CATHEDRAL CEILING / ROOF SUPPORT REQUIREMENTS - CONTENTS: how do we best support a cathedral ceiling roof structure? With a structural ridge beam or by other specially-engineered solutions. How does the building department evaluate your roof structure when giving a building permit? When do you need the services of a design professional, architect, or engineer to figure out the roof support system? How can a structural ridge rescue a fouled-up roof structure?
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Cathedral Ceiling & Roof Framing:
Roof structure choices for cathedral ceiling roofs. Why do so many otherwise bright construction people confuse collar ties and rafter ties? And why do the same people compound this error by framing a cathedral or a vaulted ceiling on a gable roof without using a structural ridge? Despite the training programs for construction supervisors now mandated by many state inspection bureaus, this misstep in the framing of a single family dwelling seems to happen as much today as it did twenty years ago.
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.
Structural Ridge vs. Ceiling Joists vs. "Nothing" for Cathedral Ceilings
- Paul DeBaggis
Paul DeBaggis, a Massachusetts building code official having a particular interest in wood framing standards, describes what goes wrong in cathedral ceiling roof framing SNAFUs and what can be done about it.
When a framer installs collar ties, he or she must place them horizontally against one side of the roof rafters and in the top third of the vertical distance between the ridge board and the plane of the top plates of the exterior walls. This is done is to stabilize the connection between the ridge board and the tops of the rafters. The collar ties do nothing to hold the exterior building walls together.
Rafter ties, on the other hand, create a rigid triangle that presses straight down rather than pressing outwards on the outside walls. Without rafter ties, the ridge sags onwards, and the top of the walls supporting the lower ends of the rafters push or “kick” outward. Inspecting a building constructed without rafter ties and that also lacks a structural ridge beam, we will often notice first that the ridge has sagged downwards, mostly in the center between the gable end walls.
[Click to enlarge any image]
From outside, take a second look with great care by sighting along the top of the front and rear walls on which the lower ends of the roof are resting. You may be shocked at the amount of outwards bulge seen at the top center of these walls. Inspecting inside the building where a cathedral ceiling design has been used, you may notice separation or cracks at the top of the front and rear walls and in severe cases you'll easily see that the top of the walls lean mysteriously "out". You may find this roof sagging, wall bulging in new construction that was not properly designed, or you may encounter it in an older building whose owners decided to "open up" the top floor rooms by tearing out all of the ceiling joists to "raise the ceiling".
The International Residential Code prohibit omission of rafter ties unless a ridge beam or an equivalent design has been provided.
“Where ceiling joists or rafter ties are not provided, the ridge formed by these rafters shall be supported by a wall or girder designed in accordance with accepted engineering practice.” - 2012 IRC, Section R 802.3.1
For the past several code cycles, the IRC code commentaries have also contained a detailed explanation, including sketches, of this crucial rafter and exterior wall relationship.
Structural Ridge Beams Required for Cathedral Ceilings
Above: this detail shows the vertical post supporting a built-up structural ridge supporting a cathedral ceiling spanning 27 feet and constructed by one of the authors [DF] in 1979. The rafters rest atop this ridge beam and were strapped together using steel strap-ties nailed along the rafter upper edges before the roof sheathing was set in place. Below one of the authors [DF again] stands at the lower edge of this roof in 2010: the rafters "hung" over the structural ridge beam and the walls upon which they terminate never moved at all during the ensuing 31 years.
To help permit applicants at our office [PD], whoever approves the plans for a cathedral or a vaulted ceiling clearly marks them, “structural engineered ridge required.” If the contractor seems uncertain about the meaning of this, we add a verbal explanation. But once in awhile, even as we explain, we have the uneasy feeling the applicant has already planned the roof framing, detail by detail, in his or her brain. In those cases, the nod or the verbal agreement is more of a, “Just give me that permit. I KNOW how to build it,” kind of a yes, as opposed to a yes that means, “Right. The way I showed it would not have worked.
Whenever I find this flaw on a job and point it out, the contractor usually says, “But I have collar ties, there.” I suppose it should be written as a building inspector’s commandment:
“One shall not use collar ties to try to hold exterior walls together.”
So when the inspector finds this, should he or she issue a notice of violation or a stop work order? To date, in the dozen or so times I have pointed out this error, each has been corrected by the framer or builder responding to my verbal order.
Clearly, the code says either a violation notice or a stop work order shall be in writing. Despite this morsel of legalese, in my opinion the answer depends on the contractor’s response to the verbal comments. Keep in mind the language of the ICC book, Basic Code Enforcement, which I paraphrase
here: At times, it is better, faster, and easier to persuade the party to make corrections than to pursue formal (legal) action. If you use good judgment, you should get a good result.
Below: at this vaulted ceiling the rafters are hung over and rest upon a structural ridge beam. The end of the beam is carried to the foundation by a built-up post.
Regardless of whether or not the rafters but into the ridge beam or rest upon its upper surface, the building inspector will typically require that steel strapping connectors be installed along the top edge of opposing rafters to form a mechanical tie so that while hanging on or from the structural ridge, the rafters do not separate at their upper ends.
Above: in conventional roof framing the rafters may be nailed-through the ridge board into the rafter end (the weakest possible connection) or they may be toe-nailed through the rafter sides to the ridge board or beam, or steel rafter hanger-connectors may be used. But as you can see from our photos above and just below, without strapping and rafter ties, the spreading forces effected by roof loads can easily separate a nailed rafter from the ridge.
How to Fix a Cathedral / Vaulted Ceiling Roof that Lacks a Structural Ridge
How does a contractor fix one of these mini-disasters? He or she will probably ask this question. You should clearly indicate you are not in the design business, and it is the contractor’s job to present you with a proposed solution. Having stated this, I am happy to point out the three types of successful fixes I have seen.
First, through the gable end, the contractor can slide an engineered structural ridge beam into position under the non-structural ridge framing. As a part of the beam package, many lumber dealers will work with a licensed engineer and provide a stamped report specifying the loading capacity and a fastening schedule. At the job site, set the beam tight under the framing using cant strips, if needed. Be sure both ends of the new beam are secure and posted directly or indirectly to the foundation, then install plenty of approved framing connectors.
Second, across the clear span of the room, at the top plates and parallel with the rafters, install “beams.” These will act as rafter ties. Use four by sixes or four by eights based on the span of the room—very large spans will require an engineer’s design-- and put them four to six feet apart (the code does state a maximum of four foot centers for rafter ties). Be sure the contractor uses approved connectors—screws or bolts as usually nails won’t work-- to secure these beams to the wall plates. The beams will keep the two exterior walls from kicking out.
And third, have a structural engineer evaluate the situation. He or she may recommend methods one or two or a different solution. On occasion, and depending on what the customer wants for a final ceiling design, the engineer may offer more than one alternative.
With each of these options, you must check whether the two long walls are plumb at mid-span. If they have kicked out more than a quarter of an inch, you need some combination of spring braces, turnbuckles, and raising the ridge by posting where it sags, to bring the walls near to plumb, again. As to whether the contractor uses method one, two, or three, the building official has the last word. Methods one and two will remedy most situations, but in all of these cases the inspector can require the engineer’s report. If it’s your first case, and if you feel uncomfortable, call for the engineer.
Finally, when a cathedral or a vaulted ceiling (or any gable roof) is framed without either an engineered structural ridge, ceiling joists or rafter ties, or an engineered solution, some amount of ridge sag and wall kick out is guaranteed. In a worst case scenario such as the record snow loads of the winters of 2011 and 2015, the roof could collapse.
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
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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: firstname.lastname@example.org>
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: email@example.com, 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: firstname.lastname@example.org. 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.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
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.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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