Introduction: Geodetic Greenhouse (low Cost)
Last year I started an allotment and felt the need to build a green house to enjoy the delights of tomatoes and cucumbers. Having built a 30 foot diameter dome before out of steel tubing I thought that there was a good possibility I could make a smaller dome using some scrap wood I had left over from taking apart some old pallets.
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Step 1: Selecting the Dimensions
Fortunately the Desert domes http://www.desertdomes.com/ site has a handy calculator to work out the size and type for any size of dome you need.
I elected to make a 9 foot wide 3 V dome this would stand on a 3 foot high sub frame to give the necessary height I needed.
Desert domes gave me the necessary dimensions for the 3 different length struts.
Step 2: The Joints
One of the issues I had with my fairly small green house was that for larger domes it is easy to drill the ends of the struts and put a bolt through. But for my much smaller - 3/4 inch square - struts this wasn't going to work. So I devised a quick assembly method using some plastic plumbing pipe and some elastic bands.
A small hole was drilled vertically through each strut and a nail allowed the fixing of the elastic band. The bands were cut from an old bike inner tube.
Step 3: Putting Things Together
The struts were cut from the pallet wood using a band saw roughly 3/4 inch square (19mm). This is tedious and your not going to want to do it by hand so if you don't have access to a band saw or something similar buy wood that is the right size.
Perfectly good domes are made from Garden canes and provided they are wide enough you could use this method of connecting them.
The lengths of the 3 types were cut from the Desert dome information and the ends drilled.
Sitting on the lawn all of the struts were laid out and then connected. I chose (perhaps unwisely) to build from the top down because this means you have to lift the full weight of the dome to add more struts.
Step 4: Finish and Cover.
Once all of the struts had been assembled I had my dome. It was quickly evident that there was far too much spring in the elastic connections for the struts and my thin struts didn't allow me to drill thorough the plastic ring and put screws in so I decided to cut a number of connector plates from ply wood and screw and glue these over every joint to reinforce the elasticated and plastic connections. The elastic and plastic rings held everything together just nicely so I could fit the plywood connector plates.
If I were to make the dome from garden cane I would use some plastic tube to connect the canes joining the tubes in the middle with a nut and bolt. as in the diagram.
We transported it to site by the simple process of picking it up and carrying it down the road. the structure is surprisingly strong.
I made a decision to cover the dome using Saren wrap (cling film) and bought a couple of catering sized rolls from the local cheap store. This wrapped the structure nicely and I covered it with at least 3 layers all over. The access door would be cut later.
No need for glue or tape as the cling film is not called cling film for nothing.
Step 5: Results
The final positioning was done and the 15 vertical wall supports hammered into the ground at every intersection of the lower section of the dome. They are about 4 feet long with 3 feet above ground. The greenhouse dome was lifted up and screwed to these vertical posts
The bottom wall was covered in a length of blue plastic film I had laying about. More Cling film could have been used.
I cut out a door for access and although I intended to put a door into this space in fact I never did as keeping the interior cool enough was a bigger problem than it cooling over night. A simple plastic sheet would do though.
I did make sure that the door way was away from the prevailing winds.
Step 6: Success!
Was the project successful - Yes - it worked well for the whole year growing many pounds of tomatoes and Cucumbers for us. The cost was minimal - about £14 and it took about a weekend to build.
The covering proved quite substantial although I guess your probably going to need to replace it every year - this isn't a big cost or issue as it only takes about 1/2 an hour to put a new layer on. The dome withstood storms, hail stones and high winds with no problems at all.
I used the wooden frame as a support for the strings that held up by large tomato and cucumber plants - this was perhaps a little ill advised as the weight of the vegetables towards the end of the season started to pull the structure inwards. I would advise anyone doing this to use separate support canes for your plants rather than the green house structure.
Make the struts at least 1.25 or 1.5 inches (30 - 38mm) square and glue and screw the support plates on and then they will take the weight.
I have built much bigger domes - up to 30 feet wide using 19mm steel tube flattened at the end and joined with nuts and bolts. Such a structure is easily strong enough to climb over. At that size a more permanent covering such as heavy gauge polythene would be better.
Domes are a very efficient use of materials and even a large 30 foot diameter dome can be built in the UK for less than £200 - covering with polythene it will cost in the region of another £200.