Polyrope Guides • Polytunnels
Ordering a replacement polytunnel cover is easy once you have two numbers: the width of sheet you need and the length. Both come from a tape measure and ten minutes with a helper. Get them right and the new cover goes on tight, with enough spare to fix it properly. Get them wrong and you are left with a sheet that stops short of the ground.
Quick answer
- Width of sheet = the distance over the top of a hoop, from the ground on one side to the ground on the other, plus the extra you need to fix both sides. Round up to the next width: 6 m, 8 m or 10 m.
- Length of sheet = the length of the tunnel, plus the height of each end if the same sheet covers the end walls, plus the extra for fixing at each end. Round up to the next length on offer.
- Measure the frame, not the old cover. Polythene stretches over the years, so an old sheet is no guide to the size you need.
What you need
A long tape measure (10 m or more for most tunnels), a notepad and a second person. A tape pulled over a hoop by one person tends to sag or slip off the frame, and that gives you the wrong number. Write every measurement down on a quick sketch of the tunnel as you go.
Step 1: Measure over the hoop
Hold the end of the tape at ground level against the foot of a hoop, or against the base rail if your cover is fixed to a rail rather than buried. Your helper takes the tape up and over the top of the hoop and down to ground level on the other side. Keep the tape snug against the frame, but don’t stretch it.
Measure at least two hoops, one near each end, and use the largest figure. Frames settle over the years and are rarely perfectly even.
Step 2: Add the fixing allowance and choose the width
The measurement over the hoop only gets the sheet from ground to ground. You also need extra polythene on each side to fix it: to bury in a trench, or to wrap around a base rail or timber. How much depends on your tunnel and fixing method, so check the instructions for your structure, then add that amount twice, once for each side.
Now pick the width of sheet that is equal to or larger than your total. If you are close to the limit, go up a width. A sheet that is slightly too wide can be trimmed once it is fixed; a sheet that is too narrow cannot be stretched to fit.
| Width of sheet | Choose it when your total width is | 720 gauge (UV4), lengths | 900 gauge (UV10), lengths |
|---|---|---|---|
| 6 m | up to 6 m | 2 to 12 m in 1 m steps, then 14 to 30 m in 2 m steps, and 33 m | 2 to 28 m in 1 m steps, plus 30 m and 33 m |
| 8 m | over 6 m, up to 8 m | 2 to 16 m in 1 m steps, then 18 to 30 m in 2 m steps, and 33 m | 2 to 28 m in 1 m steps |
| 10 m | over 8 m, up to 10 m | not available | 2 to 22 m in 1 m steps |
The 10 m width is made in 900 gauge (UV10) only. Longer lengths than those listed are available for collection; please contact us first.
Step 3: Measure the length
Measure the length of the tunnel along the ground, from the outside of the first hoop to the outside of the last. Then decide how the ends will be covered:
- Ends covered by the same sheet: measure the height of the tunnel (from the ground to the top of the end hoop) and add it once for each end, plus the fixing allowance at each end.
- Ends covered separately, for example timber end frames with their own panels: you only need enough extra at each end to fix the sheet to the end frame.
Round up to the next length on offer. Unlike width, a little spare length is easy to deal with: it is trimmed off once the cover is fixed.
Worked example: a small garden tunnel
Here is how the sums work for a garden tunnel roughly 3 m wide, 6 m long and 2 m high, with the ends covered by the same sheet. The fixing allowance of 0.4 m per side is only an example; use the figure for your own tunnel.
- Width: measured over the hoops 5.0 m and 5.1 m, so use 5.1 m. Add 0.4 m for each side: 5.1 + 0.4 + 0.4 = 5.9 m. Order a 6 m wide sheet.
- Length: 6 m of tunnel, plus 2 m of height at each end, plus 0.4 m of fixing at each end: 6 + 2 + 2 + 0.4 + 0.4 = 10.8 m. Order 11 m.
Notice how close the width is to the limit. Had the second hoop measured 5.3 m, the total would be 6.1 m and the right choice would be the 8 m width.
Common measuring mistakes
- Measuring the old cover. It has stretched, and it may have been cut short when it was fitted.
- Measuring only one hoop. Use the largest of two or more.
- Forgetting the fixing allowance. A sheet that only reaches the ground cannot be buried or fixed.
- Rounding the width down. Always round up to the next width.
Before the new cover goes on
Once you know the size, choose the thickness. Our guide Polytunnel Cover Guide: 720 vs 900 Gauge Polythene compares the two: 720 gauge is UV-stabilised for a minimum of 4 seasons, 900 gauge for a minimum of 10.
While the frame is bare, it is the ideal moment to fit Anti Hotspot Tape (30 mm × 9 m, 3 mm foam) along the hoops. It creates a thermal barrier between the steel and the polythene, and its low-friction surface makes it easier to pull the new sheet over the frame.
Frequently asked questions
My total width is exactly 6 m. Is a 6 m sheet enough?
Yes, as long as your fixing allowance is realistic. If you have any doubt about the measurement, choose the next width up.
Can I order a longer length than the list shows?
The lengths in the table are the ones we cut for delivery. If your tunnel needs more, contact us: longer lengths are available for collection.
Does the 10 m width come in 720 gauge?
No, the 10 m width is available in 900 gauge (UV10) only.
Do you deliver?
Yes, polytunnel covers come with free nationwide delivery in Ireland.
Got your measurements?
Choose your width and length in our 720 gauge and 900 gauge ranges. Not sure which size fits? Send us the width, length and height of your tunnel and we will help you choose.