Some info on building polytunnels

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I mentioned on my Greetings from England post that I use a pollytunnel. I've had a few emails asking how I built it so I thought I'd post the info here if anybody wants the know how. As with most of my projects, frugality is a major factor.

My pollytunnel is 30ft long x 10ft wide and has straight sides up to three feet from the ground. The hoops are made from 1 1/2 inch thick walled "alkathene" plastic mains water pipe. It's blue in color and hard wearing but flexible enough to bend in a big hoop. I have scaffold pipe sections (5 ft long with two feet burried and three feet showing) rammed into the ground (spaced 5 ft apart down both sides of the tunnel).

The alkathene pipe is pushed into these scaffold poles and are wedged in with wooden wedges. These help keep the alkathene pipe in place. I also drill through both the scaffold pipe and alkathene pipe when it is in situ and push a nut and bolt through both as this stops the weight of the polythene roof from forcing the plastic pipe down further into the scaffold pipe.

I screw roofing laths along the hoops (on the inside), right along the apex, along the tops of the scaffold poles (about three feet up from the ground), and in between at about ten o'clock and two o'clock if looking from the end of the tunnel. These roofing laths are screwed from the outside of the alkathene pipe. I drill a large hole on the outside edge of the pipe, and a smaller hole on the inside edge of the pipe. I then put screws through the large hole and screw the lath from inside the alkathene pipe. This holds much better than going from the wood into the pipe, and has less chance of a screw point penetrating the polythene cover.

One other advantage of adding roofing laths apart from added stability is to give somewhere to hang straining wires from for supporting tomato plants. I also feed bubble wrap behing the laths in winter to extend my growing season.

The doors are made from two timber uprights. These are burried two feet into the ground and just meet the hoop at the top. Two pieces of copper pipe are beaten flat and bent around the pipe and over the wooden uprights and screws keep them in place. This is done at both ends of the tunnel. Cross members are added to the door frames and nailed in place with triangles of plywood. This then, is the frame for the pollytunnel.

I then dug two trenches along the outside of the tunnel. The trenches were two feet deep and two feet wide. I unrolled the polythene (it was the only part I actually bought apart from screws and nails) and placed it along the apex of the roof. I made sure it extended equal lengths over both door frames and then I used a staple gun to nail it temporarily to one door frame at the top only.

I then went to the other end of the tunnel and nailed the other end to the top of the door frame after pulling the polythene as tight as I could by hand. Once this was done, I wrapped the polythene round the ends of the tunnel and tacked it to the sides of the door frame at the bottom three feet. The curved part of the frame is done last. Once all four corners were temporarily tacked, I started to gather the loose polythene on the curved part of the hoop and scrunching it so it looked a bit like a dancer's fan, tacked it to the door frame so there was equal pressure all over the frame. Again I did this on all four corners. I then had to pull the walls down tight. That is done by laying the bottom of the walls in the trench, and filling it in again. The weight of the soil will pull the polythene down tight, and it should withstand most degrees of wind. Finally I nailed thin wooden strips all around the door frames to trap the polythene that had ben tacked. The tacks alone would have pulled themselves loose one by one in the first strone wind and the wooden strips just prevented this.

If you live in a very windy area, put some cross braces of roofing lath from fout feet up the inside of the end hoops down to the base of the second off end hoops (so you get triangulation at all four corners for strength and added rigidity). I use 170 guage clear polythene. It's good and thick, and won't tear easily, but lets light in well. Once you decide on how big you want the tunnel, measure the length of a hoop from ground, over the hoop to ground again, and add six feet to this measurement (three feet on each side for the trench (two feet down and one across)). This then is the width of the polythene you order. The length is obtained by measuring the length of the tunnel plus ten feet (five feet overlap each end for joining to the door frame). Using this type of tunnel, I would not recommend a tunnel wider than 10 ft. 8 ft wide might be better for stability.

Finally, once the tunnel is up and secure (not wobbly), you can fit the doors. These can be made from the same type of roofing laths as used for the cross supports inside the tunnel You need two uprights, and three cross members, one top and bottom and one across the middle. Join them with triangular pieces of plywood at all the corners and nail the plywood back and front of the door for strength. The upper pane of the door can be covered with fine netting and the bottom pane can be covered in scrap pieces of polythene. The netting will provide ventilation and help reduce condensation build up inside the tunnel, while a door at both ends will allow a throughput of fresh air on hot days. One tip befire I forget. When planning the width of the door, make it wide enough for your wheelbarrow to get through (believe me you'll be glad you did).

The scaffold poles were scrounged from a building site, the alkathene pipe was end pieces off the roll (long enough for my use) scrounged from a new housing project. The roofing lath was taken from the roof of a condemned building (the tiles had already been removed by someone else). I only bought the polythene sheeting, screws and tacks.

I've built a few of these for other people and some have differed slightly depending on what materials were to hand. One tunnel has the alkathene pipe attached to wooden stakes with appropriately sized muffler clamps (round the pipe and through the timber, then tightened up for strength. - a short length of rebar was inserted in the alkathene pipe to stop the muffler clamp from collapsing the pipe wall). The wooden stakes were placed on the inside of the tunnel. I've also built tunnels with opaque covers to use as lambing sheds and stock shelters. These had old pallets attached round the walls on the inside to stop the animals from bursting through the polythene. There's no end to the things you can use a tunnel for. I have one I use as a woodworking workshop. I do my chainsaw carving in there in rainy weather.

Eric

-- Eric J Methven (e_methven@btinternet.com), March 27, 2001

Answers

Polytunnels are very big in New England, in fact we have one. The big differnece here, is that we use 1 1/2 inch metal electric conduit and use a electrician's pipe bender to shape it. We then use a 4 mil plastic from a greenhouse supply company that has a UV protector added to the plastic giving it a life scan of about 5 years and allowing it to stand up to 2 to 3 feet of snow.

-- David in NH (grayfoxfarm@mcttelecom.com), March 27, 2001.

Wow, Eric!! What great information! Thank you very much for going to the considerable trouble of posting this.

-- Laura Jensen (lrjensen@seedlaw.com), March 29, 2001.

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