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Roof Framing Suggestions, Canadian guidelines:
Canadaian roof structure framing suggestions, codes, guides for gable roofs, low slope roofs, cathedral ceilings. This article excerpts from & provides copies of roof framing suggestions from Canadian building codes and the CMHC Wood Frame House Construction Guide. Framing any roof but particularly a gable roof, low slope roof, and cathedral ceilign roofs requires collar ties, rafter ties, and possibly a structural ridge beam to support the roof and to prevent ridge sagging and wall spreading.
Intermediate Roof Collar Ties as Compression Supports: Canadian Roof Framing Specifications
Proper placement & connections of collar ties and ceiling joists are required for gable and hip roofs in order to avoid sagging rafters and worse, collapsing roofs. Here we describe Canadian roof framing guidelines.
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.
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A cathedral-ceiling roof framed with no structural ridge and no ceiling joists (sketch shown above) risks suffering two problems: sagging rafters, worse on low-slope roofs, and outwards spreading of the building walls as the roof loads press down on the structure. High collar ties won't prevent these problems.
Below we illustrate that particularly for low slope roofs (slopes less than 1/3), even if ceiling joists are present to prevent wall spreading, a high collar tie may not prevent sagging in rafters.
Al Carson, Carson Dunlop Associates, Toronto, was kind enough to explain that Canadian authorities have a different view of roof sagging and wall spread prevention than that offered for collar ties and rafter ties at ROOF FRAMING TIES & BEAMS.
Below: a "collar tie" placed closer to mid-span on long rafters on a low slope roof, combined with the protection of the ceiling joist connections, can prevent both rafter sagging and wall spreading. Alternatives to this construction such as use of a structural ridge beam or a supporting dwarf wall or knee wall supporting rafters at mid span and resting on a partition wall below can also afford this protection. In the sketch below we think that the "collar tie" can be in compression (rather than tension) when it is placed at any point from mid-span on rafters to locations in the upper portions of the roof structure.
Instead of high collar ties or low rafter ties, the Canadian CMHC's Canadian Wood-Frame House Construction guide shows use of an upper 1/3 span horizontal member between opposing rafters, designed to serve as a compression member to prevent rafter sagging. These compression members, called "collar ties" in Canadian literature, are placed "near the mid point of the rafter" to help prevent rafter sagging. If the compression members would be longer than 8 feet in this location, they should be braced to resist buckling.
For rafters in roofs that slope 1:3 or more, intermediate support is usually provided by 38 . 89 mm (2 . 4 in. nominal) collar ties nailed to the side of each pair of rafters. Since these ties are in compression and subject to buckling, ties more than 2.4 m (8 ft.) long should be supported against lateral deflection. This can be done by nailing a 19 . 89 mm (1 . 4 in. nominal) continuous member at right angles to the collar ties near their centre with three, 76 mm (3 in.) nails at each end (see Figure 83). - CMHC 2014
Note: Both collar ties and ceiling joists nailed between rafters at the top plate are also used in Canada and are shown in the CMHC's guide in the same figure.
The Canadian text in the CMHC Canadian Wood Frame House Construction Guide (live link given below) suggests that those authors, describing collar ties as "intermediate roof support" but the document's illustration (Figure 83 given below) shows the collar ties in the upper 1/3 of the rafter length, where those members might indeed be in compression rather than tension. Our associate, Al Carson points out that
The rafter span is the horizontal projection of the rafter rather than the rafter's actual length. That can explain why a steeper roof resists sagging more than a low-slope roof. For different-pitched roofs using rafters of identical length, the steeper roof rafters will actually be spanning less distance.
Putting the horizontal support or collar tie (in Canada) at the "midpoint" of the rafter length makes sense, since locating the collar tie or "rafter tie" at any point other than the middle of the rafter's length will fail to minimize the rafter span.
That document's illustration (shown just below) presumes that ceiling joists are in place - a feature that, if properly connected to the rafter ends and wall top plate, will indeed prevent wall spreading. The ceiling joists will certainly be in tension, not compression. For low slope roofs or where there are no ceiling joists, all sources require other roof support such as a structural ridge beam or a supporting cripple wall, knee wall, or dwarf wall. - Ed.
To prevent the rafter ends from moving outward, nail the ceiling joist to the side of each pair of rafters (Figure 83). - CMHC 2014
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Canadian Building Code Excerpt on Intermediate Support for Rafters & Joists
Below is an excerpt from the Ontario Building Code (consistent with the National Building Code, I believe). I added the Imperial measurements in blue.
18.104.22.168. Intermediate Support for Rafters and Joists
(1) Ceiling joists and collar ties of not less than 38 mm by 89 mm (2x4) lumber are permitted to be assumed to provide intermediate support to reduce the span for rafters and joists where the roof slope is 1 in 3 or greater.
(2) Collar ties referred to in Sentence (1) more than 2400 mm (8 ft) long shall be laterally supported near their centres by not less than 19 mm by 89 mm (2x4) continuous members at right angles to the collar ties.
(3) Dwarf walls and struts may be used to provide intermediate support to reduce the span for rafters and joists.
(4) When struts are used to provide intermediate support they shall be not less than 38 mm by 89 mm (2x4) material extending from each rafter to a loadbearing wall at an angle of not less than 45° to the horizontal.
(5) When dwarf walls are used for rafter support, they shall be framed in the same manner as loadbearing walls and securely fastened top and bottom to the roof and ceiling framing to prevent over-all movement.
(6) Solid blocking shall be installed between floor joists beneath dwarf walls referred to in Sentence (5) that enclose finished rooms.
Keep in mind that a raised-ceiling or a cathedral ceiling will not be framed with ceiling joists positioned as shown in figure 83 in the CMHC document. For framing a cathedral ceiling or a raised-ceiling brought higher than the bottom 1/3 of the roof rafters you'll want a structural ridge (shown below) or other specially-engineered framing design. See ROOF FRAMING TIES & BEAMS.
Canadian Wood-Frame House Construction, [PDF] Canada Mortgage and Housing Corporation (CMHC), | Suite 1000, 700 Montreal Rd.. Ottawa, ON K1A 0P7, Canada CMHCV-SCHL (2014) Website: www.cmhc.ca , Tel: 800-668-2642 retrieved 2016/04/17, original source: http://www.cmhc-schl.gc.ca/odpub/pdf/61010.pdf?fr=1460905007730
Ontario Canada Building Code, Ontario Regulation 332/12: Building Code, under the Building COde Act, 1992, S.O. 1992, c. 23, Website: https://www.ontario.ca/laws/regulation/120332
Supporting Low-Slope Roof Rafters - CMHC Advice
A roof that slopes less than 1:3 should be
vertically supported at the peak. This can be
accomplished by providing a 38 × 140 mm
(2 × 6 in. nominal) ridge beam supported
at 1.2 m (4 ft.) intervals by 38 × 89 mm
(2 × 4 in. nominal) vertical struts (see Figure 84).
A load-bearing wall may be used instead of the
ridge beam. Since the ridge support does not
result in outward thrust of the roof members,
continuous ties between the lower ends of opposing rafters are not necessary.
are also required for steeper pitched roofs when
the outside ends of the rafters cannot be tied
together to resist thrust.
Intermediate support for rafters in roofs that
slope less than 1:3 is usually provided by a
dwarf-bearing wall (see Figure 84) built in the
same way as a load-bearing partition, except that
a single top plate may be used where the rafters
are positioned directly over the studs. - CMHC 2014
Both the CMHC guide and other expert sources offer other approaches to supporting rafters in pitched roofs, including struts nailed between the rafter side and a load-bearing partition below.
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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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