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Step 5: Covering the Dome

The sheets of plastic were initially attached with handclamps. Each patch is up to 12' x10', but the center point and interior cut have to be positioned to within 1/2" vertically, horizontally, and rotationally to place the end of the cut right in the middle of the hub and running straight up the strut. I didn't actually cut out the small wedge in the patch diagram; I slit the wedge down the middle so that the two edges would overlap when the piece was wrapped around the (non-flat) hexagon at the center.

When the positioning was right I stapled the plastic lathing onto the center seam by overlapping the cut from above, to allow it to shed water. There is 6-12" of margin around the outer edge of each piece which greatly simplifies positioning but also allows each of the corners to be secured under all of the other seams of the vertex, giving me greater confidence in the integrity of these points.

After I had two of these patches in position I secured the seam between them, reaching the top half from above with a ladder inside the dome, and the bottom half from a ladder outside the dome. On each seam I left the top and bottom edges loose to be able to slide in the adjoining top and bottom plastic.

I continued around until four out of five pieces were in place. I needed to leave one off so that I could get the pentagon at the top attached.

Next, I traced out a pentagon onto my tarp in the same way I had done the previous pieces, including the center cut. Just like the trapezoids, no sliver is cut out of the pentagon--it is just a single cut where the plastic increasingly overlaps outward from the center, in the same way you might make a cone out of a circle with a radial cut in it.

This piece was poked up through the center and positioned around all of the edges and along the cut. Note that on all of these pieces the position of the cut on one of the struts is vital, since the overlap is very small in the middle. Unlike the side pieces, the direction of overlap is unimportant since it does not need to shed water downwards.

Once positioned, I started pinning down the seam of the top pentagon from the open side of the dome where I had left off the fifth side patch. Yes, I'm pretty high in the air here. Note that I'm leaning on the dome itself, and have someone below steadying my 10' ladder.

We decided to cover the center of the pentagon, where the seam comes together in the middle of a hub on a horizontal plane (i.e. it isn't sloped to shed water) with a circular "hat". Thus, a ~14" circle was attached with small strips of lathing on each hub at the top, and the seam lathing came down over the top of that. The pentagon itself is sloped down on each face, so should shed water (though not snow!).

After the hat and seam were attached I went to work around the sides. Each edge was attached from the next triangle over, all around the pentagon. You can see the circular "hat" here as it appears from the inside. The beginning of the seam was started from within the triangle being covered itself, and then the ladder was moved and I came up from within the next triangle so that it could be pulled flat.

I was using extra long staples to be sure that the seams would hold. The electric staple gun allowed me to easily place all of the staples at full arm extension, but because of the long 9/16 staples they didn't fully penetrate the wood unless I really held the gun tightly to the surface. For this reason, I had a hammer up there to pound in anything that wasn't flush. I can't imagine only doing this with the hammer, though, since I generally had to keep two plastic surfaces in tension (opposite directions) under a tensioned plastic lathing strip, and I could not have done this if I needed both hands in order to place a fastener with one and use the hammer with the other. The hand clamps were in constant use and I occasionally tacked a bit of plastic in place with a single staple to keep it there while I held an overlapping piece.

You may be able to see on the picture that the plastic lathing at the ends of each strut is attached on either edge. This is to go around the metal straps that hold the struts to the hubs. Since the edge is thin and weak here, I used more staples. In fact, I used a lot of staples throughout. We've had quite a few wind storms (this area is famous for them) and nothing has loosened in the 65mph gusts. I will also mention here that this woven plastic is quite nice to work with--much sturdier and more resistant to creasing and kinking than any other flexible poly that I've used.

Now it was time to position the fifth side piece. Again, the clamps were used and each side checked. When we were sure of the position, the top and middle seam were clamped and the whole side was rolled up and inserted through the top middle triangle so that I could work on the top seam of the patch. I temporarily tacked the top into place.

The pentagon was folded back down over the side patch, and the lathing on each side of the open pentagon triangle was finished out to the edge over the top of the plastic of the side patch. Then, the last edge of the pentagon was finally attached.

The side patch plastic was unrolled back over the side of the dome, and I moved over to the next triangle to secure the center seam. At this point, the patch was not attached on either side, just at the top. After starting the center seam from inside the triangle, it was finished from a ladder outside the dome. After the seam was finished, each side of the patch was similarly completed from the inside first and then from outside the dome for the bottom half.

With the top half of the dome completely covered, the bottom is sealed with four trapezoids (basically the tessellated patch from the diagram above, minus the top triangle, turned upside down) and five triangle patches (just the top four triangles from the same patch). Then there is the door and the portions around it which will need to be closed up as well. Each lower piece is tucked under the top pieces, whose bottom edges and corners were left open for this purpose.

As in the top patches, the center seams have to be positioned, secured and lathed before the outer seams. At the edges, the bottom patches have to be leaved under the top patches, and the triangles have to be under the trapezoids. There are only five points around the greenhouse where the corners of the patches meet, and for each of these joins there are five corners which have to be layered in the right order. All of the edge seams around one of these vertices are under the lathing before I work on the corner. I tack down the lower layers with a single staple while I pull the upper layers into place so that they can all be pinned under the same lathing. As I mentioned before, this results in the corners all being tacked under the their two opposite seams across the hub, making it even less likely that anything will come loose.

In between the trapezoids are triangle shapes, as I mentioned before, which hardly bear mention since they are trivial to attach from the ground around the dome. The only other oddities were the two pieces on either side of the door which were small and custom shaped to fit. At this point an overlapping flap door or similar arrangement of plastic on the open hexagon would make this a functional closed structure.