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DIY: How to Build a Cheap Greenhouse

21 March 2011 26,509 views No Comment

Via DoorGarden.com-

* This Article is featured in Jan 2010 issue of Birds and Blooms Magazine!
* Want to find out if this thing works before you read all this? Read 6 months in the Greenhouse first.
* Want to see what happens when a few inches of wet snow accumulates on this? Collapse!
* Building the Greenhouse Doors is addressed in a separate article – isn’t this enough for one weekend?




Materials List

Construction Steps

Hind Sight – What I would do differently

The planning is over and construction on my hoop house greenhouse has begun. I’ve rounded up all of the materials and it looks like I’m going to end up with about $50 in a 165 square ft. green house. Granted I already had most of the materials because I’m an incorrigible pack rat, but even if I had bought everything new just for this polytunnel It would still only come to about $120-$150 (less than a dollar per square ft). Due to the fact that we are in the midst of a global economic meltdown, and the future is a bit uncertain keeping the cost of this project as low as possible is an important consideration.

After some research I’ve decided to build the structure of the hoop house out of 20 ft. joints of three quarter inch PVC plumbing pipe. Some similar greenhouse designs that I’ve run across use 10 ft joints of pipe and then fasten everything together with pipe fittings, but I’m saving quite a bit of cash with the long joints of pipe and by not using any fittings – also overall simplicity is improved. There is one thing though, you can carry 10 ft joints of pipe in the mini van, but hauling 20′ pipe requires a truck and preferably a ladder rack. However, you could just cut them in half right at the home improvement store and then put them back together when you get home with the coupling that is built into one end of the 20′ long pipe joints – 10′ pipe joints don’t have the built in couplers – just go to the home improvement store prepared with a saw or pipe cutter.

My hoop house green house is going to be 11 feet wide and 15 feet long, and will be about seven and a half feet tall in the center. You could make one of these as long or as short as you want, but using this design the width needs to be between 10-12 feet. 11 feet wide just happpened to work out with the layout of my garden which has 3 foot wide beds with 5 ft paths between (the wide paths are so that I can keep it tidy with my riding lawn mower) so eleven feet covers two beds and the path between them. This width also makes the sides go fairly straight up from the ground for the first few feet – I’ve noticed that in some hoop house / polytunnel designs the outer edges are almost unusable because of the slope of the greenhouse sides.

If your Greenhouse is too Flat it will collapse!

You might be tempted to make your greenhouse wider and lower at this point to get more floor space out of it – but be careful. If you have snow in your area it will slide off of a high peak a lot better than it will if your greenhouse has more of a flattened shape – and the same goes for heavy rains. If your hoop house shape is too flattened it will cave in the first time it snows or rains really hard!

How to Build the $50 Hoop House

I decided to begin the construction by building the end walls first – even though it would be more fun to throw up the main structure in just an hour or so and make a big showing of progress, I think that in the long run it will be quicker and easier to build the end frames first on my garage floor.

I temporarily attached a joint of pipe to a piece of 1×4 to establish the outline. You might be tempted to make your greenhouse wider and lower at this point to get more floor space out of it – but be careful. If you have snow in your area it will slide off of a high peak a lot better than it will if your greenhouse has more of a flattened shape – and the same goes for heavy rains. If your greenhouse is to flattened it will cave in the first time is snows or rains really hard!

Pre-drill the pipe and use one screw so that the pipe can swivel to whatever angle it naturally aligns to. For now just let the wood "run wild"

I used pressure treated lumber for much of the polytunnel end frames even though I usually try to avoid treated wood in the garden. In this case I think it’s called for or else the greenhouse probably wouldn’t last more than 2-3 years without rebuilding the frame. In any event I’ll try to keep it off of the soil as much as possible.

Build the rest of the frame to accommodate the door size that you want to use. My door will be 5 feet wide, but in most cases 3' wide would be adequate. If you want a more permanant greenhouse or you live where you will ever get more than an inch of snow you should use "two by" lumber instead of "one by" that is shown here.

Leave the piece that runs across the bottom of the door in place for now. Once everything is set in place it will be easy to cut out with a hand saw.

I used a bit of weather resistand glue at all of the joints to help make it all more rigid. Notice the wood recycled from concrete form lumber.

Mark the final outline once the wooden parts are assembled. Watch out for that screw when you saw to the line!

Now just trim to the line – I used a reciprocating/sabre saw, but you could also use a hand saw or circular saw if it's all you have. Just make a straight cut in about the right place.

Now re-attach the pipe to the outside of the frame. I used screws and wire ties because I'm a belt *and* suspenders kind of guy.

The end wall frames ended up being reasonably light and very rigid. BTW, you might notice that the second one is different (simpler) from the first because this is a learn-as-I go process. Both of them work fine though.

back side of the green house end frame

and the front side… Notice that the plastic that will be the roof and sides of your greenhouse are going to fold over the ends and staple to the wooden parts. If you don't have enough wooden structure in the ends you won't be able to fasten the plastic and you will have trouble with it coming lose when the wind blows – and during heavy rain or snow you will be more likely to have problems with the very top sagging and holding water (or snow). If the top sags, it holds water, that makes it heavy and it sags more, then it holds more water… eventually it collapses. You don't want that

This is the front side because it is all on one plane so that the plastic skin will lay flat on it. The back side has reinforcement gussets that stick out.

And Now for the Plastic

The plastic sheeting that I’m using is plain old non-UV stabilized 6 mil “clear” plastic sheeting from the lumber yard. There is exactly one reason that I am using this particular variety instead of special polytunnel / greenhouse plastic – it’s what I have. I cut a 22′ piece off of a 100′ x 20′ roll that I already had (I’m a contractor) which was about $90 for the roll – so in essence I used about $22 worth of plastic sheeting after you apply the 10% TN sales tax. Had I ordered real green house plastic from littlegreenhouse.com a similar sized piece of 6 mil plastic would have been about $71 with shipping. The real deal would no doubt last much longer than the “visqueen” that I’m using, and also probably has better thermal and light transmittance. If all goes well maybe I’ll get some of that next year. Also, It’s hard to buy large pieces of heavy duty plastic like this without buying a whole roll, so unless you know a contractor or Mom and Pop hardware store that will cut you a piece you might really be better off ordering some of the good stuff. On the other hand a big roll of plastic sheet is one of those things that comes in awfully handy some times.

I just rolled the plastic out on the frame… Notice the falling leaves – I'm racing against fall weather with this project.

and cut it off nice and clean with a sharp utility knife. A scrap of wood to cut over and a sharp knife make this much easier.

after stapling the plastic to the front, flip the frame over and fold over the plastic and staple it to the back. Just fold the excess together as you go. Fold in the direction that will be down so that condensation won't collect under the folds.

then trim off the excess. Be careful not to make a miss-cut!

Cut the plastic out of the door opening – leave enough to fold double before stapling it to the frame. Notice the cuts back to the corners of at the top.

Thusly

Mark out the locations of the door sides on the ground, and drive fence posts or long pieces of rebar at the sides of the door frame.

If you have much wind I would recommend using steel fence posts or rebar that is at least 5/8″ diameter in these spots. My fence posts don’t match because they’re left overs from previous projects – remember, I’m on a tight budget!

Check the fence posts for plumb and bend them a little if they need straightening.

Tie the hoop house end frames to the fence posts with wire ties, wire or rope.

The humongous wire ties made this really quick easy and strong, but If I didn’t already have them I would just use “baling” wire, and it would work as well.

Once the end frames are in place pull a string to line up the stakes for the ribs.

Drive rebar pins every 3 feet to secure the intermediate ribs…

At this point it takes about 2 minutes to install the pvc pipes for the intermediate ribs – and the polytunnel takes shape. For a stronger, more permanant structure use more ribs and put them closer together – or even use larger pipe. I haven't tried it but I bet you could use up to 1 1/2" pipe – although you might have to bend it into shape on a warm day.

As you can see it’s getting dark, and I’ll have to finish this later. Total time invested so far is about 2 1/2 hours. I believe that taking the greenhouse down next summer, and re-assembling it in the fall will probably only take an hour or so, but I guess I’ll see about that.

Since the site location where I’m building my greenhouse isn’t all that level I had to raise up one side of the end frames with some 2x6s that I ripped to fit – later I cut the tail off where it sticks out toward the fence. Also notice that this means that the PVC pipes that are the intermediate ribs are too low where they hit the ground…

So I extended them with some scraps of PVC conduit that I had – I never throw anything away. BTW, the gray PVC conduit is sunlight resistant unlike the white – although somewhat more expensive and is less expensive! If you want to do a really good job you could use it instead. You should probably use the gray conduit instead of the white pipe that I used. You could even opt for schedule 80 conduit which is much thicker if you wanted to go whole hog, or if you needed to make a structure that is sturdier, more permanent or wider.

If you live where it snows – as pictured this greenhouse structure alone won’t stand up to snow accumulation. But if you put a 2×6 or 2×8 ridge pole right down the middle to keep the peak from sagging it will be a lot stronger.

I strung it all together with 1x2s that I ripped out of some slightly used 2×4 studs. Using full 1x4s or even 2x4s for the top set of these would make the structure stronger.

I used a few wire ties to get everything located, and then drove a 1 1/4″ drywall screw at each joint to secure it. As you can see by the lay out marks, I first measured and marked all of the locations so that it would go together reasonably straight.

If you look really close in this picture you will see the wires that serve as X bracing on the sides.

I used a doubled wire that I attached at the top and bottom of the ends using a washer and a screw.

I then used some scraps of wood to twist the double wires together and tighten them up like a rubber band airplane. You just want them to be snug so don’t go nuts tightening them up. These wires really go a long way to make the whole structure more rigid and sturdy.

Now for the plastic covering – measure and cut your piece of plastic – you want a little extra in all directions – the piece that I used is 20′ x 22′.

My greenhouse is 15′ feet long so I cut a 2×2 x 15′ – Here I’m positioning it in the center of one of the 20′ edges of the plastic – leaving 2 1/2 of plastic past the ends of the 2×2. Staple it together just to hold it in position.

Now roll the 2×2 under one complete turn so that the edge you stapled is facing up under the top layer of plastic sheet.

Now screw a 1×2 on to secure the plastic. By wrapping the plastic around the 2×2, and then sandwiching 2 layers between the 2 pieces of wood you make a very secure connection, and also add some weight to the bottom edges to help keep them from billowing up in the wind. Do the same thing to the opposite edge, and then roll it all up and get someone to help you carry it to the hoop house and unroll it across the top…

Thusly. Now you almost have a greenhouse.

Roll under the edges on the ends and staple them securely (Note: now that I have taken this down for the summer, I think that when I put it back up next fall instead of “stapling it securely” I’m going to just staple it a little bit to get it positioned, and then screw battens made of 1×2 or strips of plywood to hold it in place – it should be stronger and quicker), and other than the doors the structure of your polytunnel greenhouse is finished. Total time at this point – about 6 hours. Everything is a bigger job than it seems like it’s going to be. Rake soil or mulch up to the gaps at the bottom to keep out drafts and (larger) critters. Cats in particular are likely to be attracted to such a nice sheltered spot with a bed full of soft loose dirt to dig in so pay attention to the details. Rocks, bricks or concrete stepping stones or blocks placed on top of the soil/mulch around the outside edges are probably a good idea.

Before I even started on the hoop house I tilled copious amounts of compost into the beds where the greenhouse was going to end up. So, even though I probably won’t get a chance to put up the doors until next weekend (which is Halloween), I’m all ready to plant some lettuce and spinach for (hopefully) some fresh mid winter greens. One of my goals in building this polytunnel is to have something fresh coming out of the garden or greenhouse all year long. That might be a little optimistic, but I’m going to give it a shot.

Addendum:

Complete Materials list for the “$50 Greenhouse” – As Built

Each Qty Total
$4. 23 (6) $25.38 20′ x 3/4″ PVC schedule 40 plumbing pipe
$6.70 (6) $40.20 1x6x8′ pt – ripped into 1x3s
$4.99 (4) $19.96 8′ steel “T” fence post
$2.18 (3) $6.54 2×4 stud – rip into 1x2s
$3.97 (2) $7.94 1x4x12′ pt
$5.73 (1) $5.73 2x4x16′ rip into 2x2s
$7.91 (1) $7.91 20′x1/2″ rebar – cut into 18′ lengths
$4.88 (.75) $3.66 8″ nylon wire ties – 100
$5.47 (.5) $2.74 1 1/4″ x 1lb drywall screws
$6.97 (.3) $2.09 16 guage galvanized utility wire – 200′ – for X braces
$2.97 (.25) $0.74 3/8″ t-50 staples – 1000
$79.00 (.22) $17.38 20′ x 100′ x 6 mil clear plastic
Scraps of plywood for reinforcements – scrounged
$140.27 Total

In the comments:

Deb says: August 23, 2009 at 5:36 am

I built this a couple weekends ago and it cost $136 and some change from Home Depot. While it is more than $50 in reality in August 2009, it is an easy and fun project you can do in an afternoon the first time around easily.

OK, that’s a bit more than I estimated because I didn’t count all of the minor bits that I just take for granted because I buy them in bulk and keep them on hand. These prices are what you would pay if you just bought the quantities that you need for this project. Drywall screws for example are only about a third as much when you buy a 25 lb box like I do being a contractor. However, you can shave most of the 20 dollar overage by using 5/8 rebar instead of fence posts, and gray UV resistant PVC conduit which is actually less expensive than the non resistant white plumbing pipe that I used – you can also do without wire ties, and use scraps of wire instead, etc. I already had everything on hand except for the PVC pipe and a couple of pieces of 1×6 lumber. However, surely almost anyone can get some of this stuff for cheap or free if they put some time and effort into it – so shop around and use your imagination to find what will work best for you.

Admittedly it might be hard for most people to build this for just $50 out of pocket, but then again a serious scrounger can probably do it for even less.

Hind Sight – What I would do Differently

1. As originally built this hoophouse design is subject to collapse under even a moderate snow load. It has to be beefed up a bit.
2. Use UV resistant gray PVC Conduit instead of white plumbing pipe – it should last longer and is actually less expensive
3. Use UV resistant greenhouse plastic instead of “visqueen” construction plastic – It’s a good bit more expensive, but I’m so pleased with how the polytunnel turned out so far that I’m pretty sure the investment in durability would be worth while. However, if the extra expense meant putting off the project I would go ahead and use the cheap plastic because 1) The plastic will have to be replaced sooner or later anyway and the difference in cost seems proportional to the difference in life span 2) It wouldn’t be worth putting off having a perfectly usable greenhouse.
4. I should have painted the PVC pipes with latex paint before applying the plastic sheeting – apparently this makes the poly sheet last longer, and maybe makes the frame pipes more resistant to UV.
5. Site Selection – As you can see in the pictures my garden is in a clearing in the woods and the truth is it doesn’t get as much sun as I would like for it to – however I can still grow a nice garden – it just doesn’t yield as much as it might. I can’t really do very much about this, but you should keep in mind that you want as much sun as possible as well as a sheltered well drained spot that is as close to the kitchen as possible so that it isn’t too much trouble to trot out and get a bowl of fresh lettuce for supper. All that being said – do the best you can with the spot you have, and you might be surprised with the results you can get with a little effort.
6. Next time I think I will use screws and battens made of plywood strips to secure the main skin to the end walls (I’m not talking about the end wall skin here) so that instead of using 200 staples I will use a couple of dozen screws to accomplish the same thing, but it will make it quicker to put up and take down. When I do it I’ll add pictures for clarification.

Other homebuilt greenhouses:

* PVC greenhouse – quite similar to mine but with a few differences that are very worth looking at.
* A great wood framed greenhouse design – great construction details.
* An excellent article on high tunnel greenhouses by The University of Vermont.

The Greenhouse doors are built now and my small hoop house is complete.

Update – Dec. 10, 2008 – we had 2″ of rain and 30 mile per hour winds last night and so far so good – no damage to the greenhouse.

Does it really work? Does the wind blow it away? Is it even worth the effort? Read 6 months in the Greenhouse.

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