This post is part of my Sustainable House Day submission.
Start by preparing/ levelling the site. We decided to lay 400x200x40 concrete pavers inside but it couldn’t been left bare. This helps to suppress the weeds.
We had built a similar poly tunnel 10 years ago at Longley so had a fair idea of what we wanted to achieve, what we could simplify & improve.
Each hoop is made of 2 inch rural polyethylene (poly) pipe. Shop around. We had initially tried smaller diameter 1-1/2″ pipe at Longley but it was too floppy. 2″ has the advantage of fitting over a star dropper. But beware, there are two slightly different sized star droppers – Warratah & BHP. The pipe slips just nicely over one (I can’t recall which sorry), but it is a much tighter fit over the other brand.
In either case, it’s best to use an angle grinder to round over the top three corners so they don’t snag on the polypipe as you are sliding it down over the dropper.
Each dropper needs to be anchored sufficiently in the ground and sufficiently overlapping the pipe so that the enclosure won’t blow the whole thing away in a gale. We bought droppers in packs of 10 (really heavy) 1800mm long, cut into four pegs each. So about 400mm of exposed pipe and 400mm in the ground. Depending on your soil type the end will need to be cut to a point with the angle grinder.
After a few failed attempts we settled on each hoop being 6m long. This gave adequate head room.
and a 1m hoop spacing. In other words we used 12 star droppers & 6 hoops for a 5.2m x 2.9m floor plan. You can make it smaller or larger depending on the timber bracing.
In the end I wasn’t totally happy 400mm below ground was sufficient, so I wasted a lot of time adding right angle brackets to tie the polypipe to the pavers too. 400mm above ground becomes problematic though as it gets increasingly difficult to slide the polypipe over the dropper.
We used treated pine timber for all the battens, bracing & other framing. There are steel options available, but timber is perfect as you’ll read below for stapling, and easy to work with. Much the same as why houses are still generally timber rather than steel framed.
We used 5 x 5.2m battens (40×20), one at the apex, one around shoulder height, and one along each bottom/ side, running the whole length of the tunnel to space the polypipe evenly. Roofing screws have a coarse pitch & were ideal for holding the timber against the polypipe. The bottom batten is the only surface for stapling the plastic film to (discussed later).
Make sure the battens are underneath, not on top, of the hoops. There must be nothing sharp between the hoops and the plastic film otherwise it will wear a hole through.
An 40×20 on each side provides additional diagonal-side bracing:
The plastic film is “proper” greenhouse grade clear plastic. There should be a sunny side marked which I assume is for handling UV. Even after 10+ years at Longley our original one didn’t show signs of cracking or other UV damage.
Allow about 400mm over each side and end when cutting the film to size. This is important! The edges are then wrapped over & over into a 20mm or so cylinder, about 5 wraps thick, and this provides the strengthening where the staples are places. For example below:
We used 12mm thick staples. You probably want to buy or borrow a good hand tacker as you’ll use a 1000 staples. I started with 9mm long staples but 12mm go into the softwood fine, and are therefore less likely to pull back out. We used a spacing of ~150mm.
This step requires two people, and a fairly calm day. Drape & center the plastic film over the apex of the tunnel then attach each side, only along the bottom batten, in turn. Tighten the 2nd side as much as possible. It’s somewhat tricky to get right. Tighter is better when the wind is later blowing though.
For the doors and end framing we mostly used the thicker 40×30 treated pine. Steel strapping & brackets came in handy for attaching pieces to each other. In the photo below the bottom batten is 40×20 but in hindsight 40×30 should have been used.
For additional reinforcement used rolled strips of plastic film, for example for the door bracing:
We made our own door handles from timber offcuts & threaded rod, but were time consuming & not altogether satisfactory.
We put concrete blocks along the skirt of the plastic house so that weeds can be brushcut without flicking stones against or otherwise damaging the film.
Enjoy the fruits (& vegies) of your labor.