How do I replace the polythene on a polytunnel?

Recovering a tunnel is a whole-day job for three people and the single most important factor is the weather. Warm polythene stretches and can be tensioned drum-tight, which is what makes a cover last; cold polythene will not tension and flaps itself to pieces. Wind makes the job impossible and dangerous.

Safety

What you'll need

• New cover of the correct size and grade, true
• Anti hot-spot tape, true
• Three or more people, true
• Ropes to pull the cover over, true
• Spade for trenching, or base rail clamps and timber
• Ladder or platform, true

Step by step

1. Order the correct cover.Measure the hoop girth and the tunnel length, and add the generous allowance the supplier specifies for trenching or clamping. Choose the grade — standard, long-life, thermal, or diffused — for how you use the tunnel. Order from the tunnel's manufacturer where possible, because sizes are not standard.
2. Wait for a warm, still day.This is not a detail. Warm polythene stretches and tensions; cold polythene does not. Completely still air, mild temperature, and ideally the middle of a sunny day. A forecast of light wind is a reason to postpone.
3. Strip the old cover and prepare the frame.Cut the old cover away and remove all the old tape and fixings. Check every hoop, joint and bolt, replace anything corroded, and file or tape over any sharp edge or protruding bolt.
4. Apply anti hot-spot tape to every hoop.The tape goes on the full length of every hoop where the polythene will touch it. Hot metal in direct sun degrades polythene at the contact point, and this tape is what stops the cover failing along the hoop lines within a couple of years. Do not skip it.
5. Treat any timber that touches the polythene.Chemicals in some treated timber degrade polythene on contact. Isolate timber base rails and door frames from the cover with tape or a barrier strip.
6. Pull the cover over with ropes.Lay the folded cover along one side, attach ropes to the leading edge — a soft ball tied into the polythene makes a good attachment point — and throw them over the frame. Pull from the far side with one person per rope while others feed the cover and keep it off the ground.
7. Position and tension before fixing anything.Get the cover square and centred, with an equal overhang at both ends and both sides. Then tension along the length and across the hoops, working from the middle outwards. Tight enough to drum, evenly, is the target.
8. Secure the edges by trenching or by base rails.Trenching: bury the surplus in a trench along each side and backfill firmly, which gives a very secure seal. Base rails: clamp the polythene into the aluminium or timber rail system with the fittings supplied. Trench the ends or fit them to the door frame as designed.
9. Finish the doors and trim.Fix the cover to the door frames, cut the door openings and hang the doors last, once everything else is tensioned. Trim surplus polythene only after the whole cover is secured.

Tips

• Do it in the middle of a warm day and re-check the tension a week later. Polythene relaxes slightly and a small retension early on adds years.
• Keep the offcuts. They patch storm damage, and repair tape on a small tear will save a whole cover.
• If the tunnel is large or exposed, this is a job worth paying a specialist for. Recovering contractors do it in a fraction of the time and get the tension right.

Common mistakes

• Doing it on a breezy day because everyone was available — A large sheet of polythene in wind is genuinely dangerous and impossible to tension. Postpone — there is no version of this job that works in wind.
• Skipping the anti hot-spot tape — The polythene fails along the hoop contact lines within a couple of years. The tape costs a fraction of the cover and doubles its life.
• Fixing one side fully before tensioning the other — You cannot pull the wrinkles out afterwards. Position, square up and tension the whole cover before securing anything permanently.
• Letting the polythene touch untreated bolts or treated timber — Sharp edges tear it and some timber preservatives chemically degrade polythene at the contact point.

Questions people ask