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3: A new polytunnel – the frame

Polytunnels are incredibly strong, but that strength depends on three things: A firm foundation, and a well-constructed frame, and a really tight cover. Now that the foundation poles are in place, it’s time to put the frame together.

The hoops

This tunnel is 12.8m (42′) long with seven hoops, one every 1.8m (6′). Each hoop is vertical at the sides and then curves in an arch across the top of the tunnel. They are each made from three sections of tube which are held together by self-tapping screws. Putting these in is much easier if you make a small pilot hole with a regular drill bit first. Stick a piece of masking tape over the drill spot to help stop the bit from dancing around on the surface, and put a drop of oil on the end of the bit to help it cut. You’ll need to use the drill frequently, so make sure you keep the batteries well charged.

self tapping assembly, image

A small pilot hole helps a cordless drill go further

The hoops should be assembled on a flat, level surface and then placed firmly onto the tops of the foundation tubes. Wipe any mud off the tops of the foundation tubes first, or you’ll trap grit between the sections, and getting them into position will be much more difficult.

Once the hoops are set, you’ll suddenly see just how big your new tunnel really is.

The ridgepole

The ridgepole runs along the top of the entire tunnel and is clamped to the highest point of each hoop. Don’t assemble the ridge pole before putting it into place, or you’ll never manage to get it through the clamps! Instead, put it into place one section at a time, and use self-tapping screws to hold each section together as you go. These must be put in from beneath the pole, as otherwise the screw head might rub against the cover and damage it. Wrap each end of the ridgepole with the silver tape provided with the kit, or with regular duct tape. This protects the cover from damage.

ridge pole, image

Clamps hold the rige poles in place, and taping over the ridge pole ends protects the cover

Diagonal braces

A diagonal brace is put into place with ‘Q’ clips between the end hoop and hoop adjacent to it at each end of the tunnel. First attach it to the inner hoop of the pair (the lower end of the brace in the photograph below), and then hold a spirit level against the end hoop. Move the clip holding the other end of the diagonal brace up or down until the spirit level indicates that the end hoop is exactly vertical, and then fasten it securely. Once these are in place, the entire structure becomes much more rigid.

diagonal braces, image

Diagonal braces give instant strength to the entire frame

Crop bars

These don’t add very much to the frame in the way of structural strength, but are amazingly useful things to have in a tunnel. They run from side to side above head height and can be used to hang staging, support automatic watering elements or general storage of bamboo poles etc. They are attached using ‘Q’ clamps.

If you figure out exactly how many ‘Q’ clamps you need on each hoop, you can slide them on before setting the hoops onto the foundation poles. Otherwise, you have to pull the clip open enough to push it onto the hoop, and then push it closed again. While this isn’t difficult to do, you’ll wish you’d thought of it beforehand after the first ten or so.

storm braces, image

Storm braces (this one shares a Q clip with one of the crop bars) hold the hoops even more firmly together

Storm braces

These are short sections which bolt onto the hoops either side of the join between the vertical side section and the curved ‘roof’. Their purpose is to add additional strength to the hoops and make it much more difficult to pry the sections apart – as might happen in a really severe storm – but also make useful attachment points to hang things from. If you live in an area known to have severe weather, or if you just want the additional peace of mind, add them to your order.

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2 Responses to “3: A new polytunnel – the frame”

  1. Trevor S. Crockett says:

    Hello Mark!

    So, I live by the Salish Sea, (think Seattle, Washington, U.S./Vancouver, B.C. Canada) otherwise known as Puget Sound; which means that my climate is much like the U.K’s, maritime temperate. I want to build a ploytunnel/hoophouse to help extend the season and to grow some hot weather crops. Problem is thus: the vendors selling the pole bender are asking $80(US) for a tool that I would use ten times. Do you (or anyone else) have a suggestion to get the poles bent to about a 4.5 meter diameter?

    Thanks in advance,
    Trevor

    • Andy McKee says:

      Hi Trevor,

      You can’t really bend the poles without a bender, but you could look for a second hand one; you could sells yours on again afterward, or rent it out; you could enthuse a friend or two to put up a tunnel themselves and split the cost; or you could buy a ready-made tunnel kit (in your neck of the woods they’re called ‘high tunnels’.

      If none of those appeal you could always change your construction materials. In The Polytunnel Handbook we outline how to make a tunnel from thick plastic water piping, and it’s still pretty damned strong. There are also a few sets of plans out there on the net, although they’re mostly fairly vague.

      Good luck in your quest!

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