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How defects in roof framing or damage to roof connections or structural members lead to building damage & collapse.
This article describes & explains the directions of forces in roof structures, illustrating downwards force, horizontal thrust, and the causes of rafter sags, rafters that disconnect from the ridge or top plate, and why a sagging roof pushes the building walls outwards at the wall top.
We include photographs of collapsing wood-framed roofs and photo sequences showing the stages in a roof collapse as well as the direction of forces that bend and finally destroy a building.
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Forces & Direction of Movement in Sagging & Collapsing Roofs
Increased Horizontal Thrust on Low-Slope Roofs
This article series describes and illustrates the different types of support that prevents roof sagging and wall bulging at buildings, including definitions of collar ties, rafter ties, and structural ridge beams.
Without the proper support of rafter ties or a structural ridge, a typical gable or sloped roof will sag downwards while pushing the building walls outwards towards a catastrophe.
We include sketches of collar ties, rafter ties, and structural ridge beams as well as illustrations of collapsing and collapsed structures where these roof rafter ties were lost or omitted.
Watch out: collapsing roofs or structural damage from over-loaded or under-built roofs or other structural components can cause dangerous building collapses that can injure or even kill building occupants.
Watch out: also for increased horizontal thrust loads on low slope roofs.
More examples of the direction of forces that push down on roofs and out on walls for low slope roofs are found in this article series
This horizontal load - or thrust - can be considerable, especially on a low-pitched roof.
To resist thrust, the IRC calls for a structural ridge (required for any roof with a roof pitch less than 3/12) or for each pair of rafters to be securely connected to each other [at the lower end of the rafter] by a continuous joist. (R802.3, 2006 IRC).
Code does allow joists [serving as rafter ties] to be installed above the top plate, but only under certain conditions.
Previous building codes permitted rafter ties to be placed as high above the plate as two-thirds the distance between the top plate and the ridge, but the 2006 IRC now limits this height to one-third the distance between the plate and the ridge (see footnote A, Table R802.5.1, 2006 IRC). (Truesdell 2008)
At ROOF SLOPE DEFINITIONS we comment that this photograph illustrates a roof whose slope has become irrelevant after the building collapsed.
I suspect the very low-slope roof in the photo was a bit steeper before the building fell in, and that the connections of its rafter ties to the rafter ends were inadequate or failed from rot or insect damage to the structure.
Let's look a bit more at the collapsing roof and walls of the Amenia New York Barn.
The downwards load on the roof structure is combined of dead loads: the weight of framing, roof covering, and other things that might be mounted onto a roof surface, plus the live loads of snow, wind, water, and an occasional worker or animal clambering around on the structure.
These forces press downwards or in the case of strong winds, the force may combine both horizontal and downwards forces on the roof surface. How these forces are carried down to the foundation and ultimately to earth determines what happens to the roof and to the rest of the building structure.
Even without doing the engineering or the math, we can see in photos below what happens to a building wall and roof when its support is incomplete, lost, damaged or missing.
Let's press our imaginary thumb down on the roof of an old barn whose rafter ties have been cut out and carried off by someone who's decorating their Los Angeles apartment.
As the ridge sags down and the centers of the front and rear walls push outwards at the wall top, the photo above provides a graphic illustration of direction of the forces at work.
The gable ends of the structure want to hold the two ends of the ridge up.
That's why most of the ridge sagging occurs in the center of the roof (page top photo).
As the wall tops are pushed outwards at their center, either the entire wall leans over or, as the wall is held up at the building corners (gable ends), the wall center may break and bend over as in my photo above.
Ultimately as the roof collapses downwards starting at the center of the ridge line, the tops of the gable end walls are pushed inwards (photo just above).
Just below we trace another building through sagging roof to ultimate and total collapse.
Snow Load Roof Collapse Example
In March 2023, a section of roof collapsed on the Miller Hill Mall, Duluth MN, as was widely reported in the state.
The photo below, adapted from a new report by Corin Hoggard from Fox 9 News on 14 March 2023 illustrates a snow load collapse.
OPINION: Considering that this was a very specific and neatly-rectangular section of roof that collapsed out of a much larger flat roofed shopping mall, one might expert forensic structural collapse experts to find errors or omissions in connections or support in this area, or some other factor that caused higher weight of snow loading in this roof section than its neighbors.
Watch out: when there is deep snow accumulation on a building roof, particularly if followed by rain, the combination of the added weight of water held on the roof as now wet snow may significantly increase the risk of a collapse.
Source:
Hoggard, Corin, Duluth Miller Hill Mall roof collapse spotlights potential statewide dangers, Fox 9 News, 14 March 2023, retrieved 2023/03/19, original source: https://www.fox9.com/news/duluth-miller-hill-mall-roof-collapse-spotlights-potential-statewide-dangers
Excerpt: The city got more than a foot of snow over the weekend, adding to their near record total this winter and the mall’s wasn’t the first roof to cave in. ... [Duluth fire chief Shawn Krizaj] says they’ve had a few roof collapses in the area this winter while accumulating almost ten feet of snow.
Also see these studies of roof failures due to snow loading
Abé, Masato, Kazuki Nakamura, and Makoto Shimamura. "Structural failures due to snow in Japan." Proceedings of the Institution of Civil Engineers-Forensic Engineering 171, no. 4 (2019): 163-170.
Aksoylu, Ceyhun, Yasin Onuralp Özkılıç, and Musa Hakan Arslan. "Damages on prefabricated concrete dapped-end purlins due to snow loads and a novel reinforcement detail." Engineering Structures 225 (2020): 111225
Alemdar, Zeynep Fırat, and Fatih Alemdar. "Progressive collapse of a steel structure under expected snow loads." Engineering failure analysis 125 (2021): 105378.
Bean, Brennan L.; Maguire, Marc; Sun, Yan; Wagstaff, Jadon; Al-Rubaye, Salam; Wheeler, Jesse; Jarman, Scout; and Rogers, Miranda, THE 2020 NATIONAL SNOW LOAD STUDY [PDF] (2021). Mathematics and Statistics Faculty Publications. Paper 276.
https://digitalcommons.usu.edu/mathsci_facpub/276
Abstract
The United States has a rich history of snow load studies at the state and national level. The current ASCE 7 snow loads are based on studies performed at the Cold Regions Research and Engineering Laboratory (CRREL) ca. 1980 and updated ca. 1993.
The map includes large regions where a site-specific case study is required to establish the load. Many state reports attempt to address the "case-study regions" designated in the current ASCE 7 design snow load requirements.
The independently developed state-specific requirements vary in approach, which can lead to discrepancies in requirements at state boundaries. In addition, there has been great interest to develop site-specific reliability-targeted loads that replace the current load and importance factors applied to 50-year snow load events as defined in ASCE 7-16.
This interest stems from the fact that the relative variability in extreme snow load events is not constant across the country, leading to a non-constant probability of failure for a given design scenario.
This report describes the creation of a modern, universal, and reproducible approach for estimating reliability-targeted design ground snow loads for the conterminous United States.
This new approach significantly reduces the size of case-study regions as currently designated in ASCE 7-16 and resolves discrepancies in design snow load requirements that currently exist along western state boundaries.
Excerpt from Project Aims:
The final product of this project is a modern, universal, and reproducible approach for generating design ground snow loads for the conterminous United States.
Buska, James, and Wayne Tobiasson. MINIMIZING THE ADVERSE EFFECTS OF SNOW AND ICE ON ROOFS [PDF] In International Conference on Building Envelope Systems and Technologies. 2001. Retrieved 2019/11/01 original source: https://www.erdc.usace.army.mil/
Abstract excerpt:
Snow load design criteria in the United States are established in ASCE Standard 7, “Minimum
Design Loads for Buildings and Other Structures.” The information in that standard documents
how dramatically the geometry of a building can influence snow loads on its roof.
Çeribaşı, Seyit. "Reliability of steel truss roof Systems under variable snow load profiles." International Journal of Steel Structures 20, no. 2 (2020): 567-582.
Croce, Pietro, Paolo Formichi, and Filippo Landi. "Extreme ground snow loads in Europe from 1951 to 2100." Climate 9, no. 9 (2021): 133.
A research project was conducted to explore construction failure investigation techniques and processes to determine whether they were adequate and to develop failure investigation guidelines.
Data was collected on failures and failure investigation techniques from surveying 115 members of the engineering and construction industry.
Construction failure case studies were created using documentation provided by the federal Occupational Safety and Health Administration, state offices of safety and health, and forensic engineers.
The construction failure case studies were analyzed to determine how these organizations conduct their investigations and to develop guidelines that can be used for construction failure investigations. This article provides:
-1 description of the methods used for the research;
-2 results obtained from the industry survey;
-3 summary of the results of an investigation into case studies on construction failures;
-4 analysis of the results;
-5 discussion on construction failure investigative techniques;
-6 guidelines developed during the research project for investigating and documenting failures; and ~
-
7 recommended format for reporting the findings of failure investigations.
Stages in the Collapse of a Roof Without Rafter Ties
[Click to enlarge any image]
What happens if rafter ties are omitted and the builder simply frames rafters butting against a ridge board with or without collar ties but with no rafter ties nor ceiling joists?
Unless the building included a structural ridge beam the downwards loads on the roof rafters will cause the building walls to bulge outwards, most noticeably at the center of the walls, and the roof rafters will sag.
Our photo above shows a collapsing building is part of a study that I [DF] watched for years as the structure slowly settled to earth.
Our sketch above illustrates that as the ridge sags the rafters, either also sagging or even if relatively-straight, will push out the top of the wall.
This modest farm house served as a home to farm workers probably from the 1930's until around 1960. I watched its condition from 1969 until its disappearance a few years ago.
As the roof ridge sagged downwards (red line and arrow), the centers of the front and rear walls of this little house bulged outwards, telegraphing that movement into the outwards lean of the front porch as the front walls of the home also pushed the porch roof outwards (blue line and arrow). Above in snow is the sagging roof and bulging wall story of this home in 2003.
By 2007 the front porch had collapsed, the roof had sagged further and the front wall leaned outwards precariously. In fact it's remarkable how plastic building materials can be when bent over a long timer.
By 2011 the roof and most of the structure had collapsed (below).
And the next year, in 2012 this little house finally bit the dust, or I could say snow-dust (below).
Today all traces of the building have been removed and the cows seem to have wandered off as well.
Straighten Sagging Roof Structure, Surface, or Rafters
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In addition to any citations in the article above, a full list is available on request.
Paul DeBaggis is a building inspector and certified building code official with special interest in the history of and standards for wood products. (The American Wood Standards Committee). Mr. DeBaggis has served in the Easton MA. Building Department since 2002 , has worked as a building trades instructor, and also writes about land use regulations, building regulations, and standards. He is a past president of Southeastern Mass. Building Officials and is currently writing a book on the Massachusetts building code. Email: paul@jenick.net
Huber, Gregory D. "Framing Techniques as Clues to Dating in Certain Pre-Revolutionary Dutch Barns: Major and Minor Rafter Systems, Lapped Dovetail Joinery, Verdiepinghs and Other Traits." Material Culture 29, no. 2 (1997): 1-41.
Liu, Henry. "Calculation of wind speeds required to damage or destroy buildings." The Tornado: Its Structure, Dynamics, Prediction, and Hazards (1993): 535-541.
Marshall, Timothy P. "Lessons learned from analyzing tornado damage." The tornado: Its structure, dynamics, prediction, and hazards (1993): 495-499.
Mayo, A. P. "Trussed rafter roofs-Load distribution and lateral stability." Structural Survey 2, no. 1 (1984): 12-15.
Palma, Pedro, Helena Garcia, João Ferreira, João Appleton, and Helena Cruz. "Behaviour and repair of carpentry connections–Rotational behaviour of the rafter and tie beam connection in timber roof structures." Journal of Cultural Heritage 13, no. 3 (2012): S64-S73.
"The Mathematics of Rafter and Collar Ties", [Web article], Math Encounters Blog, (November 2010), retrieved 2016/04/15, original source: http://mathscinotes.com/2010/11
/the-mathematics-of-rafter-and-collar-ties/ posted by un-named web author using web name mathscinotes.
Truesdell, Jordan, P.E., "Rafter Ties and Shallow-Pitch Roofs", Q&A, The Journal of Light Construction, (October 2008) posted as PDF at http://mathscinotes.com/wp-content/uploads/2010/11/JLC.pdf
Utterback, David. "Common Engineering Problems in Frame Construction." Fine Homebuilding (2000): 110-115.
"Evaluating OSB for Coastal Roofs," Paul Fisette, Coastal Contractor, Winter 2005, online at coastalcontractor.net/pdf/2005/0501/0501eval.pdf . Fisette cites: "Jose Mitrani, a civil engineer and professor at Florida. International University in Miami, was ... Florida’s official damage assessment team. ... After Hurricane Andrew, Florida code advisers ruled OSB sheathing inferior to plywood
GluLam Structural Wood Products, U.S. GluLam Inc.,
4245 W. 166th St.,
Oak Forest Il. 60452 -
email: bevusg@aol.com, 708-535-6506
I-Joists: "The Evolution of Engineered Wood I-Joists",
Paul Fisette,
Building Materials and Wood Technology
University of Massachusetts, Amherst, MA 01003, 2000 - see U. Mass online article at umass.edu/bmatwt/publications/articles/i_joist.html
Laminated Beams: Radial reinforcement of curved glue laminated wood beams with composite materials", Kasal, Bo and Heiduschke, Andreas, Forest Products Journal, 1 Jan 2004
OSB: "Evaluating OSB for Coastal Roofs," Paul Fisette, Coastal Contractor, Winter 2005, online at coastalcontractor.net/pdf/2005/0501/0501eval.pdf . Fisette cites: "Jose Mitrani, a civil engineer and professor at Florida. International University in Miami, was ... Florida’s official damage assessment team. ... After Hurricane Andrew, Florida code advisers ruled OSB sheathing inferior to plywood."
OSB: Timberco TECO is located at 2902 Terra Court,
Sun Praire, WI 53590 USA, 608-837-2790. TECO provides a reference library of .PDF files that can be downloaded by consumers, homeowners, builders, and architects. The association refers to industry standards for oriented strand board OSB products as:
"DOC PS 2, Performance Standard for Wood-Based Structural-Use Panels. Certified to CSA 0325, Construction Sheathing, or CSA 0437, OSB and Waferboard, OSB is accepted in the National Building Code of Canada, certified to EN 300, Oriented Strand Boards and recognized for structural use in Europe and certified to meet the JAS standard for structural panels in Japan."
OSB: "Performance of Wood Shear Walls Sheathed with FRP-Reinforced OSB Panels", J. Struct. Engrg. Volume 132, Issue 1, pp. 153-163, Jan. 2006 provides a study on the development and structural testing of a hybridsheathing panel designed to improve the lateral resistance of lightwood-frame shear walls. "FRP" refers to fiber reinforced polymer material that was sandwiched between more conventional exterior OSB layers.
Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
Building Failures, Diagnosis & Avoidance, 2d Ed., W.H. Ransom, E.& F. Spon, New York, 1987 ISBN 0-419-14270-3
Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724
ISBN-13: 978-0471331728
ISBN-10: 0471788368 ISBN-13: 978-0471788362
In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.