How to build a poly tunnel (in Tasmania)

Building a poly tunnel in Tasmania

Years ago, I wanted to build a poly tunnel prior to the 15m long one used for my Dave’s Seedling business but did not find a whole lot of guides in Tasmania, so after building the latest 6m by 3.6m poly tunnel, I thought I would add one. This post is as much for me to refer to in the future and if others get some tips then bonus. Note I am no expert, I have a carpenter friend who initially helped (more told me what to do) years ago build my 15m long poly tunnel which is where I learnt, and for this latest poly tunnel I got him in to ensure it was plumb and straight before continuing on and to help get the plastic on. There are many ways to put together a poly tunnel. This is just how I do it!

First thing: A flat level area helps.

My property is not flat, so I got an area excavated flat for the poly tunnel – doesn’t take much if you use a good person. Probably costs $300 to $400. Then I marked out the four corners with the steel droppers. I went for 3.6m wide to get a bit more height for this one. The plastic is 7.5m wide (numerous places sell it but Roberts is close to me so I use them – in the Huon Valley). This width means you want your poly pipes to be 7m long to fit the 7.5m plastic. The previous poly tunnel which is a long one at 15m is 4m wide. Perfect for its purpose as a germinating and hardening off house, but just a tad low in height for my 6 foot height. You want to get as close to a perfect rectangle as you can. I run string around the bottom of the four corners and this string remains there for some time. A second pair of hands is very beneficial to measuring out.

I then lay black plastic down. I use 20mm blue metal as the base for my tunnels as this suits my requirements (No weeds a benefit). If you want garden beds in there no problems at all. I grow in grow bags and use benches for holding hundreds of seedling trays.

Then I start putting the droppers / star pickets in with a 1m gap and have the pointed side facing into the poly. I go a 1m gap as it helps keep the plastic strong and allows rain to not pool on top (I have had this when doing 1.5m gaps before). When banging in the droppers, keep checking with a level you have them vertical. This really is important. Take your time getting the droppers in as vertical as you can. I bang them in till 1m of height is left out of the ground (they are 1.8m droppers). The string will help you do a straight line. Make sure each opposite dropper is the same distance from the previous pair (cut a garden stake to 1m length is what I did). The poly pipe connects to these so you don’t want a strange angle! Get all the droppers in and take  a break.

Poly tunnel pipe:

I use 50mm green line. The blue line does not fit over the dropper as they are a bit stronger structurally and thus fatter internally. They are 7m long cut with a handsaw. The poly has each metre marked on it like 070MTR, so it really is easy to cut 7m lengths. I cut one at a time then put them over the droppers taking my time. Put a small amount of pipe over one dropper, then put the other side down a little. It is easier I find putting each side down a little at a time. They should go over nicely but sometimes you have to muck about getting them down. It is important you cut each pipe to as close to 7m each as you can. Do all of them then take a break!

At this stage you have all droppers in to 1m out of the ground and the 7m poly pushed over the dropper to the ground. If you have blue metal or like down, you need this out of the way along the edges.

Box the base:

Next, I screw in 150x30mm wood around the base (outside bottom edge of the droppers). Screw two 50mm bugle screws into the poly pipe. Sometimes the screw may hit the dropper so you have to try again on an angle. This is why it is best to face the dropper in (If you look down a dropper it has three edges like a triangle so the top faces in allowing the bottom with a gap facing outside for the screws. Make sense? Probably not. The base box is where you will screw the plastic onto with another piece of 40X21mm wood and it adds to the structural integrity. Bracing and others also attach to it.

Now I add some decking around the two long  ends of the poly at about 1m high attached to the inside of the poly. Using quick grips is useful if you are doing this on your own. Use a spirit level to get it level and to quick grips around two pipes will help if you are on your own. Before attaching this piece to the poly, it is a good chance to check the gap to the previous pipe as well as the vertical aspect of the dropper and correct before screwing together. You can fix up more when the top centre beam is attached. Do both sides then take a break.

Plumb:

A carpenter is handy to help with this bit. It is about getting one side nice and perfect and braced so everything else from this point forward is built off it. Basically, you start at one end and attach a brace internally on both sides (see photo). You screw it in when the end is perfectly vertical. You also want a piece attached to the top centre of the end poly you are getting right and braced to bottom side where the dropper is. Lastly you want the centre high point at the correct height. You know the pipe is 7m so at the 3.5m mark you attached wood onto the poly and to the base boxing you did (a level will make the brace perfect and then you screw it in. Again a 2^nd pair of hands is needed and if you know a carpenter or someone switched on with this bit then perfect, otherwise do you best. It isn’t a house so it doesn’t have to be perfect, but getting close helps the end result.

Top centre wood:

After the one end is braced plumb, The rest is not too bad as you work off that end. I run a 6m piece of wood the same as that used for the base boxing down the centre of the high point of the poly. You know where to start from the bracing, and a 2^nd pair of hands is required, plus quick grips. A wooden garden stake cut to 1m is useful to get the gap from the first poly pipe. Stick that above the plank, bend the poly you are attaching to meet it. You want 3.5m for the centre, mark it on the poly so it is easy to centre on the board. Then screw the board up into the poly. I repeat this for each poly working down to the other end. Stand back after each one and see it is looks pretty accurate. Standing 5 to 10m back is useful.

Note most screws for this are 50mm long bugle heads 14g. And it is wise to pre-drill all spots to avoid splitting wood.

Once you get to the other end, you will want to check the levels and brace also, one to keep the end piece up to the correct height (this is a temporary brace) and to for the side of that last piece (temporary also – connect to the vertical brace you just did. They will come out when a door frame is done as the final brace will connect to that frame, once that frame is braced, you then remove the temporary brace).

Centre of poly tunnel propped up. Given the weight of the top centre piece, the middle will sag. I attach a 100×100 post to this. Use some decking temporarily to prop centre up to find height needed (use level of centre plank to find the right height (horizontal level). Measure what length post you need and cut and put into position. I screw into the post from above so no screw shows potentially ripping the plastic. Consider this always. For the base of the post, I bang in a small dropper and screw that into the post (back of a block splitter is useful to bang in). Again always stand back and check you are happy with how it all looks.

Then I add two more decking planks around the 45-degree angle mark of the long sides. This is another chance to get the poly in the perfect spots and provides more structural integrity. Plus I run string off these or add irrigation piping later on.

Two ends:

For this one I just wanted a door at one end and a window at the other for ventilation in summer. Basically, I have netting staple gunned on the inside of the window and door. I roll up the plastic on the door and window in the warmer months and drop it back down when cold. After two years I replace that plastic on the door and window and it gets tatty. No doubt there are many ways. I have used Velcro with a sticky back also. Ventilation is important. It does get hot in there over summer!!. So I frame up the ends including final braces before removing the original temporary braces (some original braces will remain at in the photo. Always consider where the main strong winds come from and brace for that at one end.

Poly tunnel plastic:

The frame is done, so you want plastic. For 6 metres long, I got 6 plus 2 for either end plus 2 extra (actually I got 1 extra, but should have got 2 as I had to work around being a touch short for one end – you can see my fix in an earlier photo)

Two people are needed for the plastic. There is an up and down, so ask when buying and remember that. Either end, to people walk the plastic over getting about the same overlap for both long and short sides. Outside of this, focus on the long sides attachment for now.

You connect the bottom plastic to that bottom box boarding you did. The plastic goes between some 41x20mm wood and the boxing, screw in with bugles. Start at one end and screw together with a helper pulling the plastic tight. Take you time to get it right. Do the same on the other side and you will then have both long sides attached. The same process happens on the short sides. A Stanley knife is useful. There are two ways I have attached the ends but this time I just folded together into one or two spots and stapled to the door frame. See photo. Cut excess off with the knife, and you cut a piece off the excess and staple to the door. Same happens on the other end for the window, or door if you want one at both ends. An earlier photo shows my error in not getting 1 extra metre and how I got around it.  Put in plenty of staples – 10mm deep ones I used.

The ventilation is important as in summer it can get too hot in there on 28 degree or above sunny days. I also add shade cloth over the top on these days. I have a system of string and droppers to pull over and drop back. 

And that is it. If you liked this please consider liking my facebook page or have a look around at my store. Between August and April I sell organically grown vegetable, herb and flower seedlings largely from home and the Cygnet Market. Outside of this I now also sell my seeds on this online shop.

Many Thanks

Dave

Poly Tunnel Equipment used approx.:

• Treated Pine decking 90x22mm, about $3.17/m 20m approx. used
• Centre Post 100x100mm pine $21/m 4m used
• Treated Pine for door 70x35mm $4.22/m 7m used
• Rough sawn pine 150x30mm can’t remember price but not expensive, 26m used
• Treated pine for plastic joining 40x21mm 22m some used for door brace.
• Hinge for door
• Padbolt for door
• Staple gun staples 10mm box  2500 rapid $20
• 14G*50 bugle screws box $60
• 14G*100 small box for a few spots you need more length
• 14 star post/droppers $9 each
• 50m roll of 50mm green line poly pipe $3.30/m $165
• Poly plastic 7.5m wide – $17/m
• source gravel if required – black plastic $40 and excavation if required.