# FM Global Wind ratings explination?



## Grumpy

So way back when I was told the FM wind ratings, the numbers were the MPH. For example the Fm i-90 was a 90 mph wind rating. I don't do much in regards to projects requiring FM ratings, but have been doing some research. I've come to discover that the number "90" has nothing to do with miles per hour but actually has to do with the wind uplift pressure, where as i-90 is 90 lbs per sq ft (I believe).

I'm trying to translate that into Miles Per hour which is more easily understandable. I've so far been unable to find the necessary information to make a translation. So far the best I found was some roofer's website had "Fm i-90 is approximately 110 MPH".


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## Pie in the Sky

Grump, 

It is 1-90 not i-90. 

The 90 is actually 45 lb uplift (usually equates to corners). 

There is no Simple way to equate it to MPH because there are several factors which go into that calculation including "design wind speed", roof height, "Exposure" and "importance Factor".

All of this info can be found in ASCE-7 (american society of civil engineers)

*Design Wind speed* - 90mph is typical in the US. it goes from 85 - 184mph (this is 3-second gust) Please note most warranties end at 55mph even though they are designed to 90mph

*Roof height -* it is more roof geometry for this one. THis determines the area of discontinuity or how far in from the roof edge you increase your fastening. its pretty simple "10% of the least horizontal dimention or 40% of the height, whichever is smaller. but not less than 4% of the least horizontal dimention. 

*Ecposure - *this is B, C, or D B is an urban area, C is a rural area, D is if you are on a lake or next to a large field. These are typically dictated by localities. 

*Importance factor* - Just a multiplier.. more weight is given to a hospital or school than a hog house... 

These are all plugged into a Formula i will not list it here because it is complex and I wouldnt know how to type it.  

This should be done by an engineer. Its Funny cause FM stands for Factory mutual, an insurance company. These are standards they have developed to aid in design. the only time these HAVE to be used is on an FM insured project. Funny thing is they are WAY over designed. They dont want a loss. Manufacturers speced them for a while then realized it was almost a hinderence and tried to not use it anymore.. it proved too hard to get away from it. A properly designed set of plans will not use FM ratings. They are overkill. 

Im not saying they dont work, they do, but they are not necessarry.. also see FM Property Loss Prevention Data Sheets 1-28 (google it and sign up for free if nothing else its good information:

http://www.rci-online.org/interface/2005-03-wells.pdf


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## Pie in the Sky

this is a good photo that illustrates a means of failure for blow-offs. NOTE - most major blow-offs I look at believe it or not are structural failures! not sure the statistics it may just be I see those because they are a larger loss for an insurance company. 

Anyway, look at the field next to this place. Wind can gain momentum and then when it hits structural members unsecured to hangers or simply toe nailed in, this is where the failure occurred. One thing to remember catastrophic failures usually are several things lining up at the right time, like poor nailing pattern, and a building sitting on a lake and someone had an overhead door open on the windward side of the building... (BIG LESSON TO ROOFERS - install your blocking with screws, 16" o.c. 2 at each end - DONT nail it.. Trust me, they fail all the time)


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## Grumpy

I'm finding it much simpler just to use the manufacturer wind warranty options than screwing with all this FM stuff. 

Situation is I speced a roof, customer emailed last night asking wind rating, I spent hours trying to figure that out (after hours). New manufacturer not familiar with their specs yet. Sales rep referred me to Technical, technical has yet to reply. 

I just want to know what wind warranty my spec qualifies for and what I need to do to increase that spec if necessary. I confused myself, like you said, placing too much weight in the factory mutual designs. But they have a FM-I-240 listed. WTF? Never heard of such a thing. By the way that's an "I" in their documents, not a 1. I missed this last night, because I was exhausted (note to self don't do work after 6pm) "(1) Does not constitute an FM system. For reference only."


I'm really kind of pissed because I sat down with the property manager and asked her if she had any requirements like warranty, R value, or wind (I ask every custome,r, makes my life easier if I know what parameters I need to design around)... She said wind is an issue. Ok, I submitted my proposal with specification. Now she wants more R value, longer warranty, etc... Why didn't she tell me when I asked? So I wasted a couple hours writing the spec, making material list, pricing it, and writing proposal. That pisses me off. I've got to start all over and waste a few more hours.

After a little back and forth about these things, I said; "Just tell me your list of requirements and I'll rewrite the proposal to meet those requirements." I'm not going to do anything until I hear back from her.


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## Pie in the Sky

Is this Versico? I know Carlisle list their requirements for windspeed right on their website. But it will be manufacturer dependent. they all do their own testing.


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## Grumpy

Yeah I saw that with Versico but their wind ratings are rather low, I thought. No not Versico. If I get approved I'll post about it for now shhhh


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## Pie in the Sky

I think its funny how we have to design to 90MPH minimum but there warranties say 55. If it meets the Design itll be good till 90mph!!! Writing a blog on this now!!!


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## Grumpy

Dustin I agree with you. I was thinking that myself. However people understand warranties. How do I say it's a 90 mph design, but the warranty says 50 mph? Where do I find the documentation to back up my claim that it is a 90 mph design? 


It's not unusual for me to install a 20 year design, but sell a 10 year NDL. People understand this because I can show them the 20 year specification.


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## Pie in the Sky

Its a never ending battle. If that roof blows off in a 88MPH gust, and their warranty says 55, Im pretty sure they would still be replacing the roof. I once heard a manufacturer say "Thats when their insurance policy kicks in." Stay tuned for my Blog, Ill post it on here...


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## Grumpy

Pie in the Sky said:


> Its a never ending battle. If that roof blows off in a 88MPH gust, and their warranty says 55, Im pretty sure they would still be replacing the roof. I once heard a manufacturer say "Thats when their insurance policy kicks in." Stay tuned for my Blog, Ill post it on here...


I have an exclusion in my labor/workmanship guarantee that states somethign to the affect of "not responsible for acts of God which may be covered by insurance." I'm over simplifying it, but never had a customer complain about it. 

By far the majority of the failures I see, on any roof flat or sloped, are most often contractor error.


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## brittjon

Grumpy,
FM use to put "I" as in Roman numeral 1. Every body called it I90 now I hear it both ways, it means the same. Dustin can probably back me on this.


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