How to Build a Polytunnel
Polytunnels are an increasingly common sight on an allotment. They are a cheaper and larger alternative to greenhouses, and without the problems associated with broken glass.
How To Build A Polytunnel
The work to build a polytunnel is significantly more than for a simple plastic greenhouse, but the strength and durability can be much greater, and they provide significantly more growing space.
The advantages of using a polytunnel include easier germination in spring and a longer growing season. They are especially good for mediterranean vegetables like tomatoes, chillies, aubergines, peppers, and cucumbers.
Step By Step – How To Build A Polytunnel
See the video on how to build a polytunnel, or continue reading my guide below.
Introduction To Building A Polytunnel
My polytunnel arrived in 13 packages, and the construction took nearly 40 hours.
An experienced DIY’er can undertake nearly all the tasks alone, but the final stage of covering the polytunnel with plastic is a two person job.
Building a polytunnel can be divided into three stages or steps:
- Preparing the frame (all the metalwork)
- Fitting the doors (all the woodwork)
- Covering the polytunnel
Preparing The Frame
The first stage is levelling the ground ready for the construction of the polytunnel. This task is best done during late autumn, winter, or early spring when the ground is still soft and easier to work.
Once level, the position of the feet are marked. The easiest way of doing this is to measure one side and place two markers at each end. Using some maths, it is possible to work out the diagonal length of the polytunnel (the length from opposite corners). On the ground, string or tape is cut to these measurements, and used to accurately mark out all the remaining positions of the feet.
The foundations of the feet are then dug out. A base plate is attached to a foundation tube using clamps positioned above and below the plate, and the plate is buried in the bottom of the hole. The hole is then filled with earth, burying the base plate. The weight of the earth is what stops the foundation tube being lifted out in strong weather.
It is worth spending time to ensure that all the feet are correctly positioned and inline. Any mistakes at this stage will become obvious when you start to erect the hoops that form the skeleton of the polytunnel. The metal parts of the hoop are simply slotted together. When all the hoops are correctly aligned. the joints are sealed by a self-drilling screw.
A centre rail is used to strengthen the metal hoops and stop them moving side to side. The centre rail is attached using p-clips, and again once properly aligned, fixed into place using self drilling screws that go through the p-clips themselves to stop them moving.
Crop bars are then added. These do not add strength to the frame, but are useful for providing support to plants as they grow.
The last element, and one that really adds strength to the frame, are side bars. These go diagonally at each corner and significantly stop sideways movement of the end hoops and helps keep them vertical. P-clips are used to attach the sidebars.
The complete metal frame is a strong structure that is resistance to rocking, and strongly anchored to the ground owing to the base plates covered in earth.
Fitting The Doors
The door frames are a critical element in the construction of a polytunnel. Positioned at each end of the polytunnel, the door frames are used to attached the plastic and hold it in place.
The plastic of a polytunnel needs to be stretched very tightly. This is important as it provides strength to the polytunnel structure, stops water pooling on top of the tunnel, and reduces the amount of movement and damage caused by strong winds.
The doors and door frames do not come pre-built. They need to be constructed and then fitted to the frame. This needs to be done twice, one at each end.
When fitting, the first step is centering the door frame under the end hoop. One door post is positioned correctly, ensuring that it is below the end hoop and vertical. One end of the door post goes into the ground, the other is attached to the end hoop using a p-clip. When all is aligned, the door post is fixed into place.
The door is made from timber. It uses right angle brackets to ensure the door is square, and is designed with two square panels. The upper panel is covered with netting. This helps considerably with ventilation, as it creates a draft at head height between the two doors at each end of the polytunnel. The bottom square is made from plastic.
After the door post is correctly positioned the next stage is to fit the door to the frame. Once this is in place, the remaining door frame is fitted around the door. The second door post is positioned in a very similar way to the first, rooted in a hole and fixed to the hoop using p-clips. A mantel is them nailed in place above the door, creating a rigid structure.
An important feature of the door frame is that battens are nailed around the outer edge of the frame to create a rebate . The plastic is stretched over this, and then held securely in place by a second batten that is nailed into place over the plastic, effectively pinching it between the two battens.
On my polytunnel, the end hoop actually contained a false door – or in other words, a door frame that does not have a door in it. Rather, two squares were created within the door frame itself, the upper part covered with net and the lower part covered with plastic.
I chose to use a base rail to secure the plastic along the sides of the polytunnel. The reason for this is that by securing the plastic to a wooden base rail, it makes it easier to weed around the edges of the polytunnel. The base rail is attached to the metal hoops using p-clips, bolts, and nails. Again a lip is created by nailing an upper batton around the base rail. A second batten is nailed below this to secure the plastic.
Before attaching the plastic, all rough and square edges should be sanded or sawn away. This is to ensure there is nothing sharp to create a rip into the plastic, either during fitting, or afterwards as the plastic moves and rocks with the weather.
Covering The Polytunnel
This two person job should only be attempted on a day with little wind and no rain. It starts with the fitting of hot spot tape, a kind of insulation tape that forms a barrier between the plastic and any areas of metal frame it touches. This is to prevent the metal getting hot under sunlight and burning holes into the plastic cover.
The plastic needs to be rolled out on a flat surface without sharp objects to accidentally snag it. We found it easiest to roll it out fully lengthways, but without fully folding out its width. This made it easy to carefully lift the plastic into the correct position over the frame until it was centred. It is worth noting that the plastic does not reach down to the floor at each end. This is because the two ends are covered by plastic that wraps round from the sides.
The first step in attaching the plastic is to secure it at the top of each door frame. Next, the plastic is attached to the wooden base rails along the side. At every stage, the plastic is pulled as tight as possible to create a smooth surface. Finally, the plastic is attached to the sides of the door frame and the wooden side rails at each end. The pleats in the plastic near the door occur naturally as the plastic needs to be folded like this to neatly go over the rounded corners of the frame.
Once the plastic is fully secured under the wooden batons, the plastic is safe to be trimmed with scissors.
The last step was to lay a floor inside the polytunnel. All my plants will be grown in pots and grow bags. By not using the ground itself, it avoids the build up of pests and diseases. Instead, fresh soil will be used in the pots each year.