FORCES on ROOFS CAUSE MOVEMENT - CONTENTS: what are the forces that cause sagging rafters, collapsing roofs, leaning walls? How does a missing collar tie or rafter tie contribute to roof damage, sagging, or collapse? What are the sequences of movement in a collapsing or sagging roof?
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Roof Sag or Roof Collapse Forces:
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.
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: 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 at COMPRESSION BRACING for RAFTERS (Canada).
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.
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. 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.
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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: email@example.com
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: firstname.lastname@example.org, 708-535-6506
I-Joists: "The Evolution of Engineered Wood I-Joists",
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.
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Design of Wood Structures - ASD, Donald E. Breyer, Kenneth Fridley, Kelly Cobeen, David Pollock, McGraw Hill, 2003, ISBN-10: 0071379320, ISBN-13: 978-0071379328
This book is an update of a long-established text dating from at least 1988 (DJF); Quoting: This book is gives a good grasp of seismic design for wood structures. Many of the examples especially near the end are good practice for the California PE Special Seismic Exam design questions. It gives a good grasp of how seismic forces move through a building and how to calculate those forces at various locations.THE CLASSIC TEXT ON WOOD DESIGN UPDATED TO INCLUDE THE LATEST CODES AND DATA. Reflects the most recent provisions of the 2003 International Building Code and 2001 National Design Specification for Wood Construction. Continuing the sterling standard set by earlier editions, this indispensable reference clearly explains the best wood design techniques for the safe handling of gravity and lateral loads. Carefully revised and updated to include the new 2003 International Building Code, ASCE 7-02 Minimum Design Loads for Buildings and Other Structures, the 2001 National Design Specification for Wood Construction, and the most recent Allowable Stress Design.
Diagnosing & Repairing House Structure Problems, Edgar O. Seaquist, McGraw Hill, 1980 ISBN 0-07-056013-7 (obsolete, incomplete, missing most diagnosis steps, but very good reading; out of print but used copies are available at Amazon.com, and reprints are available from some inspection tool suppliers). Ed Seaquist was among the first speakers invited to a series of educational conferences organized by D Friedman for ASHI, the American Society of Home Inspectors, where the topic of inspecting the in-service condition of building structures was first addressed.
Defects and Deterioration in Buildings: A Practical Guide to the Science and Technology of Material Failure, Barry Richardson, Spon Press; 2d Ed (2001), ISBN-10: 041925210X, ISBN-13: 978-0419252108. Quoting: A professional reference designed to assist surveyors, engineers, architects and contractors in diagnosing existing problems and avoiding them in new buildings. Fully revised and updated, this edition, in new clearer format, covers developments in building defects, and problems such as sick building syndrome. Well liked for its mixture of theory and practice the new edition will complement Hinks and Cook's student textbook on defects at the practitioner level.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
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The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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