The basic structure of a polytunnel (hoophouse) is extremely simple, but there are a few variations on the basic theme. As well as ‘regular’ ones, there are other types of polytunnel including rigid panel models, which offer much better insulation. If you’re a practical type, you could also choose to make a tunnel yourself using rigid plastic piping and purpose-made poly sheeting.
Having made the decision that polytunnel ownership is for you, the temptation is to rush off and order one straight away. Catering for customers who do exactly that, there are some friendly-looking sellers around who keep everything looking very simple and offer you a choice of only a few different kits. Don’t be fooled; this simplicity is for their benefit, not for yours. At best you will miss out on some of the choices that make polytunnels such a flexible asset: at worst you will end up with a completely unsuitable product. (The Polytunnel Handbook, A McKee & M Gatter, 2009)
The basic structure of a polytunnel is extremely simple; a row of hoops fixed onto stakes, a ridge-pole, a door frame or two, and some means of securing the cover. But given just how simple the concept is, there are quite a few variations on the basic theme. Without going into too much detail about accessories, here are the main types of tunnel.
A basic polytunnel is a single-span structure. Avoid models without straight side-sections if you can; curved sides create an area at each side where it is awkward to work, because of the lack of head room. This is not important in very wide tunnels. Always opt for doors at each end unless your tunnel is very small, because ventilation is your main defence against moulds and fungi that can be troublesome in humid conditions. If you choose a regular polytunnel, you have to choose between two methods of fixing the cover in place; trenching, and fixing to a base rail.
Trenching is the least expensive option and gives an extremely strong result, with the tension on the cover provided by burying the edges into the soil. However, it requires digging a 30cm x 30cm (1′ x 1′) trench all the way round the tunnel footprint – no joke in heavy or stony soil. Tunnel manufacturers tell us that it is very common for customers to telephone to ask for a base rail kit once they realise how much work trenching in is going to be!
Base rails, clamped to the tunnel uprights at low level, offer a way to fix the cover to the tunnel frame rather than to the ground. This is a lot less work, but the loss of strength to the tunnel structure has to be offset by concreting the corners of the frame in place, or by using ‘anchor plates’ which clamp onto the uprights.
Solar tunnels offer a standard straight-sided tunnel design, but the cover is quite different. Instead of being wrapped in a single sheet of polythene, their tunnels are clad in modular sections of double-layer film reinforced with green nylon mesh. This means that they are more resistant to damage and insulate better than traditional film, and look better too, but light transmission is poorer. The modular design makes it possible to extend a solar tunnel without re-skinning the rest of it, but the price difference between solar and regular tunnels probably negates this advantage.
A successful hybrid between polytunnels and greenhouses, these structures use a variety of polytunnel-type frames to support plastic panels. The best of these are of dual-wall construction with embedded bubbles trapped between the layers (marketed in the UK as Keder houses), giving a remarkably strong cladding that insulates better than polythene or glass. The drawback, however, is the considerable cost.
Building a tunnel from rigid plastic piping and lumber is not difficult, although the action of ultraviolet light will ultimately make the piping brittle. There are no shortage of accounts of how people approached the problem on the internet, and we give a step-by-step guide of how to make a simple and strong tunnel in The Polytunnel Handbook. Once again bear in mind how the whole structure will be anchored to the ground, as if you get things wrong you could find yourself flying the most expensive kite you have ever owned!