Hydroponics - Take 2
Building a Greenhouse

We've moved into a new home now, and this one not only has much more sun than the last, but also has room for an outdoor greenhouse.

Materials list:

Pat and Stacie working - one triangle at a time When we arrived at the new dome home, we found a much sunnier yard and an inviting hillside with space for both a garden and a greenhouse. There was one madrone tree leaning precipitously over where we wanted to put the garden and greenhouse, and out over the driveway. Since about a dozen madrones had fallen in the yard a few months earlier (in the El Niño storms of early 1998), contributing to all that sun we now enjoy, we felt that tree was a likely candidate for falling, and had to have it removed. This set us back about 6 weeks (finding reliable contractors in the mountains is no small task) and it wasn't until mid-summer that we got the deck built and started on the dome.

The deck was made with shipping pallets that we got from places that throw/give away shipping pallets, and after shimming the various corners with small pieces of wood to get them to sit (reasonably) flat on the hill, I secured the plywood to the pallets with self-drilling bolts. These things were a great timesaver, and allowed for quick assembly. (And disassembly. I've had to remove bolts occasionally while working on various structural things in the greenhouse. I highly recommend them over nails.)

There was a somewhat gentle slope to the hillside, so we decided to use that to our advantage and have a natural gravity feed for the water return in the hydro system. The pieces we made the dome from were 3/4" PVC tubing that I bought at the local hardware store. The 3/4" stuff was a bit tight for getting our fingers into while attaching the hardware, so I recommend getting 1" tubing. (It has to be big enough to get your fingers into comfortably, and be able to hold a nut in place while you fasten a screw from the outside. Details and picture below.) You end up doing this several hundred times, so make it easy on yourself. :-)

I used the dome calculator to help with determining the strut lengths. This greenhouse is a 2-frequency alternate dome, and only has 2 different-lengthed struts. This made cutting and assembly quite easy. In fact, it took us many hours of assembly (over the course of 2 days) to fasten the plumber's tape to the PVC pipe, and only about 2 hours to assemble the whole thing once the struts were prepared. (This picture and the next were taken 53 minutes apart by our friend, Eric Messick.)

Almost finished with first version of structure That brown tarp you see was a bad idea. We were going to use it to try and protect the wood of the deck, but it turned out to be a massive solar collector which promptly heated the greenhouse up to 100°+ F (probably more like 120°+ F) and killed most everything in it. (The late start on building the thing meant that we didn't really get our seedlings in until mid-July, and that was just the wrong time to start. We also only had the half-dome at the time, so it was somewhat low, and you had to bend over to work in most parts of the greenhouse. There was also no vent mechanism, nor really any proper door at this point. And the first layout of the trays had 5-6 parallel trays, with hose going into the top of each one. This caused many problems as we had 5-6 points of failure, and fail they did. Hoses would slip out of position, clog up, or have some other mishap. What happens to plants in an extremely hot greenhouse with no water? They fry. :-( As noted elsewhere, all the plants except for 2 died in this setup. We had to bring those two into the house, and they were scrawny. A very educational year, but also very disheartening.

We eventually removed that tarp, and used Thompson's Water Seal for the deck, which is a clear sealant that causes the water to bead up and roll off the wood. It's pretty amazing stuff. I think we put 2 coats down in late 1998 or early 1999, and put on another coat or two for touch-up in late 2000. It didn't *really* need it, but while we had it all torn apart, it made sense to do it then.

Dome greenhouse
hub closeup Here's a close-up view of the hub structure I decided on. It's a variant of the connector designs I'd seen on the net, and I decided to use plumber's tape because 1) it's pre-drilled with lots of holes, and 2), I had lots of it left over from the previous hydro setup. We cut the tape into pieces ~3" long, pre-drilled holes in the ends of the PVC struts, then fastened them in place with machine screws and nuts. The fitting order was screw-pipe-plumber's tape-nut. We had to pre-bend the plumber's tape a bit to get it to fit into that 3/4" tubing. Again I'd recommend at least 1", perhaps even 1.25". The machine screws worked out very well for this, as you could thread them on by hand and finger-tighten, then fully tighten with a screwdriver. I think they were about 3/4" screws.

At each hub intersection, we brought together 4, 5, or 6 struts, depending on where they were in the dome. (Only 4 on the part that touches the ground. 5 or 6 elsewhere.) For these we used a longer machine screw. Perhaps 1"-1.5", and wing-nuts that we could hand-tighten. We passed the bolt in from the outside, so that the rounded edge would face outward (where the plastic cover was going to be) and the wing-nut was inside, where it wouldn't catch the plastic, and we could reach it.

During the contstruction period, the whole thing was pretty wobbly and flexible, due to the highly flexible nature of that plumber's tape. However, once it began to take shape, it was better. However, the bottom edge of the dome becomes it s weakest part unless that edge is secured in place. We had a variety of mishaps with it sliding down that sloped deck, and one morning, after a large windstorm, came out to find the entire greenhouse had been flipped off the deck and was sitting, upside-down, crumpled in our driveway! (That was fun, let me tell you.) Once we'd wrestled it back into place, I got more of the ever-present plumber's tape, and made little "omega" shapes that would fit over the tubing and lie flat on the deck. Then I used those self-drilling bolts to anchor the thing down to the deck, and we haven't had a single problem since. (I'm sorry it took me a year to learn that trick!) In fact, one thing I noticed was that as soon as we fastened that bottom edge, the entire dome became rock solid. Sure, the PVC will still bend if you grab a strut in the middle and wiggle it, but if you try to slide them around, you find that each hub no longer moves. The whole thing synergetically locks in place, and is surprisingly strong.

I have since discovered that the hardware store sells galvanized-metal "omegas" for about $.35/each, in a variety of sizes to fit different pipe diameters.

version of greenhouse This is our current setup. First off, you'll probably notice that it's taller. After the first year, we decided to add another row of triangles to the bottom, so we made up another batch of struts, and attached them in about an hour, with the help of our friend, Martin Roby. Martin was the tallest one of the three of us, so we gave him a tall pole and had him suspend the dome from the center on that pole, while Stacie and I ran around the bottom and attached new struts to the perimeter. (This is before we'd anchored it to the deck, as noted above.) This was much easier than trying to deal with all the floppiness, and I'd recommend that you try having someone with a pole, or ladder or something to hold up and raise the dome as you build it. (Bucky always recommended building domes this way, suspending them and then jacking or craning the dome higher. Then the people can stay safely on the ground and walk around the bottom edge, rather than climbing on scaffolding, or dealing with all the awkward bracing you see in many dome-home raisings.)

We also put in a door once it was taller, merely by leaving out one of the struts and creating a diamond entryway. Stacie found a really cool Zipper door that you just attach to your greenhouse plastic, unzip, then slit up the middle. Works great. (The zipper is nylon, though, and after about a year and a half of daily in-and-out, we found it to be pulling apart in that annoying an non-repairable way that nylon zippers do. So this year we got 2 of 'em, and have a spare. They're cheap. :-) )

Automatic top 
vent openener You'll also notice the vent on this version of the greenhouse. We made an opening and installed an automatic roof vent opener, which uses no power, and runs on heat which expands paraffin in a piston, opening the vent during the day, and closing it at sundown. We're using the "Seasam Solarlift" model, and have had no problems with it. I found that it does need a drop or two of oil on the piston about once or twice a year. That's probably because my arrangement has it sitting where it catches rain-drip. Other than that, it's been maintenance free, and opens whenever the greenhouse gets warm. (I've got it set to start opening at about 74° F.)

I also have a small 5 Watt solar panel on the greenhouse. I had originally hooked up 3 DC fans that I got from a surplus PC dealer so that they would just keep the air circulating around in the dome. You can see one of the (currently inactive) fans in this picture. I've re-done the fan arrangement this year so that the panel runs only one 4.5" fan hooked to an exhaust tube. It will now blow warm air out through the back of the dome, drawing fresh, cooler air in through the vent. The dome is 12' in diameter, so it has ~120 ft2 of floorspace and about 700 ft3 of air volume. I haven't done an air-volume exchange rate calculation on that fan, yet, but when I do, I'll post it here. I suspect that the fan is forcing a complete air-change every 5 minutes (or less) whenever the sun is out.

 used to keep rain from pooling This is a simple stellation that we added in the Fall of 2000, just before our rainy season began. This triangle is almost perfectly level, and as such it catches rain, causing it to pull in and pull the plastic sheeting loose all over the dome. The first winter or two this was very problematic, and we had to check and empty with a broom it every day or two during the rainy times. Just adding 3 shorter struts on allowed me to turn the triangle into a tetrahedron, which pokes up just enough to give a runnoff slope to the plastic. We haven't had a rain-pooling problem with it since putting this in.

Some of these things just took time to learn, but this year has already given appearances that it may be our best year yet in the greenhouse. I feel much happier, overall, with the general physical layout and the small changes that I've put in this year such as the stellation and exhaust vent seem to be making a big difference in the things that were problematic last year. That, coupled with our new hydroponics setup make it seem like we may have a very successful setup, now. We'll see what the future brings...

Norm Rohrabaugh's
8' dome greenhouse On a related note, Norm Rohrabaugh, (who runs the Hobby Hydroponics Webring that you can find on our Hydroponics Index page) was inspired by our dome-building, and used some of our pages here to construct an 8' diameter dome greenhouse in his yard. (He also invited us to join the webring! ;^) ) It came out pretty well, by the looks of it. As we did after our first year, Norm added an extra level of triangles to the bottom of the dome, to create more headroom for working inside.

I'm not sure if this dome was ever put into use or not. At 8' diameter, it only has about 50 sq. ft. of area, which isn't the most spacious. (Ours, at 12' diameter has about 113 sq. ft., and we're running out of room as you can see on our 2002 season page.)

The funny thing about circular and spherical geometry is the way a small increase in diameter rapidly increases the floorspace, surface area, and volume of the dome. As we see above, a 4' (50%) increase in diameter more than doubled the floorspace. (Actually, it's 2.25 times more space.) If we were to further expand our 12' dome to a 16' one (we're now talking about building a bigger dome, since we've completely filled the current one) we'd have a 201 sq. ft. space, which is 1.78 times larger. If we increased it by 6', to 18', (a 50% increase), we'd get 254.5 sq. ft., which is once again 2.25 times larger.

When dealing with domes, it really makes sense to make 'em as big as you can afford the space for, since the incremental cost for materials is going to be so small compared to the payback in space that you get.

Back to the Hydroponics Index.

Barefoot & Patrick Salsbury

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