Putting up a polytunnel is easier than erecting flat-packed furniture, and provided you get a tight fit on the cover they are immensely strong. If you are buying a kit from a manufacturer the frames are extremely simple, but ask to see a copy of the instructions before you buy because these are very variable in quality. The exact procedure varies from manufacturer to manufacturer, but the basic installation goes like this.
Prepare the footprint
Allow at least 1m (3′) spare on all sides of the tunnel ‘footprint’. You will need this space to put the cover on and also to clean it, and you will need to keep it clear of any tall growth so that plants inside are not shaded.
Prepare the site by levelling off the footprint off as much as you can. Polytunnels can cope with gentle slopes as long as the ridge pole faces downhill; slopes running side to side are a problem as each pair of ground tubes (which anchor the structure to the ground) have to be exactly level. Don’t underestimate the work involved in levelling even a fairly small area by hand; if possible, work with what you’ve got.
Position the ground tubes
Use a 3,4,5 triangle to mark the positions for the ground tubes accurately, and check your work before you put the tubes in place. Ground tubes can be hammered or dug into the ground, or anchored more firmly using anchor bolts, anchor plates or concrete footings. Make sure that you understand any planning permission issues before you think about using concrete.
Construct the frame
Provided the instructions for your tunnel kit are adequate, construction is a simple job which can be done alone but is easier with two people. Generally the hoops are put up first, then the ridge pole, then additional bracing. Make sure that you understand which side of the frame all the clamps have to go on before you attach the ridge pole.
Attach door frames
Whether you have purchased door frames or made them yourself, door frames are attached by clamping to the frame at the top and burying them at the bottom. For this reason they need to be made of long-lasting, treated timber, and any cut ends painted with rot preventer.
Trenches and base rails
Depending on which method of securing the cover you have chosen, at this point you will either need to dig a 30cm x 30cm (1′ x 1′) trench around the tunnel footprint, or install a base rail onto the hoops at low level. If you are using a base rail you may still wish to bury the edge of the cover to prevent draughts, but if so you will only need a very shallow trench to hold the polythene down.
Covering the tunnel
After covering the upper surfaces of the frame and any rough areas with anti-hotspot tape (a cushioned adhesive tape, sometimes just called ‘hotspot tape’, the tunnel cover is pulled over the structure. This really needs at least four people and should be done in dry, windless conditions. The cover is fixed to the door frames using battens, and buried in the trench or fixed to the base rail, providing as much tension as possible. Commercial tunnel kits should also allow you to tighten the cover further once it is on, as a tight fit prevents movement which shortens the life of the cover.
Fitting the doors
Think about which way the doors on your structure should open before you fit them. It is particularly important to avoid funnelling wind into the tunnel, but remember that if the tunnel is on a slope the doors can only open downhill unless you are prepared to do a little digging to accommodate them. Fit catches to the doors to keep them shut, remembering to leave yourself a way of getting out if the doors close while you are inside. stake hammered into the ground as a doorstop will help to prevent accidental damage to the film and frame if the doors are caught by the wind. Finally, put the manufacturer’s instructions away safely where they can be found if they are needed again in future years.