Source: DOE/Oak Ridge National Laboratory
Date: October 13, 2007
Science Daily â€â€ A study of roofing damage incurred by Gulf Coast structures following Hurricane Katrina has found that buildings with steep sloped roofs held up better against the high-wind storm damage than buildings that had low sloped roofs.
The study â€â€œ conducted on behalf of The Roofing Industry Committee on Weather Issues through a cooperative research and development agreement with Oak Ridge National Laboratory’s Buildings Technology Center â€â€œ determined that steeper sloped roofs held up better due to the fact the building materials composing the roof structure defend better against wind uplift forces that occur during hurricanes.
The study, led by ORNL engineer Andre Desjarlais, concludes that construction of these structures should adhere to current local building codes that have been upgraded over previous codes, closely following manufacturers’ guidelines and using compliant edging systems.
The funding source is DOE’s Office of Building Technologies.
Note: This story has been adapted from material provided by DOE/Oak Ridge National Laboratory.
Replies
bump
I saw that argument in a book once--that a low sloped roof acts like an airplane wing and creates uplift over it (low presure) whereas a steeply pitched roof creates turbulence, especially on the leeward side, which breaks up the low pressure. Makes sense. I suppose on a low pitched roof you could add the equivalent of dimples on a golf ball, to break up laminar flow and prevent uplift. (I would think that shingles are rough enough to cause enough turbulence, but apparently not. Maybe there'd be an advantage to "dimensional" or "architectural" shingles over 3-tabs in this instance. Metal might be worse than shingles.)
I seem to recall that FHB had an article on the subject some years back.One answer to the aircraft wing was to put a vertical spoiler on the ridge to break up the air flow.The ToolBear
"Never met a man who couldn't teach me something." Anon.
In the older settlements in the carib, the OLD houses are all square with steep (8-10) in pitch roofs. Post and beam const with pier foundations (coral block) and chains going from the upper wall corners out to anchors buried in the ground.About 45deg. on the chains.Heavy sheet metal roofs with no overhangs. Been there 3-4 hundred years so it must work. Jim. Also search good 'ol FHB for an article back around '90 'bout a hurricane that trashed the old part of Charleston- same findings. said article also showed where nice old houses had been lovingly restored by well meaning "experts" and they were trashed while the rough looking one next door was untouched........
I was pondering the same thing a few weeks ago when I had to replace some roofs on the Gulf from Hurricane Humberto. Most of the roofs in this town (90%) are very low sloped roofs that have been there through many hurricanes. The one that has been there the longest though "The Breakers House" has an 8/12 pitch suprisingly. It was the ONLY house on the pennisula that survived the "Great Storm" of 1900.
Probably something to the steep pitch theory but What I saw last month was the only houses that had roof damage were the ones with open cornice. Even one that was an 8/12 pitch which was my sisters house though it did have less damage. IMO the edge/eaves would be the most important aspect with pitch being a close second.
Where there's a will, there are 500 relatives
the study is not quite true in their results. First seventy percent of damage from a hurricane is from the water, add about ten percent for fallen trees and poor workmanship. that leave about 20 for wind. The study claim that steeper roof are better. they are but not because of the steep. the wind will have a down force on the wind, not a uplift. a steep roof has more area than a low pitch roofs.like you say low pitch roofs of south fla has handle many storms. have a proving track record.but less area than a steep roof..Know BOB, Know Peace
I did notice that the roofing industry paid for the study. More slope = more shingles. :)It is hard not to take into consideration though that the only house standing after the Great One of 1900 in a 10 mile stretch was one with a steeper slope though. It could be that it was just built a lot better than the rest. It is still standing after all these years.Where there's a will, there are 500 relatives
brownbagg,
I'm sure you are right about the companant of damage that factors other than wind affect. However once we speak about a roof the steeper the roof the sooner the wind reaches stall speed. In a stall, wind becomes extremely turbulant and loses it's lift companant In fact that very turbulance affords a strength componant because in addition to losing the lift, that turbulance creates a bubble which deflects much of the strength of the wind from affecting the structure..
That turbulance and the steep roof means the force of the wind becomes more of a straight line. We all know that wood is able to carry a lot of load if it's pushing lengthwide. Load a wall sideways and it starts to deflect with less than 100 pounds (think of floor loading) load a wall vertically and it can carry a lot more weight. the same applies to a roof..
hurricane damage is like a pet project of mine, remember I have a concrete house. The daily after katrina, I was in waveland, gulfport bay st louis area. I am still in that area. The house that was damage by the wind are few than those by water. out of the less group. is those that was damage by poor workmanship.But there are a lot that had no damage and only couple hundred feet from the gulf itself, They was small frame house with low pitch roofs.Now my statement is not saying high pitch roof are NOT better but all the hurricane engineering books are claiming the down force of a fouse when winds go over, a high pitch roof would have more area than a low pitch so more down forceand then it the fact that if the roof is not attach to the walls correct then all of this down force doesnt matter..Know BOB, Know Peace
Brownbagg,
Your last statement is the best one so far.. How the roof is attached is the critial differance not the pitch of the roof..
lose the roof, lose the house.
Roof attachment has been a major interest of mine.
ps you mentioned you live in a concrete house, may I ask poured walls or block walls? What method did you use to ensure your roof remained attached?
its block with 100% filled with 5000 peagravel and rebar, gable icluded to peak. and a couple hundred hurricane ties. both sides of wall and porch..Know BOB, Know Peace
I can't comment on the structural aspects--but from a strictly roofing material standpoint-------------- after wind storms I am MUCH more likely to recieve phone calls from prospective customers with shingles blown off from steeper roofs-----say 14/12--usually within the top 3-4 feet.
virtually NEVER do I find blow-offs on something like a 4/12.
Stephen
I would agree with that.But the other factor is even more likely to indicate losing shingles -
if the installer placed the nails right, or used enough of them. When I loved in Florida, I saw darn few steep roofs. Almost every repair job I did replacing a few wind-blown shingles was because the shingle layer used 2-3 nails per shingle.Now, the majority of those is because they were gun installed and driven too high up on the shingle and/or driven too deep into the material
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I think you are right about that Piffen,
but -- mis nailed shingles would be pretty consistent across the whole roof----so that doesn't explain why the blow offs seem to be exclusively within the top 3 feet or so
A "Bernouli effect" uplift near the ridge SEEMS to explain it though.
Stephen
There is definitely excess uplift near the ridge Steve. That is why collar ties are required faming in high wind areas.I have personally experienced this. I was roofing a cabin on a lake with a steady wind blowing. I was regularly getting higher and higher through the morning while I worked across the side facing the lake. My ladder was on the other side of the roof, and once the wind got up to around 40-50 MPH, I decided I had had enough for the day. I started climbing up to the ridge ( about an 8/12 as I recall) and just as I got about 4' from the top, I was actually lifted up and over it.
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Did you say: "WHEEE!!! I can fly!!!"?Rebuilding my home in Cypress, CA
Also a CRX fanatic!
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" Oh my ....Whew!"
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Frenchy, Brownbag, you got it right on the nose with your concern about the roof to wall fastening. When I do any building I must do a wind load analysis. "Open your hymnals to Chapter 16, Structural Design, and follow the formulas and charts and you shall be saved (or at least your building will be)."
They assign all sorts of values to the use of the building and even go so far as to dictate the size and number of fasteners and on and on.
Bottom line for any structure is the depth of the roof overhang, the connection of the rafters to the exterior wall, the exterior wall construction(and its anchoring system) and then the pitch of the roof.
Concrete block walls are without a doubt the strongest system to build on and attach to in a high wind area. Friend of mine (Florida home) just got his insurance reduced because the walls are reinforced concrete block, it has a low pitch roof with minimal overhangs and all fasteners were installed correctly. Now that's cool!
The devil is always in the details
ciao, ted
"Concrete block walls are without a doubt the strongest system to build on and attach to in a high wind area"With the exception of ICFs
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If 'ICF' stands for "Integrated Concrete Form(or Foundations)" then yes, I agree.
I would think that soil bearing capacity would have some bearing on the system used but you can say that about any structural system in FLA. I has to be difficult. Not impossible but.....
caio. ted
dirtyturk,
I'm afraid concrete block walls must take a back seat to ICF's for wall strength. They give ICF walls a 200 mph wind speed capability and if you use j bolts which go under the rebar the wind speed required to remove a roof would most likely exceed 400 mph (assuming the rest of the roof was built to that strength level)
This is clearly unscientific but I've toppled reinforced concrete block walls with sledge hammer blows and not been able to budge a ICF wall built according to the rules.. with much more than sledge hammer blows. The rebar is the prime strength adding agent..
frenchy, given the choice of ICF wall construction, reinforced concrete block, and even heavier wood wall construction I wouldn't hesitate to use the ICF. This is assuming we're talking a high wind situation.
Up here on the north coast, northern Ohio, we are required to design for 90mph for a 3 second interval. This is not quite the potential conditions one would find in FLA or any coastal area so up here ICF construction is really rare for an above grade wall. Big thing here is roof loading and location in relation to Lake Erie which kicks in special loading requirements.
Most people don't fully appreciate the importance of, and function of, rebar. Masons and foundation subcontractors could no doubt tell endless stories.
{??? CMU and rebar. I'm asking this withour knowledge of your chosen craft. Have you ever heard of the 'Ivany' concrete block system? }
And there is always, always, the cost to consider. A structure, be it a residence or a commercial project, has someone who is questioning these additional requirements for wall strength.
ciao, ted
Ted,
Regarding costs, here in Minnesota lately contractors who've been building with ICF's for a while are able to offer ICF's over stick built for a 2% premium*. That's not much considering how durable /airtight/ sound proof ICF's are..
Having built with ICF's myself I know just how quick and easy building with ICF's is for the average do-it-yourselfer.
Having built above grade with SIP's and below grade with ICF's if I were to build over again I wouldn't hesitate to go to the roof line with ICF's in leu of SIP's and just use SIP's for the roof or ceiling..
In fact if I had used ICF's instead of SIP's I suspect I could have saved myself $150,000 over what this house cost me.. Plus it would have been built a year sooner and been far less maintinance.
In retrospect building a double timberframe was far more expensive than I'd anticipated and I'd simply stone the exterior rather than stone and timberframe.
The black walnut timbers fade and will soon need to be painted rather than varnished. There goes all the beauty, so "Next" house I'll avoid that mistake. ;-)
* a few contractors desperate for work are offering to build at the same cost!
frenchy, your numbers are interesting in that I can easily "sell" a 2% premium. I think your observation about the contractors who've been building with ICF's for a while has a lot to do with that cost containment.
The thing about northern Ohio is that there are not a lot of ICF or poured concrete walled structures, at least not in residential construction. And that is not to say there are none. They just don't get out there and get noticed. Thus very few outfits that you could get to bid on a project. Spec'ing out anything but CMU nowadays gets howls of terse questions. Most asking if they can build in a method they 'normally' do.
Up until about 5 -- 7 years ago you never saw anything but concrete block walls. Almost against the local religion. Been seeing more and more poured walls. I don't understand the resistance.
Hey, enough. Thanks for the comeback.
ciao, ted
dirtyturk
I see an oppurtunity there then.. Sit down with the contractors that you normally build for and make the suggestion.. offer to learn enough to make a presentation to potential customers. It's selling......
This is hard stuff, Practice with family friends and ask for their feedback.. selling is tough and easier to mess up than do right (nationally less than 5% of those who enter the sales field are still in it after 5 years.. ) So accept critism but make it work for you..
The rewards are that you will be the go to guy and leader in the area.. that puts you in the drivers seat sort of deal..
You have a real value here with ICF's and properly presented it can take yopu into a great position.
frenchy, your suggestion is duly noted and will be acted upon.
Going back and forth with you it got me to thinking on the same line of thought. Done my share of cold calls and hated it but learned a lot.
Every time I meet a new client or discuss a project I, as are you, am selling.
Thanks for the thoughts.
ciao, ted
I'm working on an article about lessons I learned from Hurricane Fran that hit us back in '96 and what changes I made in the way I design and frame houses as a result of that. I had lots of pictures of two of my houses covered in downed trees that I was hoping to use as illustration but that was back in the kodachrome days and I've been through four boxes of great old photos but none showing the trees on the houses I was looking for. Do you have any 'trees laying on/in houses' photos that you would be willing let me submit to FHB? I'll give you a cut of my writing fee gladly if they accept any for publication (warning, it's not much) and any photo would be credited, (we can probably even get them to write the credit "Brownbagg from BT" if you like.) thanksMichael------------------
"You cannot work hard enough to make up for a sloppy estimate."