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mathey
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cupola info?
02/11/05 09:56 AM
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I'm looking for plans or design ideas (hopefully free!) to build a cupola for my little barn (24x24). I have some thoughts in mind, but have never done this before...
I'd also be interested in hearing others opinions on sizing of a cupola, as "common consensus" is either 1, 1.25 or 1.5 inches per foot of ridge length. My ridge length is 28 feet so my cupola should be either 28", 33" or 42" depending upon who you ask!
many thanks!
Mark
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slapshot
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Re: cupola info?
[re: mathey]
02/11/05 10:05 AM
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I'm currently building myself. I too will but a cupola up. My architech suggested making one out of cardboard. A pretty easy way to check sizing.
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egon
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Re: cupola info?
[re: mathey]
02/11/05 10:09 AM
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Think I'd go for the 24 in. size. Will this be working cupola or one just for looks?
Egon
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mathey
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Re: cupola info?
[re: egon]
02/11/05 10:22 AM
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just for looks...
I like the cardboard idea...right now i'm leaning towards 30-33" at the base, i can always make it smaller... 
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mathey
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Re: cupola info?
[re: mathey]
02/11/05 11:12 AM
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OK, I lied...my ridge is 26'... so i went in the middle at 32" wide at the base...here's sorta what it will look like...look ok?
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LazyJ_Arabians
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Re: cupola info?
[re: mathey]
02/11/05 07:20 PM
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I had functional one on a 24x24 garage I had at my previous house. It was 36"x36" so I could fit a $10 box fan in the hole in the summer. Even though the garage had a 2-sided roof I built the cupola with a 4-sided "hip" roof. My first attempt was too tall and looked like a chimney so I ended up cutting it down from about 20" to 14" on the taller sides, stuck a weathervane on top and it looked great.
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slapshot
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If a weather vane is installed should it be grounded?
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RobS
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Re: cupola info?
[re: slapshot]
02/14/05 11:38 AM
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You don't need to ground it. In fact, if you did it may attract MORE lightning. Check out Denningers site. I got a finial from them and they were very helpful. I think he talks about lightning on his site
Rob
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Re: cupola info?
[re: RobS]
02/14/05 06:35 PM
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Thanks much! I knew someone here would know the answer.
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mleininger
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Re: cupola info?
[re: RobS]
02/16/05 03:31 AM
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In reply to:
You don't need to ground it. In fact, if you did it may attract MORE lightning. Check out Denningers site.
I bet the National Lightning Safety Institute will agree to stop making finials if Denninger will agree to stop giving lightning advice. Here's a reputable source that describes lightning rods:
Lightning rods
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Pat
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I am not biased by having read either lightning info source...
Sharp pointed things with good earth grounds high above surrounding structures do not attract lightning. In fact the job of a lightning rod is not to be hit by lightning but to drain away the charge to ground and prevent a lightning strike. The pointed thingy increases the field strength of the electric vector to the point that it exceeds the ionization potential of the air (roughly 10,000volts per inch) which allows an electric current to flow through the pointed things and drain off the charge much more safely than a lightning strike. Lightning rods do sometimes get hit anyway and need to be designed for that possible eventuality.
It is EXTREMELY important how the conductor running to ground is constructed, formed, and connected. bends must be of LARGE radius or the inductance of the "partial turn coil" will have so much impedance that the lightning is likely to find an alternate path to ground. You might not like its choice.
Lightning story: I did some repair/replacement to electronic equipment on a fishing boat (tuna boat over 100 ft in length with a helipad on top of the pilot house) that took a massive strike at sea. Every one of the crew members was wearing a digital watch with microelectronic guts. Every one of those watches was totally inoperative after the one hit. Think EMP (Electromagnetic Pulse as from a nuclear weapon)
My garage/shop on the east end of the house is 36X48 with 12 ft wall height and 12:12 pitch roof. I think the cupola we built is 4 ft square with a 4 sided pyramidal roof 12:12 pitch. It is decorative, not an air vent as I have a continuous ridge vent and vented soffit. I may put an alarm siren up there later. You can see it in the house building thread "Oklalhoma Farm House." I will eventually build or buy some artwork, the typical north south east west pointers and a wind vane to put up there, just haven't decided what I want yet.
Pat
"I'm not from your planet, monkey boy!"
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egon
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Re: cupola info?
[re: Pat]
02/21/05 05:27 AM
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It has to be a Rooster vane with a real live rooster perched on it anouncing the joys of the day to come.
Egon 
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Re: cupola info?
[re: egon]
02/21/05 05:38 AM
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Well, all I have to say is if you don't want to see your cuppola blown to pieces, especially if your in an area that has a lot of thunderstorms, you would be smart to ground it!!!!!
Dick
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Is it best to mount the cupola before or after the shingles are on?
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Pat
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Re: cupola info?
[re: RobS]
02/25/05 09:14 PM
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RobS, the site you referenced claims "Regardless of whether or not a lightning-rod system is present, the strike will still occur."
This is just not true.
A lot of what they had to say was correct as far as it went for the most part but this statement was just plain ill informed.
There is a phenomenon called a "brush" discharge where the ionization potential of the air is surpassed by the local electric field vector. This brush discharge is formed near the pointed part of the lightning rod where the radius of curvature concentrates the electric field. Ionizing the air makes it into a much better conductor which can in fact drain off the charge and not let it build up enough to make a lightning "strike." The conductive path formed by the brush discharge is like a leak in a balloon which prevents your building up enough pressure to burst it.
In the balloon analogy, if you add enough air quickly enough you "outrun" the leak and burst the balloon. Same story with lightning rods. Although many hits may be averted by shorting out the charge difference between cloud/air and the ground a sufficiently rapid producton of static charge can overwhelm the lightning rod's ability to drain away the charge and the potential will build up sufficiently to cause a strike.
The draining away function is not why lightning rods are connected with such large wires. The wires are sized so the rod system has a decent chance of surviving a hit.
Regarding a brush discharge: See also, St. Elmo's fire.
There aren't nearly as many knowledgeable lightning protection system designers or installers as there are folks who will sell you a system and install it. Caveat emptor!
Oh, by the way... You can get a fair degree of lightning protection without a lightning rod on the building. A lightning rod may be instaled on a tall pole (flagpole) or tower and it will confer a good deal of protection to anything within a 30 degree wide angle umbrella zone under the lightning rod.
Pat
"I'm not from your planet, monkey boy!"
Edited by Pat (02/25/05 09:20 PM)
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John_D
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Re: cupola info?
[re: mathey]
03/03/05 12:04 PM
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I built a 24x28 garage that begged for a cupola. I got conflicting ideas on size, and decided to mock one up in cardboard. That helped a lot to determine what was right for both width and height.
I envisioned a much fancier cupola - my ideal was on a barn in Vermont I saw years ago - dual louvered "windows" on each face, each with a pointed arched top. Very Gothic.
What I wanted and finally got were quite different - Factors: $$, what was appropriate for the building style, and shallow roof slope.
I ended up buying out of an online catalog - one of their standard units; nothing fancy, but functional.
What I did right - avoided pine and spent the extra $ for redwood - it won't rot, and to date (4 years) the finish hasn't peeled.
If you want to do what you want to do, then you have to do what you have to do.
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