Thought I'd do a short blog on the Polycarbonate tunnel we built.
Our big field suffered with no storage, and location meant containers were very expensive to transport. so one option was to build a polytunnel, which we have planning permission for. again due to location, 500m from the coast 73M AMSL I wanted something sturdy. so looked at off the shelf packages. in the Highlands and Islands the PolyCrub is seen as the mecca. Designed in the Shetland isles, guaranteed to withstand 100MPH winds etc etc. due to the ability to get CROFT grant assistance on these, the price is, IMHO, artificially high CIRC 6K for 4M *6M.. this wasn't going to happen, so after looking at many photos I designed my own..
Basic principle is hoops of MDPE pipe secured to posts in the ground, then 3*2 timber used as horizontal support, with Polycarbonate sheets secured to this timber. the bottom sides are then clad.
My design was to use 65mm Black MDPE pipe, and secure the timber with coach bolts and penny washers through the pipe. this way I get a guaranteed fixing that won't pull out , and it still allows for flex in the whole structure.
4m wide x 6M long, but this can be extended.
My ground has bed rock close to the surface, 300-500mm below ground, so just knocking in post wasn't going to be secure enough to withstand the winds, also where I was sighting the tunnel its on sloping ground, so I couldn't get it level.
I cleared the area to removed the top vegetation and a bit of the top soil. and dug 14 holes. then due to rock levels I then core drilled the rock to allow for the post to be deeper.
We then set these in concrete, (at the same time as doing the slabs for the cabins.)
The next day we had a look and I wasn't too happy with the security of the posts, so slight change of plan, we created a plinth. basically I used some 6x2 either side of the posts, with a slight angle, then filled this with concrete (again we used ready mix at the same time as back filling the treatment plant)
this added about 300kg of concrete to each side, and joined all the posts together, and gave us a solid plinth at the side.
No Photos of the next stage but we cut 7M lengths of 63mm pipe, this when placed over the posts gives approx. 500mm dwarf walls and a 6M polycarbonate sheet, giving a head height of around 2.3M.
we used a plumb bob to get the centre line on each hoop and drilled an 8mm hole through the pipe and secured the top 3x2 treated timber to al the hoops.
One thing we found was that the MDPE did not bend uniformly, this may have been due to slight errors with the posts being plumb. The resultant timber (which was not very straight), snaked from hoop to hoop. after head scratching and re thinking. I decided it was a poly tunnel and to get on with it. we used 7 lengths of timber jointed with half lap joints to extend the length to 6.5M. if using a clock analogy the timber was placed at 9,10,11,12,1,2,3 positions, with the 9 & 3 o'clock being 500mm off the ground, also ensuring the 9 & 3 o'clock timbers were approx. 5.8M apart circumferentially.
Then it was a case of securing the polycarbonate sheets to the timber. each sheet is joined with some soffit H joint strip. (you can buy a clear joining strip for the sheets but for a 6M length it was around £65. the H strips proved extremely difficult to connect the two sheets together, or should I say impossible. so I cut off the back edge on one side and used some glazing sealant. this way we could attach the strip to the polycarbonate prior to bending over the tunnel. and each sheet 'H' strip basically lapped over the previously installed sheet.
day 2 we had the basic structure.
It was noticeable that the curve was not symmetrical, so putting the door frame header in by securing to timbers at 11 and 1 o'clock was not level. I overcome this by using a ratchet strap attached to the 11, and 3 o'clock timbers and tightening until timbers at 11&1 o'clock were level. I then put the two door jambs in (concreted at the base) and cut to the length required and secured to the header. This worked and the tunnel was now more cylindrical.
I framed the rear by baring 3x2's off the horizontal timbers.
Last steps were to clad the base, I used 4*1 treated timbers that were screwed into the MDPE pipe and wooden posts. then used strips of visqueen to create a more sealed dwarf wall, this will help prevent driven rain from the winds getting inside. over this I used Larch timber backs* to create a vertical cladding
All in with hardware, polycarbonate, timber, concrete the project cost £1200. not cheap, but its solid as a rock. It withstood the first Autumnal storms this week with winds hitting 50MPH.
Its also very warm inside, even now as the temperature is falling.
Larch timber backs are the offcuts from the saw mills and are reasonably priced as scrap. we purchased a pack of timber backs, approx.40 lengths of larch timber with bark in 4.8M lengths for £100. these are not uniform and taper etc, but as i only needed 500mm lengths it was straight forward to make it work.
Edited by Jenki