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Forum Index : Other Stuff : Earthing your roof.
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
My house has a corrugated iron roof on a timber frame. 2 story house. We get a lot of dry lightning around here, that is, lightning without any rain. Now my thinking is, if its raining, the gutters and down pipes are wet, they provide a electrical path to ground for a lightning strike to the roof. But if its not raining, a lightning strike to the roof would still be looking for a nice path to ground. The ceiling space is full of wires for the light circuit, and these are earthed. Therefore, there's a good chance the lighting will jump from the underside of the roof to the closest wire, and then to earth via the switch box, basically blowing the crap out of anything along the way, and possible anything on a branched circuit. So on my house I have two lengths of 4mm galvanised wire ( cloths line ), one at opposite corners of the house. Each wire is screwed to the roofing iron, then run over the gutter and down to ground where it's attached to a ground stake. The wires sit about a foot clear of the house. This was a temporary set up to give me peace of mind, but I wonder what the correct set up is? I've spoken to a couple of electricians and they didn't have a clue. What happens if you have a tiled roof? Surely you would need a earthed lightning rod at the apex? Otherwise what would stop a strike going through the roof and the top of your head? Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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rustyrod Senior Member  Joined: 08/11/2014 Location: AustraliaPosts: 121 |
Good thought Gizmo, The lightning usually exits a house through the telephone wires where they enter the ground. poof. All churches have the spire earthed usually a copper strap about 75 mm wide and 4/5 mm thick. (Helped fit one in my youth to the New Chinchilla Methodist Church) The robbers in an English detective mystery were stealing the lead off the roof of a church and the close up was them prizing a copper strap from the side of the bricks. When I moved here I could get a good shock from the roof to ground. I had a 1" pipe for a TV aerial, if I moved it away from the gutter I could see the small spark between the surfaces. I never was able to locate the source. But after the TV was replaced the electrification vanished. Was the TV charging up the pipe and the roof somehow is grounded, but the pipe was standing in the ground. Ah, sweet mysteries of life. On our farm there were numerous lightning strikes into trees. I remarked to my old Dad about this and the possibility of the large box trees growing over our house would be struck. He said "Lightning does not strike closer than 1/2 a mile from the creeks and it prefers ridges" with some annoyance in his voice that I was dumb not to already know that. Our house is 300 yards from the creek on the flat. I observed that in the 25 years I was there there were NO struck trees close to the creek. If you had a decent strike (I have been struck driving a bulldozer, long way from creek, middle of big empty paddock) I think your clothes line would vanish. But it would have taken the bulk of the lightning and set up a path for the remainder to follow. A simple method would be to strap the roof to metal down pipes then run straps to a rod 500mm in the soil. I think a couple of 1" gal pipes standing up to the gutter and 300 mm in the soil would do the trick quite well. Connect to the roof with a few gal flat straps. You know the 25mm wide straps with holes in them for stabilizing buildings. I am now thinking if I get a strike here the hot water panels are the highest point then the solar panels. The wires of which hang over the side of the house and dangle 300mm off the ground as they go under the house The hot water panels have copper pipes all the way down to the pressure pump under the house. Either way or no way I expect lightning would fry everything here. But I am lucky so far it usually hits the high power lines out on the street and still fries everything. I have had 3 so far and all have been some distance from this creek. Anyway Gizmo, your house is near the creek and there are mountains nearby. So going on the old timers, the half mile and ridges rule you are safe. Always Thinking |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hi Rod Your dad may be right about the creek. The strikes closest to me are over the other side of the road, about 400m from the creek. Started a grass fire once. Nothing near the house, which is about 250m from the creek. Good theory, but there would be exceptions, I do know of a tree near here that was about 300m from a creek. The tree was turned into splinters. Yep half expect the cloths line to be vaporised if the roof was struck, but once the path was created, the arc would continue down that path of ionised gas until its discharged. Thats one reason I've kept it off the wall of the house, reduce the risk of burn damage or a fire. The sparks on the aerial feed for the TV. This is pretty common, depends on the TV. Some TVs ( rare these days ) had a live chassis, meaning the whole circuit was at mains potential. The aerial socket incorporated a couple of inline caps, isolating the AC from the aerial cable. These still leaked a little AC, and you could see a tiny spark or get a little tingle when plugging in the aerial lead. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Warpspeed Guru  Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
The gal pipe standing on end is a very good idea. Ideally it should go as deep into the ground as possible, and the top be higher than the highest point of the roof. A sharp spike or spire will always break down before a flat surface. Cheers, Tony. |
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| domwild Guru Joined: 16/12/2005 Location: AustraliaPosts: 873 |
What I saw recently in Belgium in a poshy area with thatched roofs, which must be an insurance nightmare, was the following: A wire parallel to the gable and two vertical "antennae" at both gable ends. Then one or two wires must drop down from the gable(s) and be connected to a "good" earth. Murdoch Hospital in Perth has a fancy gold-coloured ball on a mast! Sad and funny story from Perth: When they switched on the Van de Graaf lightning generator for the first time for the gawking school children in a West Perth discovery centre, it blew all the computers at the Archie Martin shop next to it, because they had forgotten to build an earthed Faraday's cage around it! Taxation as a means of achieving prosperity is like a man standing inside a bucket trying to lift himself up. Winston Churchill |
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| robert.rozee Guru Joined: 31/12/2012 Location: New ZealandPosts: 2350 |
just browsing and spotted this post. an unearthed iron roof represents an electrical hazard if a nail through the roof happens to penetrate the live (phase) of any electrical wiring running under it. people have been killed by this scenario, and as far as i am aware earthing the roof is a strict requirement of the electrical wiring regulations. if you are off-grid and your electricity comes from solar/etc this may well not apply, but is still something worth thinking about if you are generating your own 230v AC and reticulating it round the house. do you try and keep both sides of your 230v AC floating, or earth one side to create a 'live' and 'neutral', and then tie the neutral side to earth? cheers, rob :-) |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Thanks Rob I have tied neutral to ground at the source ( inverter ), then split the active and neutral through a couple of RCD's, one light circuit and one power circuit. After the RCD's the neutral is floating so to speak, and not connected to ground. This is all in my shed, about 15m from the house. The shed is earthed with a couple of earth stakes. The two feeds ( power and light circuits, plus a earth ) are fed underground to a distribution box at the house, where they are split up via a few circuit breakers to supply the house. Another earth stake is used for the house power board. The house roof itself has two earth stakes of its own on opposite corners. Kind of makes you wonder how may unearthed roofs are out there? Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Gizmo Several years ago my house got hit with lightning on the satellite dish mounts, it blew the fridge and computer and microwave but the satellite device survived there was no earthing on the roof but when it struck there was a blue glow around the windows for several seconds, my place is on an ironstone ridge so is prone to strikes, but usually on the power lines about 60 meters from the house all other wiring is underground, my phone is the first to go as it usually cooks the cable even though underground. I think the discharge from the roof went down the cyclone bolts as they go up to the top batten just under the iron roof there is 12 of them around the house and they are tied to the reinforcement in the slab, my theory anyway. I have seen my side fence light up like a sparkler in a storm it is an 8 foot high mesh deer fence so is a good conductor. There have been several trees blown to matchwood over the years, there are two large trees in front of the house that I left there to act as lightning rods but they have not been struck so far but the house roof got hit only about 5 meters away. Lightning is very unpredictable as I have seen it strike in the middle of a wheat field and set the whole thing on fire, must be iron outcrop under the field. One night I was coming home from Brisbane in a bad storm and the car in front of me was struck. when I was a little kid a norfolk pine in front of our house got hit and it was blown to matchwood, I was told it is caused by the moisture in the tree being turned into super heated steam and blowing up the tree, same thing happened to a stringy bark tree down the back of my place. scary stuff. All the best Bob Foolin Around |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Wow Bob, sounds like a really good QTH you have there. Tony VK3ALY. Cheers, Tony. |
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