how to build a polytunnel

A polytunnel using the following materials can be made to any size within reason, the smallest on our allotments is 8ft x 6ft and the largest is 14ft x 40ft. A polytunnel built as described below will stand up to storm force winds that would reduce a greenhouse to a heap of rubbish. Most materials can be begged or bought cheaply.  The most costly item is the cover £86 for the one to cover my 10ft x 20ft tunnel (see photo on left) but, they have a guarantee of 5years and will last for 7 or 8 years. Tghe Scaffolding boards shown in the photo where used mainly because the site is on an incline and they were needed to hold the soil out on one side and in on the other, but they do help to protect the cover from accidental damage from spades and forks while gardening inside the tunnel.

Scaffolding tubes 48mm OD 5ft long tubes will give you about a 3ft straight-sided tunnel. 2 are required for every 5ft length of tunnel. Example…..a 15ft long tunnel will require 8. Source. Building sites, scrap metal merchants, derelict allotments or scaffolding firms where the will cut you some bent or damaged pipes for the price of a few pints if you ask nicely Tubing Mains gas (yellow) or mains water pipe (blue) 50mm ID. A 19ft length will span a 10ft wide tunnel giving a height at apex of about 8ft. Example a 15ft x 10ft tunnel will require 4off 19ft lengths of pipe Source. Large industrial building sites, new housing estates, council building yards where there will often be pieces left from a roll after jobs have been completed. Cost is usually nothing as they are glad to get rid of it. Alternatively, if a few people are building a tunnel a roll could be bought and shared. It does not cost a great deal, so I am told. Timber 8off 10ft lengths of 50mm x 50mm rough sawn timber to make the two frames and door Source Builders merchants about £2.50 a length Timber 50mm x 25mm slate battens, about ten are required for a 20ft tunnel. These are required for the stiffeners between tubes, to hold the cover to the frames and to make removable vents for the door and end frame. Source. Builders merchants. Cost about £1.50 for a 5m length Optional Old scaffolding boards or old floorboards. Enough to go round the perimeter of the tunnel Source. Scaffolding firms or reclamation sites. Cost….. depends how charming you are. Wood screws (6mm x 25mm roofing bolts optional) A trade pack of wood screws from ScewFix for about £6 will cover all requirements, roofing bolts can be purchased from there as well. Polytunnel cover Measure over the hoop plus two metres that are buried in the trench x the length of the tunnel plus the height at both ends. Covers are cut by the running metre from set widths …..7.2m…9.2m…11.2 Source Mail order

Knock the first tube into the ground so that it will stay upright, measure in 5ft increments to the furthest tube and knock this one in. Put a tight string line between these two tubes about 2 inches from the bottom. Knock the other tubes in using the string line at 5ft intervals. Making a large square from three of the slate battens (using the 3,4,5 rule) and working at one end, temporarily position the first tube of the other side. Go to the other end and do the same again. (Check the distance between these two tubes is the same as the other side and then drive them in) Put the string line between these two tubes and knock the other tubes in at 5ft intervals. Now check by eye and measuring corner to corner that the tubes are square. This is not critical but needs to be as near as you can get it. Using a spirit level, a 12 or 14lb hammer and a block of wood (so as not to damage the ends of the tubes) knock the tubes into the ground until they are firm, about 2.5ft depending on subsoil. Start at one end; use a length of wood and a spirit level to make sure that all the tops of the tubes end up at the same height. Inspect the tops of all the tubes; remove any damage with a metal file, using a scrap piece of poly pipe check that it will slide over the end with ease. 

Cut all tubes to the same length. Using a sharp knife or half round file remove the swarf from inside the ends of the pipes. Assuming the scaffolding tubes are at the same height, put a mark at least 12 inches down from the top each scaffolding tube, this is were the plastic pipe will be slid down to. Place each loop of plastic tubing over the scaffolding tubes as far as the marks, if the plastic tube has gone oval, use a G clamp to make it round until it is over the scaffolding tubes, also a little grease may help. This is a two handed job. The plastic tube needs to be fastened to the scaffolding tubes to stop it slipping farther down. I have used two different methods, Drill and tap a 6mm hole through the plastic and scaffolding tube and screw in a 6mm roofing bolt (See Photo on the Left) or drill a 6mm hole all the way through and insert a 6inch nail, cutting off all but an inch and then bending it over to stop it being pulled out.

I have fitted boards round all the tunnels for two reasons, it stops you sticking the garden fork through the plastic cover and it’s an extra means of fastening the cover.
I have used old scaffolding boards but old floorboards would do just as well.
Place the boards around the outside of the scaffolding tubes using a spirit level so that it is just above the top of your finished soil level. Hold them in position using G clamps, Drill right through the boards and into the scaffolding tubes with a 5mm drill bit, remove the boards when all the holes are drilled and put a thread in each hole using a 6mm tap and wrench. The boards can now be fitted in place using suitable length roofing bolts. 

Both the end frames are made out of 2×2 rough sawn timber, one to take a door and one to take opening vents. If you can get a second hand door and window frame with an opening part from the municipal tip all the better.
Place the two 2×2 uprights so that your door/window will fit between or 30inches apart if you are making your own, cut the top of the 2×2 at an angle to meet the curve of the pipe, fix the 2×2 to the pipe by driving a 3inch screw at an angle through a pre drilled hole in the wood and into the plastic pipe, one from each side. See Picture & Inset below. The bottoms of the uprights need to be securely fastened, either by screwing to scaffolding boards (if used), digging a hole and concreting them in or driving a stake or angle iron into the ground and fastening them to this.
Two cross pieces need to fitted at both ends using 3inch screws as before.
Before fastening the bottom of the frames get the two end frames and tubes as near vertical as possible.
Doors and removable frames can be made easily from 2×1 slate battens and 2inch screws.
Two small vents should be left one at each end covered with mesh or windbreak netting to allow permanent ventilation.
Fasten slate battens using 2inch screws and pre drilled holes in the battens top centre and half way down each side making sure that each tube is as near vertical as possible. Polly tube has a mind of it’s own and you will never get the tubes perfect. Additional stiffeners can be fitted at each end as in photo (belt and braces).
This is a good time to dig/level the beds and lay paths. 

It makes fitting the cover a lot easier if Anti-Hot spot tape is purchased with the cover and fitted to the plastic tubes.
Pick a warm calm day to fit the cover.
Cut all the battens that hold the cover to the end frames and bottom boards (if fitted) pre drill the battens every 6inches.
Dig a trench all round the tunnel at least a spit deep and the same in width.
Unfold the cover down one side of the tunnel and pull over the tunnel, centre the cover (use the fold lines) making sure that there is sufficient at both ends to fasten to the frames.
Put a little soil back into the trench, straighten out any creases, add more soil and by pulling on the spare cover that is sticking out of the trench the cover can be stretched quite tight over the hoops. Refill the trench with soil at both sides, if boards have been used round the bottom, screw a slate batten along each side at the top of the board trapping the cover.
At one end and starting at the top pull the cover tight and fasten with the pre-cut battens, work down both sides and then repeat at the other end.
When you are happy with the cover, the spare can be trimmed off at both ends and down both sides, fill in the trenches at both ends burring the some off the spare cover.
Refit the door and any meshes/vents that have been made.
Get the plants in.


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