I was offered an allotment in June. As it was late in the sowing season I decided to spend my time getting it in shape for next spring. Having struggled with a cheap walk-in type greenhouse for the last couple of years I immediately thought 'Polytunnel'. After research I decided on a size of 20' x 10'. At around 800 euros it was way out of my price range so I looked into DIY options using recycled and scrounged materials. I found instructions that used 2 inch PVC water pipe sleeved over lengths of cut scaffold tube for the main structure. I aimed to do it for around 200. This is the result.
First step was to clear the site of weeds and debris that I inherited. I did this by hand using a fork and a Chillington hoe. It was hard work but worth it. I have very few weeds coming up now whereas I believe if you rotavate you can churn up a lot of weed seeds and chop up stems and roots and increase your weed problem.
The tunnel would have 2 raised beds along each side with a central path. Using 9" high scaffold planks would give nice deep beds (plus single dug beneath) but it would cost a few hundred euro to import topsoil to fill them. So I excavated the topsoil to a depth of 7 - 8 inches and set that aside to go back into the raised beds once the tunnel was complete. Stones would be screened out and compost/manure/seaweed added later in the year.
Once that was done I marked out a level plane, levelled the area and then marked out the tunnel footprint leaving plenty of space around it to work.
It is important to get the right angles correct for the stability of the tunnel so I used the 3-4-5 rule (pythagoras' theorem). Then I measured out the distances for the 5 scaffold tubes on each side and marked their positions with rebar.
I managed to scrounge a few 21 foot lengths of bent scaffold pipe for free. I cut 10 x 5.5 foot straight lengths from these and used them as the foundation tubes. Each one was driven a meter into the ground using a sledge hammer. Bloody hard work getting them in and keeping them straight. I would say it is a must to have a helper with you on this part. Make sure they are all in line and distanced the same as the one on the opposite side. Ensure that the tops are all level with each other.
The ends of the scaffold will get mashed out of shape from hammering so will need to be taken off with an angle grinder or hacksaw so the PVC pipe will sleeve onto the tube. I didn't have that problem ...
I tried to get my hands on free/second hand 2" internal diameter PVC water pipe as it is a perfect fit to sleeve over the scaffold tubes. A bolt or nail is then driven through both of them low down to tie them together. But I couldn't find any. It can be bought new from builders and plumbers merchants but is expensive at around 100 euro for 30 metres. But persistence paid off and I did manage to do a trade for enough electrical duct with a chap who had some surplus. Thanks R!
The problem with the electrical duct is that it is too narrow to fit over the scaffold tubes. I decided on a plan to span the two using 3 foot lengths of rebar. Again I got the rebar free.
6 inch nails were pushed through 6mm drilled holes in the pipes. Two lengths of rebar with bent ends were taped together in such a way as to hit the inside of both the scaffold tube and the PCV pipe so the pipe wouldn't move North, South, East or West horizontally. Just before I pushed the PVC pipe down on to the rebar a third shorter length of rebar was pushed in to give added stability. It worked, and there was no give on the pipe.
At last I could see how big the tunnel would be. Each piece of pipe was 6 metres long. They need to be exactly the same length so measure twice and cut once!
The pipes were tied to the scaffold tubes using builder's band which I had already ...
The next step was the internal woodwork to give the tunnel rigidity and stability against the weather. But my wife headed off for the weekend with the camera. So fast forwarding ...
... the nearly finished tunnel with labourer doing a great job hammering some topsoil into shape.
Scaffold boards were fixed to the exterior of the scaffold pipes using the same 6 inch nails that hold the rebar in place to span the PVC pipe. Builders band was also used to give additional hold and then the nails were flattened down. The boards will also serve as the perimeter of the internal raised beds.
I also drove wooden stakes into the ground at intervals and the scaffold boards were screwed and nailed onto these for added stability.
Thinner scaffold pipe, also sourced free, was driven 3 foot into the ground and the gable end scaffold boards were secured to these using builders band (great stuff! get a roll if you are doing this).
Most of the wood and screws were scrounged from a local demolition and I was lucky to get enough to make the diagonal bracing, gable ends (door frames and stiffeners) and other bits and bobs.
I had to buy 2 x 1 inch baton for the stiffeners that run the length of the tunnel, the ridge pole, hinged door, removable vents and for rolling the polythene cover to be attached to the scaffold boards. It is very cheap at 1.25 per 8ft length.
This is a picture of the back end.
The door frame (goal post) is sunk 2 foot into the ground for stability and held at the top with 4 inch screws screwed directly into the PVC pipe. The stiffeners either side are also screwed in. This end does not have a hinged door but it does have removable polythene vents. The top one is complete. I will staple debris netting to the inside of the door frame for hot days when I need to remove the polythene vents for ventilation. The netting will reduce any wind chill if it is windy and keep out unwanted animals/birds. There will be a permanent open vent at the top to allow for some heat release from the top of the tunnel and also allow pollinators in.
This is the front gable end with the hinged door.
The debris netting is in place and I need to make the movable vents that will sit into the door top and bottom sections. The door doesn't hang quite right so I need to refit the top hinge when I get a chance.
I also put in builder's band on each of the three middle hoops so I can hang timber crop bars at a later stage.
The cover is tricky to install.
Firstly you need anti-hotspot tape at any contact points between the cover and the frame. This protects the cover from wear and increases its life-span.
You need a still sunny day where you have the time yourself to complete this and also be able to gather a few people to help. There were three of us and we only had half a day so I didn't get it finished in one go. Luckily there was no bad weather between then and the time I was able to get back and finish it off. Three people was hardly enough. Six would be ideal, more would be even better.
A G-clamp is essential. The first couple of batons I screwed in above the lintels were not tight enough and the cover began to tear at the screw holes. Nerve-wracking moment! I was able to adjust the lintel height up and pull the torn piece under the lintel and start again. This time with a clamp pulling the baton in tight I was able to screw it to the frame correctly so it held the polythene in place without pulling at the screws. Use the clamp wherever possible to get a tight fit between baton and frame and then screw the baton on.
These give a pretty good idea of the size ...
So nearly there. I need to finish off the door furniture and get the raised beds sorted.
I would say it cost a tad over 200. 130 for the polythene, 25 for the anti-hotspot tape, 40 for timber baton and wood screws, and a few quid for sundries like hinges and staples. It took about four 6 hour days.
Big thanks to this chap who was a great help ...
Bring on the storms!