From Pasture to Polytunnel – Part 2 – Cladding a Polytunnel

by Joy Michaud on February 5, 2012 · 0 comments

in General Information, Product Reviews

This is the 2nd part of a 2 part article click here to see the 1st part if you have not already read it.

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.

Hotspot tape

29. Sticking hot spot tape to the tunnel to ensure the plastic never touches the metal hoops.

putting on hot spot tape

30. Every part of the tunnel hoops that might touch the plastic should have hot spot tape.

hot spot tape on all tunnels

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.

use wood to dig ditch

32. The ditch should be about 8 inches wide. The spade should be used to cut the outer side of the ditch.

cut 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. digging ditch
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. ditch
35. The ditch goes all around the polytunnel frame except where the doorframes are.

tunnel and ditch

Cladding the polytunnel

Lifting the plastic sheet over:

A polytunnel cannot be covered or ‘clad’ on just any day. The weather has to be still – any wind will make the plastic sheet act like a parachute. In addition, the sun has to be shining – the warmth from the sun’s rays trapped in the polytunnel make the plastic more supple and easier to handle and stretch. If there is no sun the plastic is prone to tearing.

If the weather is not right do not attempt to clad a polytunnel – even if a team of people have arrived to help or you have taken a day off work. Any attempt to clad a polytunnel in inappropriate weather will, at best, result, in a loose cover that will flap in the wind and may shorten the life of the plastic by years. At worst bad weather could result in a complete failure.

36. Before starting it really helps to make sure all the necessary tools and equipment are available. In particular, a knife is needed for cutting the plastic, spades for filling the ditch, and hammer, nails and wood for attaching the plastic to the doorframe. check equipment
37. The plastic sheet needs to be unrolled along one side of the polytunnel frame. Once unrolled the sheet will still be folded several times along its length. The different layers tend to stick together, so before lifting over the polytunnel frame the layers should be separated. lay plastic out
38. The sheet should then be pulled over the polytunnel frame. At least two people, preferably three or more, are needed for this job. The bigger the polytunnel the more people are needed. Holding both ends of the plastic sheet it should be gently lifted onto the frame and eased up. pull sheet up
39. The plastic sheet should never be forced, just gently manoeuvred to the apex of the frame. There may be times it seems impossible, but it will eventually get there. The only problem that might occur is if there is a gust of wind. The plastic sheet will act like a parachute. If it is too windy, give up. If there are occasional gusts of wind it might be exciting but it is not really a problem. a puff of wind
40. As soon as the sheet is over the polytunnel frame it should be weighted down with soil. This must be done quickly if there is any risk of there being more gusts of wind. and down
41. The plastic sheet must lie evenly over the frame, with equal amounts of spare plastic on each side and each end. Checking that the creases run parallel to the ground is a useful way of ensuring that the plastic is not skew. To secure the sheet further the spare plastic at the ends can be tucked in through the doorframe.

plastic over polytunnel

Attaching the plastic sheet to the polytunnel frame:

42. The plastic is first attached to the top of the doorframe at both ends. The plastic above one doorframe should be smoothed out, and a cut made diagonally, down and towards the centre from both corners, creating a flap of plastic that comes to a point 2–3 feet below the top of the doorframe. attach to top of door frame
43. The flap should be smoothed out, but not pulled. It should be folded inside and attached to the top of the doorframe on the inside of the polytunnel. attach to door frame
44.The door at the other end is attached in the same way, but before being attached it must be pulled as hard as possible. If the sun is shining the air inside the tunnel will quickly heat up, warming the plastic as well. It is worth letting this happen – take a break, have a cup of tea – as warm plastic stretches better. pull plastic hard
45. If the plastic is warm it becomes supple and can be pulled extremely hard without any risk of damage. If the sun is not shining, particularly if the air temperature is cold as well, the plastic is brittle and will easily tear. Even if the plastic is warm, though, it must be held correctly for pulling. The best way is to bunch the plastic together and hold that. Clutching a single layer of the sheet when pulling is risky as fingers easily make a hole in the plastic. torn plastic
46. The plastic can be attached to the doorframe by sandwiching it with wood strapping. Use several strips, and many nails. strapping
47. Alternatively, in exposed sites where the polytunnel may experience strong gales in the winter, the plastic sheet can be attached to the wooden frame by cutting it into strips and rolling it around 1×2 inch pieces of wood, which are then nailed onto the frame. This method will not allow any slippage or tearing of the plastic. stronger attachment
48. After the plastic has been attached to the top of the doorframe at both ends, it must be buried in the ditch on the sides. This is done in a very particular order. Bury the plastic at the centre hoop on one side, do not pull the plastic down hard. fill in ditch
49. Then bury the plastic on the other side, at the exact opposite position. From then onwards, the plastic must be buried, hoop by hoop, first one side then the equivalent opposite side, moving gradually from the centre to the ends. stand on soil
50. When burying the plastic the first time it should be smoothed out but not pulled. From then on before the plastic is buried it should be stretched as hard as possible over the hoops. To do this put soil on the plastic sheet; lift the soil up by holding the loose plastic on the outside; and then stand on the soil (still holding the plastic). Your body weight will slide the bulky soil down into the bottom of the ditch, thus stretching the plastic. The warmer and sunnier the day the more pliable the plastic is, and the better the results. stand on soil
51. Work from the centre to the ends of the polytunnel. To save time at this stage it is only necessary to wedge soil into the ditch where the hoops are situated. The ditch between the hoops can be filled in later. Only bury the plastic on the sides, the ends will be done later. attention on each hoop
52. If the site is particularly vulnerable to strong winds, the plastic can be buried in the ditch even more securely by folding the spare plastic over, like an “S”, and adding more soil. fold plastic over
53. With the ditch done the next step is to attach the plastic to the sides of the doorframes. Starting at the top the plastic is pulled around the last hoop and the doorframe and nailed onto the frame on the inside of the tunnel. The job really needs two people, one to keep a tight pull on the plastic, while the other does the hammering. secure door frame sides
54. The plastic must be pulled as hard as possible around the corner of the last hoop. pull plastic
55. Towards the bottom of the doors there will be a lot of spare plastic to deal with. This is quite natural and just has to be folded out of the way. Folds must be introduced to accommodate it all. attach to side
56. The plastic at ground level at the ends can now be buried in the ditch. The soil should be tightly packed in, and the extra plastic sticking up form the ditch should be cut just at the soil surface. ditches at the ends
57. When all the ditches are filled in and the doorframes done the tunnel sides should look neat, and there should be no loose plastic. tidy tunnel ditch

Making the door

There are as many designs for tunnel doors as there are tunnel owners. They can range from very sophisticated methods using hinges and proper door latches to just hanging a sheet of spare plastic from the doorframe. Generally, the simple methods are the easiest to maintain.

58. A simple but effective door is a sheet of plastic wrapped around a piece of wood and nailed onto the top of the doorframe. The plastic should be larger than the doorframe. putting the door on
59. Two planks of wood sandwiching the plastic at the end and at a couple of places in the middle will give the plastic ‘door’ extra weight and strength. making the door
60. The door is opened by rolling the plastic up and hooking some string around it. the finished door
61. In situations where strong gales can be a problem it is important that the door is able to prevent the wind entering the tunnel. In these cases a slightly stronger door is necessary. One method is to make a wooden frame that is slightly larger than the doorframe. This can then be covered with plastic and kept on with string crossed over the door. other door designs
62. A more sophisticated system involves hinges and doors that fit accurately within the doorframe. These systems are easy to manage day-to-day, but do require more maintenance over the years. other door designs

Preparing the inside of the polytunnel

63. Many polytunnel crops, such as tomatoes and cucumbers, need some sort of support when growing. It is much easier to put the supporting wire up before any crops are planted. The wire should be laid out along the length of the tunnel. laying out the wire
64. The wire should be attached to the end hoops with attachments slipped onto the end hoops when the polytunnel was erected. attached the wire
65. With the wire firmly secured at both ends of the polytunnel, it should then be attached onto each internal hoop. A short piece of wire wrapped around the hoop and wire does the job nicely. attaching wire to hoops
66. When the four lengths of wire are secured to each hoop they are unlikely to get in the way of anyone working in the polytunnel. The polytunnel is now ready for planting. polytunnel ready for planting
67. The same polytunnel a few weeks later. growing crops
finished polytunnel

The finished polytunnel

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