Re-skinning The Polytunnel

It’s March and the allotment continues to slumber, with only the perennial daffodils poking their heads above the damp soil. With nothing much here to prune, water or harvest it’s the ideal time of year to work on all things structural, which is great for somebody like me who is much more proficient with a screwdriver than a trowel.

So far this year I have re-laid paths, installed guttering on the shed and started preparations for a fruit cage but this blog is about my refurbishment of the polytunnel this time around 2 years ago, which begs the question – why did our 18 month old polytunnel need refurbishing?


A polytunnel is a frame covered by a plastic film. Unfortunately my tunnel consisted of a good sturdy 32mm frame wrapped up in an exceptionally poor quality cover which disintegrated after just 1 season. If that wasn’t bad enough the allotment treasurer on a neighbouring plot followed our lead and bought the same tunnel, which deteriorated at the just same rate. So much for my recommendations.

Sieve
Sieve

The only available off-the-shelf cover would have been identical to the one that had failed so I threw myself into weeks of research involving supplier YouTube videos and amateur allotment blogs before arriving at my preferred design. This was going to entail not only a new high quality cover but the construction of a bespoke new base frame that would hold the cover in place. Perhaps most excitingly for me was the opportunity to develop a multi-facetted spreadsheet capturing all of the build requirements, components and project sequencing!

Poor abused car
Poor abused car

Over the period of a couple of weeks I became quite intimate with the local builders merchant, conducting numerous trips to collect unfeasible quantities of timber. It’s surprising how much you can fit into a Ford Focus if you don’t mind not being able to see out of the passenger side window. It might also help if you are relaxed about your car’s resale value.

No left turns
No left turns

A polytunnel performs best when it is wrapped in a tight membrane, both in terms of thermal characteristics and structural longevity. The original cover had simply been buried into a trench around the frame and the new cover was instead going to be held tightly down by a wooden base rail. I marked the beginning of construction by levelling off the soil around the perimeter of the frame to provide unhindered access to the bottom of the tunnel.

Digging, it's always digging
Digging, it’s always digging

The next step was in many ways the hardest. Despite the fact that the once impervious cover was now just a very large sieve it still felt unintuitive to take a knife to it. I sheared it off 3 inches from the soil since this represented a much simpler task than digging the sides out of the ground. No going back now.

Cuts have to be made
Cuts have to be made

Since the new film had to be tensioned and secured around the tunnel extremities this entailed not only the installation of a timber base rail but the replacement of the tubular metal door frame with wooden posts and a lintel. You can see below an original metal post on the right and a new timber post on the left.

Early doors
Early doors

The whole refit, while simple in essence, demanded a great many subtle design features that needed to be identified and catered for. I had spent many hours visualising and sketching joins and fixing types and yet more hours still evolving the sequence in which it would be best to construct the various elements.

For example, the single door post in the photo above required the existing diagonal support to be rotated through 90 degrees, using a special clip while the post itself was secured by 2 special P-Clips and over-sized bolts. Listen, if you think this is dull imagine having to live with me during the design process.

You've Been Framed
You’ve Been Framed

I cut up sections of hose pipe and wrapped these between the clips and the metal frame like over-sized washers to create tension in the fitting. Then I cut a wooden lintel to complete the frame, ensuring enough height to avoid banging my head every time I entered.

One thing that no amount or reading or virtual design could have prepared me for was the dawning realisation that my plot had a mild slope to it, which in turn meant that the tunnel frame was in fact slightly elongated to one side – something you wouldn’t observe from a casual glance. Bizarrely this required the left door post to be about 5cm shorter than the right post. It also meant I could never assume any dimensions but had to measure and cut each piece of timber independently while putting all of my faith into a spirit level.

DIY SOS
DIY SOS

With the door frames in place attention turned to the base rails. These substantial beams were plated together and u-bolted to the metal frame an inch or so above ground level.

I’ve always enjoyed working with wood but never with anything on this scale. Handling timber of these dimensions was bringing out the builder in me. I started to keep a pencil behind my ear. I found myself drinking twice as much tea. I even loosened my belt a notch to experience that extra inch of “tail breeze” when bending over.

World's Strongest Man
World’s Strongest Man

My trips to the allotment were frequent and lengthy for a period of 3 weeks while the entire build continued. I didn’t want to rush a job that was supposed to pay me back for many years to come. Each trip involved the packing and unpacking of a great many tools such as saws, drills, planes, chisels, clamps, spare batteries for my drill/screwdriver, not to mention fittings like screws and bolts. Lego used to be so much more spontaneous.

Homes Under The Hammer
Homes Under The Hammer

With the door frames and base rails in place the next step was to fit a continuous length of baton to the top of each rail section, for reasons that will hopefully make sense later. Any sharp corner was cut off to avoid potentially splitting the plastic cover when it was stretched over the frame.

Great Rail Journeys
Great Rail Journeys

I had sourced pressure treated timber suitable for outdoor use but as a precaution I dabbed wood preservation over all of my saw cuts. I bet nobody else does that. I also bet nobody else would have dragged this all out for so long.

Finally, with the growing season standing behind me impatiently tapping me on the shoulder, the time came to fit the new plastic sheeting. This was going to be the moment when all of my plans either came together or ended up in a skip.

Cash In The Attic
Cash In The Attic

Research had led me to a highly evolved polymer film designed in partnership with a university and the agriculture industry for use by professional growers. It cost 50% more than typical polytunnel film yet was a no brainer for the enhanced thermal and structural characteristics. So long as I didn’t cock up the fitting.

You simply can’t do this next bit on your own so I roped my parents in. First we dragged the cover over the frame. On a film of this quality there is an inside and an outside so I was super careful to ensure the sheet was the right side up.

Celebrity Masterchef
Celebrity Masterchef

There now followed a series of precise steps that I had to follow with surgical detail. I fitted more thin batons to the timber base rail on one side of the tunnel, tightly sandwiching the polymer film all the way along. Then I repeated the operation on the other side of the tunnel, keeping the film tight over the top of the frame. With the plastic secured on both sides we moved inside the tunnel, loosened the u-bolts that were clamping the base rail to the frame and my parents stood on the base rail to tension the plastic while I re-tightened the clamp.

Supermarket Sweep
Supermarket Sweep

At least that’s what should have happened. When your parents collectively weigh as much as a moderate Sunday lunch there’s only so much tension you can get into the plastic. Most of the tension was mine as I worried about the plastic splitting, although these fears were groundless.

With the film tensioned widthways the next task was to grip the plastic under each door frame and suspend my entire body weight while my dad rapidly hammered in more battens. Again, that’s what should have happened. Instead I dislocated my fingers hanging off the film for 5 minutes while my dad pillocked around doing – I’m not sure what. I was really grateful for his help a week later once the agony in my joints had subsided.

Tipping Point
Tipping Point

And with that final act of self harm the tunnel was reborn! Just the doors to fit. In retrospect I will never again use the word “just” in any sentence related to fitting doors. It turns out that fitting doors is not something you “just” do, at least not when you are custom making a door to fit a custom made frame.

Quincy ME
Quincy ME

One of the challenges with a larger polytunnel (and at 6m x 3m this tunnel is edging towards the fringes of largeness) is providing adequate air circulation and temperature control. Polytunnels maintain a higher internal temperature by design but in the hotter months you need to get air movement inside to avoid incinerating your plants. Leave the door open for any extended time and you invite cats, foxes, birds and butterflies (hence caterpillars) inside. My solution was to attach a permanent netting mesh to the door to prevent unwanted guests and make removable film panels to keep the heat in during the colder months.

Open All Hours
Open All Hours

Two years later and we are so pleased with the quality of our refit. The build quality has proven its worth in the face of storms and settling snow. The door panels have enabled us to regulate the environment whatever the season.

A Place In The Sun
A Place In The Sun

The professional film has resulted in an astonishing improvement in growing conditions versus the old cover. I would hope to get a minimum of 5-7 years out of the cover before it needs replacing, although it’s not uncommon for this particular covering to last for 10+ years when fitted well. I deliberately used screws for (almost) every timber join in order to massively simplify such a future operation.


Job done? Well not quite. The current gravity fed irrigation system still needs further improvement or replacement with a solar pump. Also I would love to be able to harvest rainwater but that’s complicated by the fact that I’ve nowhere to mount a gutter.

These are hopefully projects for this year. There are always more projects. You never finish everything. In an allotment infrastructure is like planting. You always want to do more or do better. It’s what keeps bringing you back.

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