Once the doors are on the new polytunnel is suddenly not only a wonderful new gardening space, it’s a space that can be closed! This means you have a much better chance of keeping out slugs, snails, rodents and other unwanted guests. All you have to do is to make sure that there aren’t any in the tunnel to begin with…which may be more difficult than it first appears. This mostly depends on your tunnel’s situation. If it’s alongside a hedge and a damp ditch, for example – you may wage a constant war against these pests, whereas if you are able to put the tunnel up on a more open site you may find that they’re not much of a problem.
Sliding doors are comparatively new for domestic polytunnels. They are a great space-saver and mean that the surrounding area doesn’t have to accommodate doors flung wide. Instead, they will occupy much the same space whether open, or closed.
The doors on this new build were constructed with heavy-duty 100 x 50mm timber, two uprights and three cross beams. These were hammered together using 6” nails, then nail plates were fastened across each joint. All this makes for an incredibly strong door even though there are no diagonal braces.
Two roller units are then screwed into place on the top of each door. These will ride in a slot in the aluminium rails bolted to the top of the door frames at each end of the tunnel.
Once complete, a plastic sheet is fastened to the doors with battens nailed into place. To make the cover tight it’s best to work on all four battens simultaneously. Bang a nail through the centre of the batten on one side of the door, then pull the cover tight and bang in a nail in the centre on the opposite side. Then do the same at the top, and again at the bottom. Keep adding nails one or two at a time on each side, gradually working our way out to the corners and pulling the cover tight as you go. Once the cover is fastened to the frame, an additional batten is nailed to the cross brace halfway down each door.
Hanging the doors is easy enough but this is really a job that requires two people. The roller units are likely to need adjusting in order to make the doors hang vertically with no gaps between them. In this case the doors have to be slid off the aluminium rail, adjusted, and then slid back on. When this was done, three ‘door stops’ were added at each end of the tunnel: one in the centre of the rail, and one towards either end. These prevent the doors crossing over too far in the centre or sliding off the rail. Finally a hasp was added allowing the doors at each end to be fastened together.
At this point the additional door hardware can be added immediately, or later. In this case, there were many seedlings waiting for somewhere to grow, and it was clear that digging the tunnel beds was the next most important job. However, once the beds are finished and planted up the remainder of the work on the tunnel can take place: the door fixings, which include a retaining roller at the foot of each door to prevent it from being blown around, and a stake to stop it sliding too far along the runners. The cover will then be dug slightly into the ground on all sides, creating a truly enclosed environment – especially at night, when the slugs are out!