This is the top half a two-frequency icosahedron. I chose to go for a much stronger and permanent structure and ordered tannelised garden two-by-one especially for the purpose, along with sand and cement to put in a ‘crazy paving’ base from my garden of rocks (on the edge of Bodmin Moor).
I can’t over-emphasise the importance of having a model to work to for dome building that has more than one strut length, otherwise it is easy to get lost in the structure. Marking the different struts helps too, I painted mine with some left-over wood treatment. I connected the struts using metal discs, pre-cut with metal cutting shears and drilled using a cardboard template. The discs were made from a sheet of galvanized metal I found, and a sheet of corrugated galvanized roofing that seemed to have been flattened by a tractor. The struts were just screwed onto the discs.
Connecting the struts and raising the dome up on tyres allowed me to connect new pieces to the underside. I had to give special attention to the joints at the top because they took additional strain during the construction. I bolted on plywood disks for support.
Piece by piece, it took shape and it became clear to me that a half-sphere dome would be easily tall enough rather than the five eighths dome I had intended to build. This was fine as it meant there was wood leftover for an inside structure.
I dug holes under each of the strut nexus points at the base and put in some uprights to clear the plastic lining sections from the ground and allow me to build-in ventilation. I cemented these uprights in – making horizontal corrections with a spirit level as I went round. I used a mixture of techniques to fill-in these undergaps: engineering bricks, concrete and dog-food tins, old slates, carpet, bricks and blocks with cement (one side was next to the gas output of our septic tank and I certainly didn’t want methane build-ups in the dome).
Then the floor went in, like a mini patio made from all the flattish rocks and bits of slate I could get my hands on. I painted some of the rocks at the back black, and left them raised to absorb more heat from the sun.
Finally, it was time to put on the polytunnel cover. I ordered another £30 worth because I intended to recycle the cover from the first dome into this one. This glazing part was quite laborious as I had to cut the sheeting into rough triangles, staple it onto the struts and then trim to size. It is best to work from the bottom up, then the rain will flow down, and not into, the dome due to the overlaps. Once I got to the top of the dome it got tricky and there was lots of stretching up a ladder. I had made one top triangle as a detachable window which helped with the top glazing and is essential for airflow. I also made a detachable window on one side for wheelbarrow access. All the overlaps were then sealed using a waterproof tape made for polytunnels. I made a door by taking out a cross strut and putting in hazel sticks which pushed the space open so as not to weaken the structure. This has a porch from which I hang netting to keep out the ever-present blackbirds.
The first season I had a good crop of blueberries, picking salad, coriander absolutely loved the warmth in there, squashes, tomatoes, basil and peppers. Next growing season I will hopefully have time to concentrate more on the soil quality in my pots. All in all the wood, screws, plastic sheet, sand and cement cost me around £150, a great investment for a growdome which I hope should last five years at least.