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Should a metal roof be grounded?

Dr Mark

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Mike Holt explains why 690.47d needs to immediately be removed from the code (why multiple grounds can be bad). Long video but he explains it well. If it's a concern for your area (e.g., don't you live in one of the lightening capitals of the world?), think about how to undo it after the inspector leaves. Auxiliary grounding electrodes (like for a generator) are exempt and don't need to meet the 25 ohm rule or tied to the main ground - but you have to make it match up to what your inspector wants to get passed.
I have a metal roof without a solar array. Would there be any need to ground a metal roof? I liked the video.
 
Check your local codes to be sure... but...

AFAIK metal roofs are not usually grounded and do not attract lightning strikes any more than asphalt shingles; nor do metal roofs protect a building against lightning -- but that's just opinion... I'm not an expert. AFAIK, the only way to protect a building from lightening is with a properly designed and installed lightning protection system.

As a side note, a long time ago in a bayou far far away (actually it was the UF Lightening center), I heard an interesting theory about grounding masts on sailboats backed by a dataset too small to draw accurate conclusions from. The theory went that a grounded mast had less resistance to water, so was more likely to be struck by lightening. But, if lightening hit an ungrounded mast, it would do massively more damage to the boat - so it was still worth while to have grounding.
 
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AFAIK metal roofs are not usually grounded and do not attract lightning strikes any more than asphalt shingles; nor do metal roofs protect a building against lightning -- but that's just opinion... I'm not an expert. AFAIK, the only way to protect a building from lightening is with a properly designed and installed lightning protection system.

As a side note, a long time ago in a bayou far far away (actually it was the UF Lightening center), I heard an interesting theory about grounding masts on sailboats backed by a dataset too small to draw accurate conclusions from. The theory went that a grounded mast had less resistance to water, so was more likely to be struck by lightening. But, if lightening hit an ungrounded mast, it would do massively more damage to the boat - so it was still worth while to have grounding.

....so what happens if lightning strikes nearby and induces a charge into the roof? Is that not a concern?
 
....so what happens if lightning strikes nearby and induces a charge into the roof? Is that not a concern?
No clue really. I wondered the same thing about surface charge accumulation in dry climates, it'll build up just from the wind. I suspect the surface area is large enough to equalize the charge fairly quickly. It would be interesting to see the roof-to-ground voltage differential in different climates.

Googled a bit, general consensus seems to be don't worry about it. Most authoritative was probably this, the gist of which is grounding should help minimize damage from a direct strike (but if you're the sole high spot on a hill and worried about a direct hit then you want a lightning protection system, as Mike said in the video nothing else will cut it). Also saw an post where someone "grounded" their metal roof with metal downspouts and got flagged by the inspector for improper grounding.

So as to not further hijack ArthurEld's thread I'll move these posts into a new topic.

(Update: wonder if there's any impact from Atmospheric Electricity, about 30V/foot of elevation)
 
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I get the direct hit really does not matter much what is grounded or not. Makes sense about metal roof not being connected to the electrical. I have a 50' oak 10 feet away from house so I figure it is protecting the house from a direct hit. When I had my service meter housing/pole and aluminum wire from meter to service panel replaced the installer added a second grounded pole six feet way from the old one. It seemed like a good idea until I watched Holt's video.
 
While wandering aimlessly through the neighborhood in search of enlightenment I looked up into sky... and realized I might be wrong.... yet again...

1616691322977.png
See It? Metal roof, metal service mast...

When I wrote my original post I was under the assumption I'd never seen a house in Florida (where there are many metal roofs) with grounding.

Now, I'm not so sure.... most of them might be actually grounded incidentally as the service mast typically goes through the roof and as the service mast is physically connect to the meter box, and the meter box is grounded... so, the roof is defacto grounded (enough for static charges anyway).

image001.jpg


So much for the roof being isolated from electricals.... ;-)
 
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...It seemed like a good idea until I watched Holt's video....
Same here, I was about to sink one when I saw it... I think @Dzl posted it originally.... that's the great thing about the forums, always learning something (hopefully before I do it).
 
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Metal roofs typically have poor continuity between panels, especially over time. Kinda counterintuitive.

To accomplish a proper grounding (earthing) you would have to run multiple ground wires or bond the individual panels.

I’ve never seen anyone do it except as a bonded ground plane for a roof mounted HF vertical antenna.
 
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