How to clad a polytunnel - Sea Spring Seeds

Cladding a polytunnel

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 flexible and easier to handle and stretch. If there is no sun the plastic cannot be stretched over the polytunnel frame, and 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.

Preparation

In the days before you intend to clad your tunnel there are certain things that must be done:

Polytunnel frame with ditch dug — *A polytunnel frame with the ditch dug.*

1. A ditch must be dug around the tunnel frame.
2. Hot spot tape must be stuck on the metal hoops so that the plastic sheet does not rest on metal in any place.
3. All rough edges or sharp points must be covered with thick tape so that it will not catch on the plastic.

Do not be tempted to skimp on points 2 and 3; they are VERY important. If they are done properly the plastic will last 10 maybe 20 years; if you don’t put the hot spot tape on or if you leave any edges or points the plastic sheet will be off within a few years.

For details see the article “How to erect a polytunnel“.

To clad a polytunnel you need at least three people committed to working until the job is done, which may take most of the day. It also helps, though is not necessary, to have a few extra people when lifting the plastic sheet over the frame.

Lifting the plastic sheet over

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.

Tools necessary for cladding a polytunnel. — *Before starting to clad a polytunnel make sure you have all the necessary equipment*

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.

Plastic sheet unrolled beside the polytunnel frame. — *First unroll the plastic sheet alongside the polytunnel frame*.

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.

A puff of wind when pulling the plastic sheet onto a polytunnel frame. — The plastic sheet acts like a parachute when there is a puff of wind.

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.

Lifting the plastic sheet over the polytunnel frame. — *At least two, preferably three or more people are needed to pull the plastic sheet over the polytunnel frame.*

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.

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 temporarily secure the sheet to the frame the spare plastic at the ends can be tucked in through the doorframe. If the breeze is a little strong and the sheet is at risk of being blown a little soil can be put on the plastic in the ditches – but not too much as it will have to be moved.

Plastic sheet drapped over the polytunnel frame. — *Once on the polytunnel frame the plastic sheet should be weighted down with soil and the ends stuffed into the door frame.*

It is worth taking a little break at this point to allow the air inside the tunnel to heat up. The warmer the plastic is, the more flexible it becomes and the more it can be stretched, making the job of attaching the sheet to the frame much easier. So relax, take a break, have a cup of tea.

Attaching the plastic sheet to the polytunnel frame.

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.

Attaching the plastic sheet to the top of the first door frame — *The plastic sheet should first be attached to the top of one door frame.*

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.

The door at the other end is attached in the same way, but before being attached the plastic sheet must be pulled as hard as possible. If the sun is shining the air inside the tunnel will have heated up, warming the plastic as well so that it becomes more flexible and stretches better. If the plastic is warm it becomes supple and can be pulled extremely hard without any risk of damage, but it must be held correctly. The best way is to bunch the plastic together and hold that.

Stretching the plastic sheet. — *The plastic sheet should be pulled hard before attaching it to the other doorway.*

Clutching a single layer of the sheet when pulling is risky as fingers easily make a hole in the plastic. If the sun is not shining, particularly if the air temperature is cold as well, the plastic is brittle and will easily tear.

The plastic will tear if held incorrectly. — *When pulling the plastic sheet it should be bunched – never hold just one layer of plastic as it is likely to tear.*

The plastic can be attached to the doorframe by sandwiching it with wood strapping. Use several strips, and many nails.

Strapping attaching the plastic sheet to the door frame. — *The plastic sheet is attached to the door frame by sandwiching it between strapping.*

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 gives an extremely strong attachment and will not allow any slippage or tearing of the plastic.

A stronger method of attaching the plastic sheet. — *A stronger method of attaching the plastic sheet to the door frame is rolling it around 1×2 inch wood.*

Once the plastic sheet has been attached to the top of the doorframe at both ends (note: the sides should not be attached at this stage), it must next be buried in the ditch on the sides. This is done in a very particular order. First, bury the plastic at the centre hoop on one side. Gently ease the plastic sheet down, smoothing out wrinkles but do not pull it down hard.

Bury the plastic sheet at the centre of the tunnel frame. — Bury the plastic sheet on one side.

Next to the same thing 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. 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 of the polytunnel frame, the ends of the polytunnel should be done after the doors are finished.

bury the plastic in the ditch at each hoop, moving out from the center. — *Moving from the centre to the ends, the plastic sheet needs to be buried at each hoop.*

[Important] When burying the plastic the first time the wrinkles should be smoothed out and the sheet gently eased downwards, but not pulled. From then on before the plastic is buried it MUST be stretched downwards as hard as possible over the hoops. The tighter it is pulled the better the final result.

However, the plastic sheet cannot be pulled downwards by holding it pulling as there is nothing that you can hold onto. Instead you have to use your weight and some soil to stretch it downwards while holding the sheet’s edge; i.e. put some soil on the plastic sheet in the ditch and stand on it with one foot; lift the soil up by holding the edge of the plastic sheet; 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.

Bury the plastic sheet on the opposite side. — *The plastic sheet must be stretched over the polytunnel frame at each hoop. This is done by putting soil on the sheet in the ditch, then using your weight to slide the sheet downwards.*

Fill in the spaces in the ditch with soil leaving about a third of the soil. Fold the edge of the plastic sheet back onto the ditch and bury it with the remaining soil. By folding the spare plastic over, like an “S”, and covering it adds extra strength.

Fold the remaining plastic back onto the soil and cover.

With the ditch done the next step is to attach the plastic to the sides of the doorframes. The plastic must be pulled as hard as possible around the corner of the last hoop.

Plastic sheet pulled around the end of the polytunnel frame. — *The plastic sheet needs to be pulled hard around the end hoop.*

Starting at the top the plastic is pulled around the last hoop and the door frame 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.

Attaching the plastic sheet to the doorframe sides. — *Starting at the top attach the plastic sheet to the top of the door frame sides.*

Photo 20. 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.

Attaching the plastic sheet at the bottom of the door frame. — *There is a lot of excess plastic as you get to the bottom of the door frame.*

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.

The loose plastic at the ends are covered with soil. — The loose plastic needs to be put in the ditch and covered with soil.

When all the ditches are filled in and the doorframes done the tunnel sides should look neat, and there should be no loose plastic.

When fully buried the plastic should be tight against the polytunnel frame.

Making the polytunnel 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.

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 door frame. 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. The door is opened by rolling the plastic up and hooking some string around it.

Making the polytunnel door — *A simply polytunnel door is just a sheet of plastic weighted with wood.*

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.

*A polytunnel door where strong gales could be a problem.*

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.

Sophisticated polytunnel doors. — *A hinged, sophisticated polytunnel door is very good but requires constant maintenance.*

Preparing the inside of the polytunnel

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 wire in the polytunnel. — *Unwind the wire to run the length of the polytunnel.*

The wire should be attached to the end hoops with attachments slipped onto the end hoops when the polytunnel was erected. If the attachment was not put on the end hoops when the polytunnel was erected, then a heavy duty strapping can be slipped around the hoop. Alternatively, wire can be used, but this is tricky as it risks piercing the plastic, so is the least good option.

Wire attached to the end hoop. — *The wire must be attached to the end hoop.*

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.

Attach the wire to each hoop of the polytunnel. — *The wire should be attached to each internal hoop.*

It is useful to have four wires, two on each side, running the length of a polytunnel. When the four lengths 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 with four lengths of wire attached. — *Once the four lengths of wire are attached the polytunnel is ready for planting crops.*

The finished job… and a few weeks later

Newly clad polytunnel. — *The final product, a newly clad polytunnel*

Polytunnel with a new tomato crop growing. — *The polytunnel a few weeks later with a young tomato crop.*

For details about how to grow crops in a polytunnel click here.