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Metal roof = Faraday cage?
Would a metal roof cause problems for wifi etc?

 2:56 am on Apr 19, 2014 (gmt 0)

I just saw a commercial for metal roofing. I thought it looked good and would be practical in our climate.

Then I wondered, would a metal roof have unintended consequences for things like wifi, cell phone reception, etc.?

Is there anything I should know before I let myself fall in love with the idea of a metal roof?



 4:00 am on Apr 19, 2014 (gmt 0)

Most of those are aluminum or painted galvanized panels aren't they? I really don't know, just seems they would be less issue than in a van or similar vehicle. Maybe the seller has a FAQ? I bet others wonder too, so maybe they've been asked before.


 7:16 am on Apr 19, 2014 (gmt 0)

Is there anything I should know before I let myself fall in love with the idea of a metal roof?

You mean, other than the fact that metal in general is a terrible insulator? Seems like what you'd really want is a good solid layer of sod-- the kind you got on old Scandinavian farmhouses. Goat optional.


 9:57 am on Apr 19, 2014 (gmt 0)

Radio waves (WiFi signals and phone signals, etc.) will not penetrate the metal, they wil be reflected off it. If your WiFi is within the metal (ie, it's in the metal-roofed building), you will get a WiFi signal, but, a word of caution: Reflections from the metal can create hot spots and dead spots of signal where you have the original signal from the WiFi, and the reflected signal from the roof, all within the room. This has the equivalent effect of doubling the WiFi strength. Some would argue there are health issues, but I would not want to scaremonger over that as the signal strengths are relatively low power. But, it is a consideration for anyone close to any radio signals where strengths are increased beyond the safe limits.

Also, dead spots can cause the WiFi reception to disappear within the room. In very simple terms, this is where the original WiFi signal and the reflected signal cancel each other out.

Across the room you'd get a complex pattern as the reflections go from strong to weak.

The opposite effect with incoming phone signal occurs. Any signal that would ordinarily come through the roof would now be reflected away, reducing the phone signal strength. Some signal may still come through the non-metal areas, such as windows, etc.

Of course, you also have the transmitter within the phone and the signals there would work in a similar way to the WiFi. Your outgoing phone signal to the mast would likely be reduced.

One solution for those with a metal roof wanting phone signals from outside is to extend the range with an external antenna.



 11:53 am on Apr 19, 2014 (gmt 0)

Thank you, Engine, that's helpful.

What if the metal is not solid? Now I'm wondering about other metal placements such as the reinforcing wire mesh within a stucco wall.

Or what about metal mesh screens on the windows? Bug screens are a necessity around here.


 5:47 pm on Apr 19, 2014 (gmt 0)

What sort of climate do you live in?

In a tropical country I can tell you that metal roofed building become very hot.

I have also found metal reinforced cement floors CAN block wifi.

The effects are very unpredictable (due to the way reflection interact, as engine explained) and sometimes a fairly strong signal can be inches away from a dead spot - I used to have that in the room I am in as I write this, but that was with the added complication of sitting on a bed with an iron (not solid though) bed-head.


 7:38 pm on Apr 19, 2014 (gmt 0)

Mesh will also be an issue, depending on it's spacing. The door of your microwave over has holes that let visible light through (you can see your food), but are still small enough to block the microwave's energy (your eyeballs don't heat up).

A Faraday cage needs to nearly surround you, I doubt a metal roof will block much, the signal comes from things often ~6 stories tall, meaning the reception is mostly horizontal, not vertical (you will block a lot of the sun's energy).

To be a cage, there needs to be electrical continuity, so rebar (most office buildings, airports, etc) forms a mesh, but doesn't normally form a 360 degree cage, and it's spacing is too wide - so your reception is generally fine, perhaps just attenuated some in a heavily reinforced building. But inside an elevator, it's generally dead (or severely attenuated) - you're in a nearly perfect Faraday cage.

Rebar, like in a bridge overpass, acts like a very large antenna, not a faraday cage. So it kills very long wavelengths - drive under a bridge, AM radio goes out, but phone calls usually don't drop. Go thru a tunnel though, and once you're sufficiently deep in it, to be nearly surrounded on all sides, and your cell won't work. In the tunnel's case, it's not really a Faraday cage doing the blocking, as it is the shear mass of metal surrounding you in every direction. A Faraday cage doesn't require massive amounts of metal, a tin foil tent would kill almost all signals.

All this said, another way to think about your roof question is to think of other incomplete Faraday cages you use every day, like your car... :-)

Find a friend with a metal roof though, nothing beats an empirical evidence.


 2:03 am on Apr 20, 2014 (gmt 0)

I volunteer with an organisation that has site wifi with the router in one metal building (roof and walls, insulated metal sandwich) plus another metal building on the other side of the car park. In the building with the router wifi is great, out in the yard the signal is good to great. In the second building the signal can only be acquired if by window facing building with router. Once acquired the signal drops to 1 or 2-bars of 5 when step away from window. Further, signal can not be initially acquired if away from the windows.


 10:14 pm on Apr 22, 2014 (gmt 0)

I run into issues with clients who have 1930's-1960's homes with metal lathe for the bathroom tiles. If the bathroom walls are between the access point and the device the signal goes to heck. Same thing with some metallized wallpaper (Very popular in the suburbs here in the 70's.. ).. I still shudder at the ugly that provided.

See if you can visit other places where the roofs were installed. With some of the box stores here, I know I'll get almost non existant cell phone reception.



 11:32 pm on Apr 22, 2014 (gmt 0)

I live in a house with a metal roof (800 sq ft located in Central TX). The roof has 2 peak lines like a "T" from front to back. My router is located on the front wall of the house on a shelf 6 feet off the floor and I have no issue picking up the signal anywhere in the house or yard. I have a friend who lives 2 doors down and I can stay connected to my wifi with my phone when I am over there as well.


 12:36 am on Apr 23, 2014 (gmt 0)

My wife's parents live in the NC mountains and have a metal roof on their home. The home is somewhat L shaped, single level, 2200 square feet or so. They have their Linksys wireless router located in the junction of the L (their kitchen) on a small table about waist high.

No issues anywhere in the home with Wifi, or on any of the decks that completely surround the home. Her dad has a workshop that is below ground level (home built on side of mountain) in a partial basement and it works there as well... although the signal is a bit weaker.



 5:04 am on Apr 23, 2014 (gmt 0)

If you have any signal degradation issues, that's why they make repeaters.

FWIW, I often wonder if a local Walmart jams the signal to stop people from checking prices online because the minute you step about 20 ft. inside the front door you go from STRONG to ROAM.


 5:51 am on Apr 23, 2014 (gmt 0)

Walmart hates lawsuits and the FCC is pretty clear on this issue:

"Generally, “jammers” — which are also commonly called signal blockers, GPS jammers,
cell phone jammers, text blockers, etc. — are illegal radio frequency transmitters that are
designed to block, jam, or otherwise interfere with authorized radio communications."


 8:01 am on Apr 23, 2014 (gmt 0)

Well, that's all right then. If it could possibly be construed as contrary to the letter and/or spirit of Federal law, no major business would even dream of doing it.

What sort of climate do you live in?

See, that was my first reaction too. Wifi is a good thing-- but is it good enough to be worth $1000/month in heat dissipating through the roof?


 8:49 am on Apr 23, 2014 (gmt 0)

have there been stories of reception problems by people in mobile homes or campers?


 11:53 am on Apr 23, 2014 (gmt 0)

Electromagnetic waves will not pass through metal. If you only have a metal roof, the WiFi will not pass through the roof, but it will pass through non-metal walls and glass.

If you just want WiFi within the room you'll be fine. If you are in a room next to and above the metal roof the signal will be reduced, and will rely on reflections within the walls for the signals to penetrate.

If a store has metallised the windows the WiFi signal will not pass through. You can test it with your mobile phone. If you still get signals to your phone, the glass is unlikely to be metallised. Most likely the store has directional WiFi so the signals point into the store, and not towards the car park.


 1:52 pm on Apr 23, 2014 (gmt 0)

Climate = highly variable. The Canadian prairies can range from forty below in the winter to forty above in the summer (Celsius).

We insulate our attics heavily around here so there would be no significant difference in heat loss or gain from a metal roof versus other materials.

$1000/month in heat dissipating through the roof?

A house insulated to our current standards wouldn't lose anywhere near that amount through the roof.


 3:40 pm on Apr 23, 2014 (gmt 0)

Just reiterating what others have said...

We have a townhouse with a flat(ish) roof. It's constructed of metal sheeting and metal joists, with insulation above and below. The WiFi signal on our roof deck was a huge problem. Even when I put a WiFi router in the room below the roof deck there was no signal. The solution was to put a router in a room where the signal could get up a staircase and out some windows. That gives us a signal that's barely tolerable. Often my phone will switch over to the 4G LTE signal - which is more reliable.

We actually have metal joists throughout the house. The signal gets through those pretty easily. So, from my experience, the problem is just with solid sheets of metal.


 7:01 pm on Apr 24, 2014 (gmt 0)

Or very close meshes that have electrical continuity... :-)


 9:33 pm on Apr 25, 2014 (gmt 0)

Our neighbor has a metal roof and he can pick up our wifi signal anywhere in our house just fine from inside his house. He can also pick up the other neighbor's signal just fine who is on the side where the roof would block most of the signal.

The one thing I will say about metal roofs - I've had to go up and repair the neighbor's roofs very often with metal. The problem is that we get big storms - a meter, sometimtes two - and that presses down on the metal, which then rebounds and the bolts work their way out.

If it's a "seamless" roof without any bolts exposed, I think they fair a bit better, but if any bolts are exposed, the push and release from the snow loading and releasing works the heads up to the point that the holes start to get moisture in and weaken. Then the bolts really start to move and need to be replaced with larger bolts. I've helped a neighbor replace hundreds of bolts.

So my feeling is that if you low snow load, it might be okay. If you have a very high snow load, it seems to work better if the roof is very steep (over 45 degrees). In northern Japan they build with a very steep peak (over 60 degrees) and then a 45 degree main roof. That gets the snow started moving and then carries the rest with it.

If you have gables or a T-shaped roof, I've seen serious disasters here. One house had snow collect in the valley perhaps 2-3 meters deep. After some freeze/thaw cycles it basically had ice at the bottom that held onto the roof ridges. When it warmed up and the ice had little adhesion over most of its surface, it opened that roof up like a can opener. It just peeled the metal sheets right off.

Let's see... oh yes... if your roof sheds over pedestrian areas (walkways, entries, garage doors) you will at best end up with huge piles of dense avalanche snow and at worst end up having someone actually get buried when it lets loose. We have another neighbor who has spent his whole life adventuring in the Himalayan and she said the closest she came to getting hurt in an avalanche was when the snow on her metal roof cut loose while she was on the porch and pinned her against the porch railing.

So all in all, not a fan. Last I read up on it, most engineers who design for snow country seemed to think that the best strategy is just to engineer for the snow (our house is engineered to hold about 15 feet - though we did get minor cracks in the sheetrock when we got 12 feet of snow on the roof) and let it sit there. It does most of its damage when it moves. Stationary snow is much safer unless you can't handle the loads.


 5:06 am on Apr 28, 2014 (gmt 0)

Walmart hates lawsuits and the FCC is pretty clear on this issue:

Yeah, well something funny is going on as I had to visit a different Walmart yesterday to pick up a few things and about 30ft inside the building my phone buzzed and it had an error message on the screen:

"Mobile Network unavailable".

No roaming, nothing.

All I could say was "WOW!"

Back to metal roofs...


 5:31 am on Apr 28, 2014 (gmt 0)

I would say that not all walmarts are shielded. We have a local walmart that was completely refurbished about 2 years ago. My son stopped in there and I could see him moving through he store via google locations. So, he was getting GPS and his wifi was going out.

I think it was Bill who mentioned repeaters. I worked with cell phone and wifi repeaters. THey can be a bit tricky to set up but they can work. For wifi, I've had better luck running 2 or more WAPS. That works as long as you can get the ethernet from the router to the wap.


 7:50 pm on May 3, 2014 (gmt 0)

A metal roof is pretty bad on GPS. (But, if you're inside
a structure, don't you know where in the hell you are? :-)

Stucco, as mentioned, is pretty bad for FM radio, and not
very nice, either, for AM radio.


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