Roof Framing: collar ties, rafter ties, tension beams & structural ridge beams: some of these can support the roof and prevent ridge sagging and wall spreading.
Roof structure definitions & support choices for sloped roofs, cantilevered roofs & cathedral ceiling 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 such as that shown in our page top photo of a barn in Amenia, NY: the failure of rafter ties in this building combined with snow loading in Amenia, New York, pushed the ridge down and the walls outwards as the building slowly settles down to the earth.
Here 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.
Green links show where you are. © Copyright 2017 InspectApedia.com, All Rights Reserved.
[Click to enlarge any image]
What are those horizontal 2x members in the roof frame photo just above? I don't know if we're looking at collar ties or rafter ties,or "something else" since these horizontal 2x members are at mid-span on the rafters of this roof. In definitions below we give the locations and functions of collar ties, rafter ties, ceiling joists, and structural ridge beams.
A collar tie is a horizontal roof rafter connector that is located in the uppermost third of the span of a pair of opposed sloped or "gable roof" rafters. By upper third, here we mean one third of the length of the rafter from ridge to top plate. A collar tie is used to prevent rafters from separating from the ridge, a structural failure that can be caused by snow loads, wind loads, or un-balanced roof loading for any other event or force that causes the roof to begin to move downwards or to collapse.
[Click to enlarge any image]
Above: a roof framed with just collar ties, or with nothing but a ridge board, is unable to resist sagging at the ridge and outwards bulging at the side walls unless specially-engineered features are included. So what good is a collar tie anyway?
Collar ties help keep the rafters against the ridge board when there are un-balanced roof loads such as during high winds or when most of the snow load is on one slope but not the opposing slope of a roof. (Mathscinotes 2010 & Truesdell 2008).
Watch out: located in the upper third of a gable roof, and particularly for larger roofs with a longer run or span, a collar tie are not likely to prevent rafter sagging from uniform vertical loading and thus may not prevent spreading or bulging walls when the roof is loaded by downwards forces including its own weight.
Why do some roofs get away with just high collar ties? Take a look at the roof pitch. On a very steep-pitched roof, the horizontal loading caused by roof spreading forces is less than on a low slope roof.
Really? There is some confusion among writers about whether the upper roof collar tie is under compression or under tension. As you will read below at COMPRESSION vs TENSION in ROOF FRAMING and as Mark Cramer, home inspection expert says, " ... it depends". Until the roof begins to collapse the upper areas are pressing in against the ridge board and are under compression. But when a roof ridge begins to move downwards or to collapse, that situation changes from compression to tension.
A rafter tie is a horizontal tension tie that is located in the lowermost third of the span of a pair of opposed sloped or "gable roof" rafters. A rafter tie resists spreading of the building walls by bulges at the wall top that would otherwise occur due to roof loads. Rafter ties greatly increase the resistance of wood structures to wind damage when compared to simple toe-nailing rafters to plates. Liu (1993)
A ceiling joist that is affixed to the ends of rafters (and normally also to the top plate of the wall) is also a rafter tie: located at the wall top it forms the bottom of the roof triangle as you can see in our sketch below.
Depending on the building design, finished ceiling surfaces may be affixed to a ceiling joist at the bottom of the roof triangle, giving a flat ceiling between the opposing building walls, or the ceiling may be affixed partly to the under-side of rafters and partly to the under-side of rafter ties, giving a Cape Cod type ceiling design.
Building codes such as the ICC's 2012 International Building Code section 2308.10, Roof and Ceiling Framing typically specify that where rafter ties are used, they are required at intervals ranging from 12" to 24" o.c. depending on the roof design and the roof rafter spacing. Where rafter ties are not being installed with every rafter pair, model codes often specify that
Rafter ties shall be spaced not more than 4 feet (1219 mm) on centers and rafter tie connections shall be based on the equivalent rafter spacing in [a table given in the code, such as ICC Table 2308.10.4.1]
The following is excerpted from the International Code Council (ICC)'s International Building Code section on Ceiling Joist and Rafter Connections
2308.10.4.1 Ceiling joist and rafter connections.
Ceiling joists and rafters shall be nailed to each other and the assembly shall be nailed to the top wall plate in accordance with Tables 2304.9.1 and 2308.10.1. Ceiling joists shall be continuous or securely joined where they meet over interior partitions and fastened to adjacent rafters in accordance with Tables 2308.10.4.1 and 2304.9.1 to provide a continuous rafter tie across the building where such joists are parallel to the rafters. Ceiling joists shall have a bearing surface of not less than 11/2 inches (38 mm) on the top plate at each end.
Rafter ties shall be spaced not more than 4 feet (1219 mm) o.c. Rafter tie connections shall be based on the equivalent rafter spacing in Table 2308.10.4.1. Where rafter ties are spaced at 32 inches (813 mm) o.c., the number of 16d common nails shall be two times the number specified for rafters spaced 16 inches (406 mm) o.c., with a minimum of four 16d common nails where no snow loads are indicated. Where rafter ties are spaced at 48 inches (1219 mm) o.c., the number of 16d common nails shall be two times the number specified for rafters spaced 24 inches (610 mm) o.c., with a minimum of six 16d common nails where no snow loads are indicated. Rafter/ceiling joist connections and rafter/tie connections shall be of sufficient size and number to prevent splitting from nailing. - ICC 2012, http://publicecodes.cyberregs.com/icod/ibc/2012/icod_ibc_2012_23_par196.htm
Table 2308.10.4.1, Rafter Tie Connections, relates rafter slope, rafter tie spacing, and roof load ratings or snow load ratings and also roof span in feet.
The local building code inspector is the final authority, and your local building codes and inspectors may have different requirements.
Above: one of the author's [DF] construction projects in the 1970's, the Weeks' corrugated-fiberglass-panel covered porch in Poughkeepsie, NY, was framed placing rafter ties at mid-span.
Below, the author framing a raised-roof on the Seneca Howland historic home in Pleasant Valley, New York in 1982. Below: adding ceiling joists below this rather low-sloped roof required adding a short wood-framed wall that itself had to be anchored to the original brick wall of this building extension. Forming a very rigid roof triangle was particularly important to eliminate any risk that roof loads might push the added-on cripple walls outwards. I'm using a 2x ridge board that did not form a structural ridge. You can see temporary 2x lumber placed horizontally to tie the front and rear walls together before we added ceiling joists (yet yet installed) that were to be nailed to the top plates and to the rafters.
The roof structure above can be seen near completion near the end of our ROOF DORMER TYPES article. Below is a sketch of the original roof-wall structural details for this building. The original roof was built using purlins, beams extending between gable-ends. The roof sheathing consisted of inch-thick wide pine boards nailed to the purlins and thick enough to resist (much) sagging. That "tipped" purlin at the top of the brick wall is no mistake, that's how it was framed - tipped as shown.
Where ceiling joists are installed, if the ceiling framing is not in parallel with the rafters, then equivalent rafter ties are required to be added, or a knee wall (dwarf wall) can be used when placed on a supporting beam that sits on the building the tops of the wall plates of the exterior walls. The top of the raised beam should be at least 1" above the finished ceiling that runs below the ceiling joists in order to prevent any telegraphing of roof loads through the finished ceiling below.- CMHC 2014
Where ceiling joists are not parallel to rafters, an equivalent rafter tie shall be installed in a manner to provide a continuous tie across the building, at a spacing of not more than 4 feet (1219 mm) o.c. The connections shall be in accordance with Tables 2308.10.4.1 and 2304.9.1, or connections of equivalent capacities shall be provided. Where ceiling joists or rafter ties are not provided at the top of the rafter support walls, the ridge formed by these rafters shall also be supported by a girder conforming to Section 2308.4.
A structural ridge beam is a horizontal beam placed at the peak or ridge of a roof and is designed to carry much of the live and dead loads or weight of the roof structure by itself.
The loads on the ridge beam are carried by vertical posts at the beam ends down through the building structural walls to the building foundation. A ridge beam essentially allows us to hang a roof over the beam such that rafter ties can be omitted, giving a cathedral ceiling with a completely open space below.
These illustrations illustrate two common wood roof truss designs: the Fink Truss and the Howe Truss. The bottom chord of these roof trusses is serving as a tension tie: it is in tension and is holding the top chords of the truss against spreading. It is because the bottom chord of a roof truss is in tension that the truss member can be as small as a 2x4.
However the weight of a ceiling affixed to rafter ties of any dimension can cause ceiling sagging unless the lumber is also sized to support the vertical load or weight of the ceiling. In a roof truss design, the additional web members of the truss help resist sagging of the bottom cord and add rigidity to the truss structure.
See Framing Methods Age for the history and date ranges of various building framing methods.
Mathematicians and engineers discussing collar ties and rafter ties explain that collar ties and rafter ties (or ceiling joists) under a gable roof are in enormous tension and that the mathematics of the roof forces are the same for collar ties and rafter ties.
A Tension tie is a general term to refer to any structural member that is subject to net tension: that is, the forces or loads on the member place it in tension or "stretching" forces. The most-common tension tie observed in roof framing for a conventionally-framed gable roof is the ceiling joist that forms the bottom chord of the roof triangle shown below. By fastening the lower rafter ends to the horizontal ceiling joist and the upper rafter ends to the ridge board (or to one another in older framing), the triangular structure is very rigid and resists both rafter sagging and spreading of the building walls that would occur if the ceiling joist were omitted.
Really?: I [DF] and other sources consider that under normal conditions forces on a sloped roof's exterior surface are pressing downwards: a condition that will indeed cause the "triangle" formed by the sloped roof to flatten, pushing out the building walls upon which the lower edges of the roof are resting and to which they're normally fastened. But near the ridge, these forces tend to press the rafters inwards against the ridge board (or against the opposing rafter in each rafter pair. So the top of the roof is in compression, and collar ties near the upper roof should also be in compression, not in tension.
An exception occurs however, even near the ridge, if the bottom of the roof triangle is not secured against stretching or flattening by a ceiling joist or a low-placed rafter tie. When these framing members have been omitted, and when there is no structural ridge design over which a roof is suspended, the inwards-force on the upper roof combined with the downwards movement of the ridge will pull the lower half or more of the rafter away from the ridge board. You can see this in my photo just below.
To me this phenomena argues that a mathematician's observation that "all the roof loads are specified as if they were in the horizontal plane" could be proper, but it may ignore horizontal forces that occur within the roof structure such as those that lever or pull away a rafter from the ridge when a roof ridge moves downwards for any reason. The engineers and mathematicians define the first view as a "static" or "non-rotating" situation. (Mathscinotes 2010 & Truesdell 2008).
My concern, that seems secondary, is for a situation that includes rotating forces. But in fact it might be that rotating forces (photo above) may be the proximate cause of connection failures and more calamitous collapse in some cases.
Please see these three separate articles now founda at
We moved this discussion to its own article space now found at STAGES in ROOF COLLAPSE
We have moved this discussion to CATHEDRAL CEILING / ROOF SUPPORT REQUIREMENTS - Paul DeBaggis
“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
We moved this discussion. Please see CANTILEVERED ROOF SUPPORT REQUIREMENTS
Below we're installing a structural beam supported by posts carrying the roof loads to the foundation and footings of this building addition by one of the authors, working with builder Eric Galow in Poughkeepsie, New York.
This discussion is now found at COMPRESSION BRACING for RAFTERS (Canada)
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.
Continue reading at CATHEDRAL CEILING INSULATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see FRAMING DAMAGE, INSPECTION, REPAIR - home
Or see TRUSSES, FLOOR & ROOF
Or see TRUSS UPLIFT, ROOF causing interior ceiling & wall cracks
Or see WOOD STRUCTURE ASSESSMENT
Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Search the InspectApedia website