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Picture of Geodesic Cedar Greenhouse

In 2006 I started work on a geodesic dome greenhouse. At that time there were fewer resources on the internet for doing this, and I had to derive the formulas for some of the math myself using sites and resources that are no longer available on web. These days I would recommend a calculator like http://acidome.ru to find the exact measurements you need. Nonetheless, all of my numbers are here so that you can see the process.

This dome is a hub-strut wooden structure. To be more precise, it's a 20' frequency 3 dome, 3/5 (or 4/7) sphere using cedar 2x4s. Since eight foot lengths are the most economical, I've adjusted my strut lengths to fit this with two struts per eight foot section. 3 frequency domes have three different strut lengths, each with their own vertex angle. Since I will be connecting these together with hubs for simplicity, I also have to factor the hub size into the strut lengths.

The angles are

  • A - 10.038 degrees from a right angle
  • B - 11.641 degrees
  • C - 11.9 degrees

Internal angles of a regular polygon are given by 180 - 360/n, where n is the number of sides. In this dome, there are 15 sides, so the angle is 180 - 24, or 24 degrees from being flat. Split in two for the contribution from the miter on each strut, gives 12 degrees, which fits nicely with the calculated 11.9 degrees on the C strut which makes up most of the middle circles. The difference comes from the insertion of an occasional B strut and the contribution of the extra-planar tilt.

For practicality with my miter saw, I will simplify these to 10, 11.5, and 12 and expect my hub attachment to give enough to make up the difference.

I will use 3" sch 80 pvc conduit pipe (uv-stable) cut to 3" lengths for the hubs, attached with perforated steel strips. This adds three inches total to the effective lengths of all struts. This is actually a simplification, since the hub widths are not measured at the same angle as the struts will be, but it's close enough. I can't afford to produce materials at greater tolerances, anyway.

3.5" tan 12 degrees = .75", which is the effective overlap obtained when I cut two C struts out of the same 8' 2x4. So, measuring all strut lengths from the outside (longest) edge, I can cut struts up to 8' 3/4" / 2 = 4' 3/8"

Add 3" for the hub, and the effective C strut length will be 4' 3 3/8". I can now work backwards from a reverse strut calculator to find the appropriate lengths for the other two, shorter, strut types. I calculated with decimals for the math:

Laid out as Effective Decimal, Actual Decimal, and Actual Inches:

  • C - 4.281 - 4.031 - 4' 3/8"
  • B - 4.189 - 3.939 - 3' 11 1/4"
  • A - 3.619 - 3.369 - 3' 4 3/8"

So these are the measurements to which I will cut the lumber on the longer edge, mitering at the angles listed above.

There are some fine graphics of the layout of these struts at acidome.ru and similar sites, so I won't attempt to illustrate it here. I will need numbers of struts as follows:

  • A 30
  • B 55
  • C 80

In reality, I'm warping the bottom layer of struts slightly, both to give a more even bottom edge and to normalize the height of the first tier of triangles on the dome. The bottom of a 3 frequency dome isn't quite flat; it bulges and pulls up an inch or so every couple of vertices. In practice we can expect this small bit of warping to cause few problems. Additionally, the edge isn't flat on the ground; it contacts at an angle, since the sphere it projects would be expected to continue at an angle under the ground. Many online calculators can give you adjustments to your struts at the bottom to flatten this out, so I'm not going to walk you through the tedious process of reprojecting the bottom vertices for this.

I need a door in this greenhouse, and after much examination I'm going to put it in one of the hexagons, removing the six internal C struts. I will drop two vertical posts between the top and bottom of the hexagon with joist hangers and use half-sized C struts to brace on either side to the midline vertices. This will connect at a 12 degree angle with the bottom strut of the hexagon. I'll then add horizontal extensions from the side posts to hold a vertical door.

The final bill of cut pieces:

  • 30 A
  • 45 B
  • 10 B flat end
  • 54 C
  • 20 C flat end
  • 2 B half struts
  • 2 vertical door posts

Each individual 2x4x8 will be cut as follows:

  • 2@1 vertical door post each
  • 27@2 C struts each
  • 20@1 B strut, one C flat end each
  • 10@1 B strut, one B flat end each
  • 1@1 B strut, two 1/2 B struts
  • 7@2 B struts each
  • 15@2 A struts each
  • horizontal door braces will come from scraps from A brace cuts

This adds up to 82 8' 2x4s. Cedar is not the strongest material, but I don't think it will have to be in this structure, and it should hold up to the damp better than fir. At the time of the build it ran $6 at Lowes, for a total of $492, plus tax, not counting any extras I have to purchase for mistakes and the occasional bad section. Today it would be $9 each, for a total of $738. Redwood is much more expensive here. Pressure treated isn't much cheaper, requires more expensive hardware to resist corrosion, and introduces chemicals that I really don't want.

The plan was to glaze with greenhouse film inside and outside (two layers separated by 3.5") the first year and consider things like polycarbonate down the road. I used a radiant barrier on the north wall triangles when I redid the internal plastic the first time.

I should also mention that I deviated from this plan slightly in that I used ground contact pressure treated wood for just the bottom struts. This didn't change the cost.

Step 1: Strut Preparation

Picture of Strut Preparation
IMG_2369.JPG
IMG_2370.JPG

72 8'x2"x4" cedar

11 8'x2"x4" structural pressure-treated

Doesn't look like much in the photo, but it felt like a lot when loading and unloading it (and it felt like even more when paying for it!).

Most of the pieces are 3/16" longer than 8'. Cutting at a diagonal of 12 degrees turns 1/16" into sawdust and yields 8' 1/8" on the longer side and 7' 11 3/8" on the shorter side -- a difference of 3/4", as predicted. Apparently I can still do trig after all!

This is good, as I did all my calculations based on overlapping 3/4" on the struts. Also, the extra length in original pieces allows for loss from the sawblade, which I hadn't really thought through.

The picture of cut struts are all out of the pressure-treated lumber: 10 C and 5 B. You can see the chemical penetration on the cut ends. This constitutes everything that will be on the ground. Besides these, the doorway will be made out of pressure treated lumber, consisting of top, sides, and side braces. This stuff must have a stain included because it is orange, rather than the sickly green I associate with ground-contact pressure-treated, especially in the newer high copper formulations.

It's took about an hour to do 30 struts, mostly because I'm being meticulous about the measurements and marking all of the struts on both ends to ensure I can tell them all apart.

WoodPrix8 days ago
Learning to work with wood and build to meet your own needs may prove to be simultaneously addicting, practical, and therapeutic. If you start down this path, you may never see the bottom of your project-list again. This greenhouse is really extraordinary. I am very impressed with your work.
Great job and very good tutorial.
AngelHouse20 days ago
Beautiful job! About how many man hours does it require? And I'd like to know approximate dimensions if you can please provide. I'm thinking of it as an Eagle scout project, or as one for a math class. We're a nonprofit establishing in south Florida but my sister in Texas may establish a chapter as well. We teach students, special needs, elderly & those in recovery to grow food, and this is a very cool place to do it in. Sometimes we need a Wow factor to get the attention of would-be donors and volunteers, to get the programs funded.

In Texas they'd need the double wall construction, easily heated to 20 degrees difference, etc because their weather is Very erratic compared to out stable, warm one. Sure, we get half a dozen cold nights and quite cool days, but nothing like the rest of the country, here in south Florida. I'd think our fan could go out/up in summer to get the heat out and maybe down in winter, opposite yours. Any insight is greatly appreciated.
I'm always amazed at so Few people having heard of Instructables. And how many people tend to jump at the chance to squash the dreams of our beneficiaries. To find themselves still able to garden because someone makes it easy and accessible to them, is a joy to behold!

What we'd like to do is use greenhouses for aquaponics, so the water temp will help too, in TX and there are aquarium heaters of course. Texas winters can be brutal, with 65 in the afternoon, plummeting that same night to below freezing and it occasionally goes down to the low 20's. Also in summer, it can be 105 degrees in south Texas.
At those times, maybe south FL and for sure, Texas, will need a way to also chill the water a bit and cool the greenhouse.
I think in south Florida those "swamp coolers" tend to not work as well with our humidity. But on the other hand, I once had something in our garage, called a "water-heating heat pump" and as the heat from the air in our garage went into the water, the air coming out, was cooled so it was like a mini air cooler for our dogs.
This was on the order of 30 years ago - I'd love to find such a thing today, but may be one of those things that in the US, people have money for a "real" air conditioner, so that it's not used anymore...
If any of the Instructables fans out there can please help me find such a device, I would be forever grateful. We need to keep our greenhouses tolerable for the frailer elderly who will be gardening with us and that's the best way I can think of with our small budget.
Thank you
AH Ladies Walking.jpgEdelfina tg.pdf.jpegHella TG1.pdf.jpegDilbert Fusion.jpg
I live in Tampa and suggest you reach out to construction companies who want 'credit' for reuse of wasted materials, but also would be interested in mentoring opportunities. In downtown Tampa the city helped pay for an underground cooling system for new buildings, that didn't seem feasible but sounds promising.
gentleworks (author)  AngelHouse20 days ago
The greenhouse is about 13 feet tall and about 21 feet in diameter.

Your results will vary, but it took me about 3 hours to cut the wood and about 2 hours to bend all of the straps and cut the pipe. Construction took place over a couple weeks in the evenings, but probably took around 10 hours for two people who didn't know what they were doing and figured it out as we went along (we had a third person for occasional tricky parts). Covering with plastic took closer to 20 hours, with two people required for safety throughout the top part, and the bottom and inside done entirely by one person. So, maybe 55 man hours for initial construction and covering. This was a long time for us working only a few hours at a time in the evenings, but really not long if we had been able to just take a day or two to complete it all.

In your use cases it would probably be best to install vents in one or more of the triangles. You will need outflow at the top and inflow near the bottom. They have actuators than can automatically open them when the temperature gets too hot and close them when it cools down.

Hydronic coolers and heaters use a thermal load to heat or cool water that is then fed into tubes and circulated into the area whose temperature needs to be controlled. These can either be hooked up directly to radiators that blow air across the hot or cold coils of water, or they can be woven into the floor or walls to radiate heat. The thermal input can come from many sources, including large pools of water, underground ballast (it is always the same temperature if you go down far enough) or active heating and cooling such as water blanket furnaces or isolated stage swamp coolers. These setups are easy to diy, but are also becoming increasingly available from commercial sources.

terrefirma12 days ago
I too continue to wonder why more don't come to instructibles, but maybe that so many people look only for the 'monetizable' . Very shortsighted, however I am always amazed and awed by the dedication, hard work and creativity I find here.
If I can offer a suggestion, to post on other sites as well, like Permies.com or Physics.com it will only encourage and create more creative engineers and sustainable solutions.
Well done.
Gadisha13 days ago
Very nice, I'll keep this project in mind.
If I ever decide to build a greenhouse I think I will use this Instructable as a source of inspiration.
I need to construct a green house for my hydroponics. I live in Nysa Poland and getting my supplies is the problem as I do not speak or read Polish . I will try to follow your construction but it may be a bit too for me
Great Job you have done. I compared to some other Domes, yours is probably the easiest way to manage. What i did not really understand is how you fixed the bended metalstrips inside the tube ?
gentleworks (author)  euronight44618 days ago
In Step 2 there is a picture of how the strap is wrapped through the inside of the tube and attached with two screws on each end to the strut. When putting up a piece where no hub existed I attached it this way. When attaching a strut on an end that already had a hub I attached the strap on one end with the two screws, and then hooked the strap through the existing hub before securing it down with two more screws. As mentioned in the text, the screw needs to be angled away from the strut, but pulled outward as far as possible so that it tensions the strap as it goes in. The straps themselves are pre-bent to the size of the hubs, which themselves are cut 1/4" shorter than the ends of the struts so that the straps curve neatly around at less than a 90 degree angle.
Really well written and illustrated...and your structure is pretty amazing! Thanks for sharing so clearly!
icosflybell20 days ago
I've always been intrigued by a two piece stitched leather cover on a baseball. Do you think there is any scaled application for a dome cover?
gentleworks (author)  icosflybell20 days ago
The baseball cover works because it provides uniform stretching across the ball to hold it in tightly. At the scale of this dome that would be a very stretchy cover and a lot of tensile force required to pull it outward onto the structure. If you made it bigger so that it didn't need to stretch so much then it would hang loose at the seams and damage itself in the wind and rain. It is an intriguing idea, though, and might have some use with a shrink wrap plastic or a smaller structure, or maybe some material I'm not thinking of.
maxhirez20 days ago
Nice work! Both on the dome and the writing. Well done.
LarryF2920 days ago
Probably a very nice project but after the first few paragraphs I deceit was too confusing to follow. Angles and descriptions confusing for me to follow.
I am very impressed with your work, this is the first time I see it made so well described, step by step with good illustrations, thumbs up.
maxman20 days ago
Yours is absolutely awesome.
Here's mine. And here's how I feel after scanning over your write-up.
Mine's a trellis in the warm weather, and it will be a greenhouse in the cool weather when I cover it with plastic. Mine took an hour. I'm lazy.
arch1.jpg
belsey20 days ago
Great job, beautifully documented.
seamster22 days ago
This is excellent. I've always wanted to build a dome structure like this, but don't currently have the space. But someday! Thank you for sharing the details of your build. Very well done, loads of good info! : )
That turned out really good! Thanks for sharing how you put it all together :)