How to build a polytunnel

Video by Premier Polytunnels. Premier Polytunnels have been making polytunnel kits in the UK for over 35 years, and this guide covers the full build from unpacking to the finished structure. Particularly useful for the cover-stretching sequence — it's one of those jobs where watching it done first makes a real difference.

1. Choose the right site

A polytunnel wants maximum sunlight, so avoid overhanging trees and north-facing slopes. Orient the tunnel on a north-to-south axis if you can — it reduces shading from the hoops and keeps temperatures more even along the length.

Good drainage matters too. A site that sits wet in winter will give you a muddy path and rot problems in the beds. Check it after heavy rain before committing to a position.

2. Choose the right size

Polytunnels are sold by width and length. Common widths are 3 m, 4 m, and 6 m; you can usually specify the length in multiples of the hoop spacing. As a rule, buy bigger than you think you need — serious vegetable growing takes up more space than expected, and a larger tunnel is proportionally more cost-effective.

Check whether you need planning permission. In most cases a polytunnel for domestic use falls under permitted development, but it's worth confirming with your local authority, particularly on a listed property or in a conservation area.

3. Mark out and prepare the ground

Use string lines and pegs to mark out the exact footprint. Check the rectangle is square by measuring the diagonals — they should be equal. Clear the area of any perennial weeds before the structure goes up, since they'll be far harder to deal with once the tunnel is in place.

Level the site as best you can. Some slope is fine, but a significant cross-slope will make the structure awkward to use and harder to keep watertight at the edges.

4. Drive the ground anchors

Ground anchors are the metal spikes that go into the earth and accept the hoop tubes above. Space them at the correct intervals (set by the manufacturer for your hoop spacing, typically 1.5 m to 2 m) and drive them to the specified depth with a post driver or a club hammer and block.

Get these as vertical as possible. An anchor that leans in or out will skew the hoop above it, and correcting that later is frustrating. Check each one with a spirit level as you go.

5. Assemble the hoop frames

Each hoop is usually two or three sections of galvanised steel tube that join in the middle and at the base. Slot the legs into the ground anchors and push the sections together until the clips or bolts lock. Check each hoop is upright and that the ridge sits at a consistent height from one end to the other.

A ridge pole connects the tops of all the hoops and holds the structure rigid. Fit it before you start on the ends.

6. Fit the end frames and doors

The end frames carry the doors and any ventilation panels. Most kits come with a timber or steel end frame that bolts to the first hoop. Fit the doors and check they swing freely before the cover goes on — much easier to adjust them now than once the polythene is tensioned over everything.

If you're fitting netting vents in the side panels, cut and fit them at this stage too.

7. Stretch the polythene cover

This is the two-person stage. On a warm, calm day — polythene is more pliable in the warmth — drape the cover over the hoops and pull it tight from both ends simultaneously. The cover needs to be taut but not strained; folds and wrinkles trap water and degrade the plastic faster.

Work out any large creases by pulling from alternating sides. Take your time. A cover that goes on well will last several years longer than one that goes on in a rush.

8. Secure the edges

Most kits use a base rail system: the polythene wraps around a timber or steel base rail and is clamped in a channel. Alternatively, bury the edges in a trench around the perimeter — about 200 mm deep — and backfill to hold the cover in place.

Both methods work. The base rail is tidier; the buried trench is simpler and cheaper. On a windy site the trench method is often more secure.

When to call a handyman

Call Richard if you need help with the cover-stretching stage and can't get a helper for the day, or if the site needs levelling work beforehand. Larger polytunnels — 6 m wide and above — are significantly harder to cover single-handed, and getting the tension wrong on a large structure means expensive replacement polythene.