New load calculations for new insulated roof?
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Greetings all,
we recently had a late season, heavy snowfall, 20 – 30 inches with some drifting.
the local hardware store roof collapsed the day after. the building is >30 years old, and has often seen much more snow than this. [as you see in the photo, most all the snow is on one side but the adjacent uninsulated / unheated building roof is fine]
however, this summer, spray foam insulation [afaik] was directly applied to the underside of the [metal] roof. I do not know what kind or density. this is the first significant snowfall since then.
given the large roof size [>10 000 sf] should the additional weight of the foam have been taken into account –for the existing structure?
although residential roofs are much smaller, should the weight of the foam be a consideration on a deep energy retrofit– that involves adding much more insulation to the roof?
I also wonder whether the previously uninsulated roof shed the weight quicker through melting, as afaik the building is heated 24/7 in winter?
comments much appreciated
thanks!
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Replies
P. HA.,
There is absolutely no way that any of us can speculate about the cause of this roof collapse. All kinds of engineering errors, truss fabrication errors, or sheathing fastening errors might have contributed, and to try to guess which of these errors were most important would be rank speculation.
I'm sure that the insurance company is looking into the causes right now.
It is well known that snow can cause uneven loading on gable roofs. Engineers are supposed to take this type of uneven loading, which can stress a roof, into account.
I'm not usually familiar with texting abbreviations, but I looked up "afaik." Evidently it means "as far as I know." From the location of this abbreviation in your sentence, I assume that you aren't really sure whether this roof was recently insulated with spray foam, but that it might have been.
It is highly unlikely, in my opinion, that spray foam insulation has anything to do with this roof collapse -- if indeed this roof even had any spray foam insulation, which (evidently) you aren't quite sure about.
-- Martin Holladay
thanks for the reply, Martin
to clarify,
the roof had spray foam insulation applied. the "afaik", referred to it being directly applied to the underside-- thus adding weight to the roof. I have not been in the roofspace-- but that is my understanding-- this is my local hardware store.
my question was-- as the post title simply states; should the weight of adding insulation to an existing roof structure be taken into account?
my question was not to "speculate" why the structure failed. most basically, the structure failed because it could not support the weight of the roof, the snow and the new insulation.
closed cell spray foam at [say] 2lb/cf, at 8" [iirc R49 code minimum here] on a 4000sf roof, weighs >5000lbs-- not an insignificant amount.
it seems on the most basic level, that the amount/weight of snow now required for roof failure has been reduced by 5000lbs.
this may be the academic difference between [say] 8 feet of snow or 9 feet, to cause failure, or [say] the more critical difference between 2 or 3 feet.
this is the discussion I was hoping to have
on a separate note, a quick search of GBA finds [ https://www.greenbuildingadvisor.com/search/node/afaik?page=1 ] many people using the AFAIK abbreviation, going back more than 6 years.
Moreover Martin, perhaps you don't recall your own post , from a couple of years ago.
Oct 11, 2014 5:23 AM ET
Response to Peter Hastings
by Martin Holladay
Peter,
I agree with your reaction to abbreviations. It's hard enough to keep up with the ones in the construction industry without throwing in slang used by texting youth. Is IIRC a building code, or something else?
Anyway, as far as I can determine, this is what GBA readers need to know.
BTW = by the way; BMW = a car brand from Germany.
FWIW = for what it's worth.
IIRC = if I recall correctly; IRC = International Residential Code.
AFAIK = as far as I know.
Read more: https://www.greenbuildingadvisor.com/blogs/dept/musings/it-s-not-about-space-heating#ixzz4brgQ6Nts
Thanks
P. HA.,
There is absolutely no way that any of us can speculate about the cause of this roof collapse. All kinds of engineering errors, truss fabrication errors, or sheathing fastening errors might have contributed, and to try to guess which of these errors were most important would be rank speculation.
I'm sure that the insurance company is looking into the causes right now.
It is well known that snow can cause uneven loading on gable roofs. Engineers are supposed to take this type of uneven loading, which can stress a roof, into account.
I'm not usually familiar with texting abbreviations, but I looked up "afaik." Evidently it means "as far as I know." From the location of this abbreviation in your sentence, I assume that you aren't really sure whether this roof was recently insulated with spray foam, but that it might have been.
It is highly unlikely, in my opinion, that spray foam insulation has anything to do with this roof collapse -- if indeed this roof even had any spray foam insulation, which (evidently) you aren't quite sure about.
-- Martin Holladay
Answered by Martin Holladay
Posted Mar 20, 2017 7:09 AM ET
Read more: https://www.greenbuildingadvisor.com/community/forum/general-questions/101297/new-load-calculations-new-insulated-roof#ixzz4bsK2FgJV
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P HA,
Every few months, I encounter a texting abbreviation which is opaque to me. When it seems relevant to the meaning, I look it up. But I don't leave any room in my long-term memory for these texting abbreviations. It's all I can do to learn the meanings of real words like psychrometric and hygrothermic without clogging my brain with afaik.
-- Martin Holladay
I my past life, I designed many commercial re-roof projects. It's always a good idea to have a structural engineer look at the existing structure before adding insulation to a roof, as more insulation means added weight of unmelted snow. Post-frame and metal building structures are very often designed to meet the bare minimum code requirements, and are often built with less oversight of inspectors, etc. I suspect the additional weight of the foam itself was not critical in the collapse of your roof.
Joel,
Thanks for your comments.
-- Martin Holladay