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.
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Roof Collar Ties, Rafter Ties, Structural Ridge Beams for Cathedral Ceiling Framing
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.
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“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
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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 protected]
Continue reading at CANTILEVERED ROOF SUPPORT REQUIREMENTS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
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