Building a Polytunnel: Affordable Covered Growing for Every Garden

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A polytunnel. The allotment dream. If a greenhouse is a luxury, a polytunnel is the affordable alternative that gives you nearly as much growing space for a fraction of the price. I’ve wanted one for years. Kept looking at them, pricing them up, measuring my plot, and then deciding I couldn’t justify it. Then one autumn I just went for it, bought a small secondhand one off Facebook Marketplace for eighty quid, and spent a weekend putting it up. Best money I’ve ever spent on the allotment. Bar none.

The difference a polytunnel makes is incredible. Suddenly I can grow things that would struggle outdoors — peppers, aubergines, melons, cucumbers that actually crop properly instead of sulking in the British summer. My tomato season starts earlier and ends later. I can start seeds weeks before anyone else. And in winter, it’s a sheltered, dry space where I can potter about even when it’s miserable outside. It’s basically a big garden room that happens to have vegetables in it instead of sofas.

Now building one isn’t exactly simple — it’s a decent weekend project and you’ll want a helper (I roped in JB, which was an experience in itself — he’s very particular about things being level, which is fair, but he also stopped every twenty minutes for a brew). But it’s absolutely doable for anyone with basic DIY ability and a willingness to get a bit frustrated with polythene sheeting. Let me walk you through it.

Choosing the Right Size

Polytunnels come in all sizes, from tiny 2m x 3m jobbies to commercial monsters. For an allotment, a 3m x 6m (10ft x 20ft) tunnel is the sweet spot — big enough to be useful, small enough to fit on most plots without taking up too much growing space. Mine is 3m x 4m, which is slightly on the small side but it was what I could fit and afford. I make every inch count.

Height matters too. You want enough headroom to stand up comfortably inside, especially in the middle where the arch is highest. Nothing worse than stooping around in your own tunnel like Quasimodo. Most standard tunnels are about 2m at the ridge, which is fine for most people. I’m not that tall anyway, so it works for me. JB has to duck slightly, which he makes a point of mentioning every time he visits.

Site Preparation

The tunnel needs to go on flat ground. Slopes cause problems with water runoff and make the frame harder to erect level. If your plot isn’t perfectly flat, spend time levelling the site first. It’s worth the effort — a level tunnel is a happy tunnel.

Orient it so the long axis runs east-west if possible. This maximises the sunlight entering through the south-facing side. If your plot doesn’t allow this, north-south works too — just means slightly different light patterns through the day.

Clear the site of any stones, roots, or debris. The polythene skin sits close to the ground and any sharp objects underneath will puncture it. I also laid down a ground cover membrane on the paths inside the tunnel to suppress weeds — saves a lot of weeding later.

Erecting the Frame

Most polytunnel kits come with hooped galvanised steel tubes that slot together. The hoops are anchored into the ground either with ground tubes (short tubes driven into the soil that the hoops slot into) or with anchor plates. Follow the manufacturer’s instructions here — they’ll tell you the exact spacing between hoops and how deep the anchors need to go.

This is where a helper is essential. Lifting the hoops into position and holding them steady while you secure them is a two-person job. I suppose you could do it alone if you’re very patient and very creative with temporary props, but honestly, get someone to help. Bribe them with tea and biscuits. That’s what I did with JB, and he’s now emotionally invested in the tunnel’s success.

Once the hoops are up and secured, add the ridge bar along the top (this runs the full length of the tunnel and ties all the hoops together) and any cross bracing. The frame should be solid and rigid when you shake it. If it wobbles, something isn’t tight enough. Tighten everything before the cover goes on.

Covering with Polythene

This is the bit that can test your patience. And your marriage. And your friendships. Polythene sheeting is clingy, unwieldy, and has a mind of its own. Do this on a calm day. NOT a windy day. I cannot stress this enough. We tried to cover mine on a breezy afternoon and the polythene turned into a massive, billowing sail that nearly dragged JB across the allotment. He still brings it up. Every single time.

Drape the polythene over the frame, centre it so there’s equal overhang on both sides, and then secure it by burying the edges in a trench dug along both sides of the tunnel. Dig a trench about 30cm deep, lay the polythene edge in, and backfill with soil. This anchors it securely and stops wind from getting underneath. Pull the polythene taut before you bury it — wrinkles and slack spots flap in the wind and wear through faster.

The end walls can be covered with polythene too, or you can build timber end walls with a door. A door is essential for ventilation and access. Most people put a door at one end and an opening at the other for cross-ventilation. I’ve got a door at the front and a roll-up panel at the back. Works brilliantly for airflow in summer.

Inside the Tunnel

Once it’s up, the fun begins — planning what goes inside. I’ve got raised beds along both sides, filled with a mix of topsoil and compost, and a gravel path down the middle. Tomatoes on one side, cucumbers and peppers on the other. In winter, the beds switch to salads and overwintering crops.

Ventilation is crucial. A polytunnel in summer can hit 40°C easily if it’s closed up. Plants will cook. Open doors and vents every morning and close them in the evening. On really hot days, I leave the doors open all night too. Managing temperature is the main ongoing job with a polytunnel — it’s more about keeping it cool in summer than warm in winter.

Watering needs to be done manually or with drip irrigation since the polythene keeps rain out. I’ve got a simple drip system running off a water butt positioned at the back of the tunnel — gravity fed, no pump needed. Works perfectly and means I’m not lugging watering cans in and out every day.

Maintenance

Polytunnel covers typically last five to seven years before needing replacement, depending on the quality of polythene and your local conditions. UV degrades the material over time — it’ll go cloudy and brittle eventually. Replacement covers are available from the tunnel manufacturer or generic suppliers. It’s a bit of a faff to re-cover but it’s much cheaper than buying a whole new tunnel.

Check the frame annually for rust, especially if you’re near the coast. A quick rub with sandpaper and a coat of galvanised paint on any rusty spots keeps things going for decades. The frame will outlast many polythene covers.

Keep the cover clean — a wash with soapy water in spring maximises light transmission through the increasingly cloudy plastic. I do this once a year with a soft brush on a long handle. It’s one of those jobs that’s oddly satisfying. Like washing a very large, very awkward car.

Look after yourselves. Take good care. 🌱