Erecting a polytunnel
Polytunnels at Sea Spring Farm
From Pasture to Polytunnel
Though it might look daunting, erecting a polytunnel is not difficult. It is simply a step-by-step process. There are two main processes. Firstly the frame must be erected. This is a one-off job as the metal frame is likely to remain standing for several decades; the wooden ends will not last as long, but if perserved wood is used should last for many years before repairs are needed. This picture essay shows how to erect the frame.
The second stage is cladding the frame with a plasic sheet. How to do this is covered in the article "Cladding a polytunnel".
Preparing the site
1. The first step is to find a suitable site. There are certain requisites; firstly the land must be relatively flat or at least only gently sloping in one direction. The site must also not be shaded and have a water supply for irrigation. Shelter from the wind is desirable, as is soil that is not too stony.
2. Clearing the land. If there is plenty of time a sheet of black plastic (silage plastic bought from agricultural suppliers) spread over the proposed site will kill the vegetation; and given enough time will produce a patch of bare ground in perfect condition for erecting a tunnel. If there is no time, then cutting the vegetation will have to suffice.
3. It is easier to cultivate the soil before erecting the polytunnel.
Pounding the posts
Tunnel frames are quite simple structures, composed of posts pounded into the ground in two parallel lines, and hoops that slot into the posts, forming a line of arches. The first job in erecting a polytunnel is to pound the corner posts firmly into the ground. If they are placed in exactly the correct position the polytunnel will be easy to erect and will stand firm for many years.
Mark out where the corner posts go. If they have been measured out correctly, the two diagonals will be the same length. A simple equation called ‘Pythagoras’ theorem’ can be used to calculate the correct length of the diagonals.
If your tunnel is measured in feet (a lot still are) convert the lengths into metres before calculating the length of the diagonals; doing Pythagoras’ theorem in feet and inches is very difficult.
The Pythagoras equation is the length squared + width squared = diagonal squared.
The tunnel in the photos is 14 x 40 feet. This is 4.2 x 12.2 metres
> 4.22 + 12.22 = diagonal2
> 17.64 + 148.84 = diagonal2
> 166.48 = diagonal2
Therefore the diagonal for the tunnel is = √166.48 = 12.903 metres.
It is a wise precaution to check and double check your measurements, and remember to measure the diagonals both ways – they must be identical lengths. It is easy to make a mistake and any discrepancies at this stage will affect the quality of the rest of the job.
4. Once the exact position of the corner posts has been determined they are pounded into the ground. A sledge hammer is good for the job, but it should not be allowed to hit the metal posts directly as this will damage the rims and the hoops won’t be able to slot in. In the photo a rubber plug that fits over the post is being used. However, an old plank of wood held on top of the post does just as well.
The posts should be vertical – use a spirit level to ensure this.
If a post stops going into the ground when it is hit there may be a stone in the way. To remove the obstacle the post has to be pulled out (wobble it from side to side to loosen it). In most cases a stone can be dealt with by pounding a crowbar into the hole until the stone has broken up. Occasionally, this does not work and the only way to remove the stone is to dig it out. The hole must then be filled in and packed down as hard as possible with soil, then the post hammered in again.
5. Once the corner posts have been hammered into the ground, string should be attached to them marking out the perimeter of the tunnel.
6. The string must be attached to the top of the posts and pulled tight so there is no slack. It should be level throughout – use a spirit level, do not do it by eye. This means if there is a slope some posts might have to be knocked deeper into the ground than others.
It is possible to erect a tunnel on a slope provided it is even. In this case the corner posts should be hammered in all to the same depth, so that when the perimeter string is tied around it is not level, but rather runs parallel to the ground.
7. Once the string is in place lay the other posts along each side.
8. Using a measuring tape running the length of the tunnel, the posts can be pushed into the soil in exactly the correct place.
9. The posts are then pounded into the ground.
10. It is essential that the posts are all hammered in so that their rim is the same height as the string.
Erecting the hoops
This is the easiest stage, and is a bit like putting up a meccano structure. Provided the posts have been put in the right place and are vertical the whole structure will be assembled simply.
11. Depending on the model of the tunnel the hoops come in two or three parts. They are put together by slotting them into the posts. A nail should be placed through each hoop and bent over to prevent the hoops from sliding down into the post too far.
12. Hoops are placed in every post.
13. Then the cross pieces at the apex of the hoop have to be added. To reduce friction with the plastic make sure that the cross piece runs under the hoops rather than above them.
14 …. and the last piece!
Many crops in a tunnel, such as tomatoes and cucumbers, need to be supported. The easiest way to provide that support is to run wire along the length of the tunnel, and this is the stage to put the attachments in.
Also, to provide structural strength polytunnels need a side bar running between the last and penultimate hoop on both ends, and that has to be put in now.
15. Lift the hoops out of the base post and slide in the attachment for the wire and the T-connection for the supporting bar.
16. The attachment for the wire. Ideally there will be two attachments on both sides of both end hoops.
17. Put the supporting bar between the last and penultimate hoop on both sides and at both ends.
18. The support bar adds extra strength.
Preparing the doorframes
Tunnel doors are made out of a wooden frame. Tunnels should have a door on both ends as this improves ventilation and ultimately the quality of the crops that grow in it.
19. Tunnel doorframes can be made beforehand or put together in place. The frame should be high enough to allow an adult to walk through and as wide as possible given the width of the tunnel.
20. To make the doorframe, metal plates, which can be bought from any building supplier, are very useful.
21. To be sure that the door is placed in the centre of the tunnel a plumb line should be tied from the top of the centre of the end hoop.
22. String should be tied between the end posts, and the point that the plumb line touches is where the door will go.
23. Dig a hole for the doorframe legs to be buried….
24. and drop it in.
25. Fit doorframe so that it fits snuggly under the metal hoop.
26. Attach doorframe to the polytunnel hoop with metal strapping.
27. Then fill in the holes, firming the doorframe into position.
Preparing for the plastic
Before the plastic can be fitted over the polytunnel frame there are still a couple of jobs to do:
• Applying the insulating tape.
28. The metal hoops in a polytunnel tend to get very hot and over time will make the plastic on a polytunnel become very brittle. To prevent this happening an insulating tape should be stuck on the top of the hoops so the plastic and the metal hoops never touch.
29. Sticking hot spot tape to the tunnel to ensure the plastic never touches the metal hoops.
30. Up and over – every part of the tunnel hoops that might touch the plastic should have hot spot tape.
• Digging the ditch
A ditch has to be dug right around the tunnel frame. This is for burying the plastic sheet, thereby securing it firmly into the ground.
31. The ditch should run a few inches away from the posts. A 2x4inch plank of wood placed flush against the posts is ideal for acting as a guide. Using a flat bladed cutting spade the soil can then be cut along the edge of the wood.
32. The ditch should be about 8 inches wide. The spade should be used to cut the outer side of the ditch.
33. With the sides of the ditch already cut the soil can simply be cut out. The ditch should be about 8 inches deep.
34. Move the wooden plank as the ditch progresses. A neat, carefully dug ditch makes the burying process easy and helps in achieving a well-covered tunnel.
35. The ditch goes all around the polytunnel frame except where the doorframes are.
The tunnel frame is now completely ready for cladding. For details on how to cover the tunnel with the plastic sheet see "How to clad a polytunnel" or click here.
© Joy Michaud