the Premium Repair Tape (product #105383), which is $71.95 for a 4"x100' roll. I'd feel safest sealing both the outside and inside edge, with a 6" overlap of the tarp.

Hope your idea for securing the framing against possible racking works OK. If it didn't for some reason, you could retrofit easily with a support line and stake on either end.

If you can find a good source for purchasing the wrapping material (to cover the rebar) you described in bulk, that's probably a good bet. I know FarmTek sells something for this purpose, but I'll be darned if I can find it now. The only thing I'd suggest is to use a good-quality tape. I know duct tape is a popular do-all, fix-all, but it performs miserably whenever moisture and heat are present! For this reason, you may want to consider a wire-wrap over the packing material, as an inexpensive material to tightly secure it.

Removing the covering in the summer will expose all your handiwork to the elements, so that's another thing to consider. All that work to wrap your rebar may not survive unprotected.

Hope there was something helpful here.

-Bruce

Here is a link that might be useful: 5.2-ounce fabric at FarmTek

Bruce, I _HAVE_ done a lot of thinking about it, but despite that, you've pointed out a fatal flaw in my plan - the film.

I know a great deal about wood, rebars and building things (was a homebuilder for 15 years) but I know nothing about greenhouse film and growing in a greenhouse environment. I clearly thought that the greenhouse film I had figured to use - 6 mil CLEAR Greenhouse Film from Growerssolution.com - would do the job because it was "greenhouse film" after all, which the website reports is good for at least 4 years. But I had no experience with it, and experience is what counts.

It sounds like that would have been a pretty bad move, since I definitely want it to last more than "a few hours in windy conditions", as you put it.

So, now I'm planning to upgrade to the 5.2-ounce translucent tarp you called out. The product page calls it clear, but I assume it's really more translucent than clear.

To cover, I need a piece about 21' square. I'm thinking I will join the two pieces as you described with a 6" double taped overlap and then run that seam up and over the greenhouse halfway down the length, rather than run the seam down the middle at that ridge.

I agree with your tape comments about holding the wrapping material on the rebars. I had thought of that, and also realized that the need for wrapping them precluded removing the film in the summer. It will be tedious to wrap it all once, but absurd to do that every year.

What is the "wire wrap" that you mentioned for securing the material? Is that an actual product, or you just saying to use thin wire to spiral around it?

Bruce, thanks for all your input. You may have saved the day...

Glad that I've been able to help think through your project a little. There are a lot of smart folks on this Forum, although most have standard hoophouses or prefab gh's with polycarbonate panels. I'm one of the few who have experimented with a few different materials over the years in an effort to have a successful Rube Goldberg structure "on the cheap," and have worked out most of the kinks, so to speak!

Just to clarify a bit -- the 6-mil works fine for a standard do-it-yourself design when there are battens and multiple attachment points. IMHO, four years would be EXTREMELY optimistic, but under perfect conditions, it has a reasonable lifespan. Since you have experience homebuilding, you're familiar with the 6-mil construction film. This is the identical thickness (although with added UV inhibitors), so, you can imagine how a 20-foot wide piece would fare out in the sun and wind. The problem would be adapting the 6-mil to your unique design requirements. Errant hand pressure can cause 6-mil to tear through a staple.

I have never seen the exact 6-mil material that GrowersSolution is selling, but oftentimes when manufacturers call their product "clear," it's really translucent. I've seen 4-mil film I could call clear, but you would never use that stuff (seriously, anyway) on a gh. But let's say that the 6-mil material you were considering is *reasonably* clear, and transmits 85-90% light. (90% would be high, and I doubt it's that clear.) By comparison, I would estimate that the 5.2-ounce fabric I recommended would be in the 75-80% range. I grow orchids, so this is plenty of light...even requiring further shading for sensitive species. I've never grown vegetables inside my gh, so I can't offer any experience in that respect. They may, and probably will, do just fine with 80% light, but I can't say for certain. Watermelons might be a different story. Everything's a compromise...

As an alternative to the rather expensive seaming tape, if you have to make a minor repair, I can offer a cheaper solution. A couple years ago I had a local kid start to vandalize my gh with a knife, then thought better of it after leaving an 18" slice in the fabric. After a lot of grumbling and cursing (!), I cut a 20" strip of scrap fabric, smeared Goop over the cut, and pressed the strip on top. Then I went on the inside and placed my hands on the cut for support, and had my wife work at it from the outside to squeeze it down firmly. It's lasted well, although if it was at a place where there was constant flexing, or additional strength required, it may not do quite as well.

FarmTek is a great company, and they have always been fantastic in sending sample materials on request. If you'd like a piece before you make a final decision, give them a call. Or, send me a message with your address, and I'd be happy to send a piece from my scrap pile. (I save everything!)

As far as wrapping the material around your rebar, all I meant was a thin-gauge wire to secure it.

-Bruce

Bruce, thanks for the insights and the sample offer. Your comments are making me think. I certainly have used 6 mil poly and know it's limitations. Somehow, I was thinking THIS 6 mil poly was different - actually, I based that assumption on what I saw on their website. Here is what they say on the Grower's Solution website (I put the link below):
------
This tough polyethylene unfolds easily, installs quickly, and stands up to 4 years of UV bombardment. This Greenhouse Plastic is as good as it gets! These plastics/films are made from the toughest polyethylene resin in the business, on a machine that stretches it out and makes it thin. Many people used to buy Brand Visqueen in Plastic. This has taken its place.

Because of production methods that produce this thin, yet durable polyethylene resin, greenhouse plants benefit from optimum light transmission for optimum plant growth. Infra-red additives helps to maintain a more stable greenhouse environment and reduces heat loss.

Increase the life of your greenhouse film by cleaning it with soap and water every six months. Greenhouse Plastic has a 4 year guarantee, but I have known growers to get up to 8 years out of their greenhouse film!
* 4 year, 6 mil
* Clear plastic
* UV Treated
* 91% Light Transmission
-------------

So they're making pretty impressive claims, but if it isn't any tougher then regular 6 mil poly, then you are absolutely correct. Notice they say 91% light transmission, so it is quite clear.

With that in mind, I think I'm going to contact them to see what they say about my case, where I don't have battens to keep the material from flapping. I'll be curious to see if they still say it's tough enough, or if they'll suggest that I secure it with ropes, as you suggested early on. From a strength point of view, I sure feel better about your 12 mil product, although I hate having to seam it.

I'll let you know what I find out.

Here is a link that might be useful: Grower's Solution website

I wrote to Grower's Solution today and got a very quick reply with a name and phone number to call. I called and talked to Roger Fisk who told me that virtually all their greenhouses are like mine ie only secured around the perimeter. Roger assured me that his film material would hold up just fine, but that I should definitely be using 2 3/4" wind strapping to go over the top of the structure to secure it against wind flapping. He pointed to a picture on their site and I've included the link below.

I also found a site that gave a great deal of information about building a stoop house with rebar. It can be found at http://www.northerngreenhouse.com/ideas/how_to/rebar.htm

On the same site, I found a nifty product for securing film to a structure without putting holes in the film, providing continuous anchorage. Called Polyfastener PR-800, it's a 2 part plastic that allows the film to be held firmly, but can be released at any time with a hooked tool, so that you can remove the film or tighten it up. You screw this continuous plastic channel to your structure, stretch the film across it, and use the tool to force the second member in place, securing the film. It can also be used to splice film. It also bends, so you can use it on the endwalls. It's not cheap, but it looks very, very handy.

I hope that might help someone else looking for a way to secure their film.

Here is a link that might be useful: Wind stapping on greenhouse

Glad you were able to find the right product to make the 6-mil work for you!

-Bruce