Size of Covering for Mel's Protective Dome Supports?

rachel597(5A ME)November 3, 2009

If I use 10 foot PVC pipe as a protective dome support for a 4x4 SFG like Mel describes on page 71 of his ALL NEW SQUARE FOOT GARDENING, what size in feet should the covering be?


GrafixMuse's Garden Spot

Here is a link that might be useful: All New Square Foot Gardening: Grow More in Less Space! By Mel Bartholomew

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eaglesgarden(6b - se PA)

A 10 foot diagonal (the length of the PVC) would mean that the sides should be just a little more than 7 feet long on the edges. If you go with a 8 foot square you should have no trouble! See below for the math!!!

***Warning MATH****

If the diagonal is 10 feet across for a square, it forms a right triangle (45-45-90 degree angles). The legs of the triangle can be found using the sine relationship: sin (45) = leg length/ 10 feet. That becomes: 10 feet*sin(45) = 7.07 feet! If you go with an 8 foot square, you should have about 9 inches or so extra to pin down with bricks, etc.

    Bookmark   November 3, 2009 at 12:51PM
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Dan Staley

IMO that particular picture is an inferior design and you want the design with the two pipes that are secured across the 4' span ("covered wagon"), then attach a purlin at the top (per page 76 in the given link).

Then what you have are essentially hoops, easily calculated.

o For hoop fabric I use height x 1.57 then add 18" to make it easy to anchor at the sides.

o So presuming that's a 4' height at center in the book:

  1. 4 x 1.57 (+ 18") = (6' 3" + 18") = 7' 9".

Most row covers come in 6 or 7' widths so this means you want less than 4' height for a 7' row cover, and 3.5' height will get you there:

  1. 3.5 x 1.57 (+ 18") = (5' 6" + 18") = 7' 0".

Which is a ton of height - my regular hoops are ~ 2' at center. I'd use SCH40 only for this application and a zip tie to secure the tops of the 2 hoops to the purlin.



    Bookmark   November 3, 2009 at 3:20PM
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engineeredgarden(7, nw Alabama)

Inferior, huh? What about this one, Dan? Ha!

EG (The crazy inventor)

    Bookmark   November 3, 2009 at 10:13PM
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rachel597(5A ME)

Thanks eaglesgarden and dan_staley! I just KNEW someone could help me with the math. Now I will have something to think about over the winter as I plan my garden for next year.

My goal was to have a structure in place so I can quickly cover the garden in the spring in case of frost. I thought MelÂs Dome would be the easiest. However, when I tried to price out the row covers, I worried that the smaller ones may not fit the height of the dome. The modified covered wagon does seem to a better setup for this application.

I was looking for something simple, but I am also watching EGÂs (engineeredgardenÂs) mini greenhouse very carefully. He has more details on his blog: Our Engineered Garden.

Thanks again for your help!
GrafixMuse's Garden Spot

    Bookmark   November 4, 2009 at 8:38AM
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Dan Staley

Huh. My clever reply has disappeared and I forgot what I wrote - surely something to give the crazy injuneer a hard time. I'll try to take some pix of my overengineered coldframe soon & we can compare crazy projects.

Rachel, I was in HD yesterday and 1/2 in conduit was 1.65 for a 10' piece. Two of those are about the same price as PVC and will last much longer. I'll pick up a few lengths my next trip.


    Bookmark   November 4, 2009 at 9:47AM
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engineeredgarden(7, nw Alabama)

Dan - likely so, I enjoy receiving a good heckling from time to time. Mine is far from finished - as I may install a programmable logic controller on it, with a 4 channel type J thermocouple input module.

Rachel - Thanks for following my creative little greenhouse construction. It's gonna be pretty sweet when finished.


    Bookmark   November 4, 2009 at 12:44PM
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Dan Staley

My coldframe design and the heater for it, EG, is being developed for ease of homeowner use and low price point (likely grants will defray costs to homeowners), so I'm injuneering/testing for simplicity, the exact opposite. So that will take some cerebral contortioning on both our parts...

Rachel, in your zone you can still go to HD, get two lths. of conduit, assemble and sow for cool seasons for~ $10.00 - perhaps even incl. row cover (not incl rebar for anchoring). . I'm in Denver area and my greens are going well (albeit they may get overheated today and tomorrow as we get highs in mid-70ºs).


    Bookmark   November 5, 2009 at 10:09AM
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engineeredgarden(7, nw Alabama)

Dan - without a doubt, the simple/cheap solution is ideal for the average homeowner. I just want to use my technical knowledge to enhance my greenhouse experience. Since heirloom tomato seedlings will be sold to the public next year, it really needs to be self-sufficient.


    Bookmark   November 5, 2009 at 1:18PM
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I found the dome type easy to work with. For a 4' x'4' I just got a 25' length of clear plastic about 4-6ml thick. It is 10' wide and it works just fine with the bricks. I just stuck the half-inch (10')pvc into the corners and secured the cross with cable ties. It held up during a late wet snow we had in April. I can only get hubby to do a couple garden chores a year so I made do with the easy dome.

    Bookmark   November 5, 2009 at 1:22PM
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