If you cannot afford to buy a ready made greenhouse, it is quite possible to build a polythene tunnel greenhouse. All you need for this is some timber, a reel of 4 inch diameter polythene water pipe, 6 x 2 foot lengths of old metal water pipe, less than 4 inches in diameter, and thick, horticultural purpose polythene.
Work out the dimensions of the greenhouse and cut 4 lengths of 6 inch by 2 inch timber planks to form the four sides. Treat the timber with wood preservative. Place the planks in a rectangle as the ground plan, so that you have, for example, a 12 foot by 6 foot rectangle marked out on the ground. Hammer in a couple of temporary supports on each side and check that each corner is a right angle. You can do this with a large set square â€“ or use the 3 x 4 x 5 method. Measure 3 feet along one side from the corner and mark it. Measure 4 feet along the adjacent side from the same corner and mark it. Measure diagonally between the two marks â€“ it should be 5 feet. If it isn't, adjust the position of the two sides until it is. This is Pythagoras' theorem â€“ the square on the hypoteneuse of a right angled triangle is equal to the sum of the squares on the other two sides; 25 = 9 + 16; very useful for a DIY enthusiast! Re-inforce the four corners by screwing 2 L-shaped brackets into each corner to hold them firm.
When your ground plan is ready, hammer in 3 of your metal tubes along each of the long sides, one at each corner and one in the middle. Check that they are vertical, using a spirit level. Do the next bit on paper, as a scale drawing. Draw a horizontal line to represent the width of your greenhouse, 6 or 8 feet as it might be, so 6 or 8 cm From the exact middle of this line, draw a vertical line to represent the desired height of the greenhouse, usually 7 feet, so 7cm. Draw a third line from the top of the greenhouse to one of the ends of the width, so that you have a right-angled triangle. Measure the diagonal and add on one quarter of its length; that is, if your diagonal is 8cm long (as it will be if the total width is 6cm and the height 7), add 2 to give you 10cm, or rather 10 feet. Double this, and this is the length of the span from one side to another.
Cut a length of polythene water pipe to this measurement and, by fitting each end of the polythene pipe over a rigid metal pipe, span the greenhouse. If it is a very tight fit, oil the metal pipe first. Continue until you have three spans completed. You now need to fix either a length of timber or another length of polythene pipe along the ridge in order to keep the three spans rigid. This needs to be fixed under the spans, not on top. Do this by drilling through the highest point of each span and fixing it to the ridge with a nut and bolt. Cover the head of the bolt with padding so that it will not tear the polythene sheeting.
Make a door frame for the front of the tunnel out of timber. Bolt this on to the base and on to the front span. Make a door to fit the frame and cover it with polythene, folding the polythene round the edges and nailing it on the inside.
Now cover the tunnel with the horticultural polythene. Measure the width you will need by taking the length of the span and adding 2 feet. For the length, multiply the height by 2, add this to the length and add on four feet. Put on the polythene on a hot day so that it will expand and stretch tightly over the frame. If at all possible, have help with doing this. Pull the polythene over the frame, bury 2 feet of it all along one side and the stretch it as tightly as you can on the other wide, before buying that side also. Do the same for the back of the tunnel. At the front, stretch it tightly over the door frame, nail it to the frame and cut off the excess.