International Residential Code - Collar Tie Conflict?
International Residential Code - Collar Tie Conflict?
(OP)
Referring to IRC (2000 or 2003) Section R802.3.1;
The third sentence of the second paragraph states "Where ceiling joists or rafter ties are not provided at the top plate, the ridge formed by these rafters shall also be supported by a girder designed in accordance with accepted engineering practice." The first sentence of the paragraph limits this restriction to conditions where ceiling joists are not parallel to rafters.
The way I read this, any condition with ceiling joists located higher than the top plate (say a cathedral ceiling) will require a structural ridge beam, even if the ceiling joists/rafters meet the span requirements of IRC span tables R802.5.1 (1) through (8).
Have you all read this the same way?
The third sentence of the second paragraph states "Where ceiling joists or rafter ties are not provided at the top plate, the ridge formed by these rafters shall also be supported by a girder designed in accordance with accepted engineering practice." The first sentence of the paragraph limits this restriction to conditions where ceiling joists are not parallel to rafters.
The way I read this, any condition with ceiling joists located higher than the top plate (say a cathedral ceiling) will require a structural ridge beam, even if the ceiling joists/rafters meet the span requirements of IRC span tables R802.5.1 (1) through (8).
Have you all read this the same way?






RE: International Residential Code - Collar Tie Conflict?
However, you need to make sure you can develop the reaction of the ceiling framing into and through the roof rafter to the sill plate. You also need to check how much spread you will have at your sill elevation, which may cause cracking in your finishes and the roof to sag. This anaylsis is more complicated than pulling member sizes out of typical charts. If you are not comfortable with the anaylsis procedure, you should ask a structural engineer.
RE: International Residential Code - Collar Tie Conflict?
RE: International Residential Code - Collar Tie Conflict?
I don't see a conflict but my IRC is at the office. If I see one, I'll post it.
Below is a link to an interpritation by the state of Connecticut Building Inspector.
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RE: International Residential Code - Collar Tie Conflict?
Structurally a rafter ceiling joist system is an three hinged A-frame with a horizontal tie. When you raise the ceiling joist you start to develop a horizontal thrust at the top of the stud wall. In a lot of cases after you have raised the joist 2'-3' the thrust is high enough that the stud walls are unable to resist the horizontal thrust.
RE: International Residential Code - Collar Tie Conflict?
A structural ridge IS NOT required if the ceiling joists are parallel to the rafters, connected per table T802.5.1(9), and the adjustment factor from footnote "A" of Table R802.5.1(1) & (9) is used (Hc/Hr < 1/3) This was 2/3 in the 2003 IRC, but is now 1/3 in the 2006 IRC.
A structural ridge IS required if the ceiling joists are located more than 1/3 the height of the rafter height.
Also, a structural ridge is required if no ceiling joists or rafter ties are used.
I hope this helps. RARSWC is correct that this is an A frame system with a horizontal tie. I have seen in the field where the walls were pushed out because the builder did not design the ridge beam as structural for a cathedral ceiling. You definatly want to account for the horizontal thrust produced when the system deflects!
RE: International Residential Code - Collar Tie Conflict?
I am currently evaluating the failure of one with no collar ties, and the ceiling joists do not tie to the rafters, just the walls. Lateral thrust appears to have overloaded the rafter-to-top plate connection. The ridge beam was a 1x8.
RE: International Residential Code - Collar Tie Conflict?
I am not sure how to interpret your posting. Are you questioning wheter or not there is a lateral thrust if you have a large enough ridge beam?
I am confused because you refer to an 1 x 8 as a ridge beam. Athough as in your case, I have seen contractors and engineers try to use 1 x 8's as ridge beams. The 1 x 8 is intended to be a nailer and to provide a flat surface for the rafters to bear against.
RE: International Residential Code - Collar Tie Conflict?
Ron - with a properly sized ridge beam and no ceiling joists or collar ties, there should be no thrust. It sounds like you're saying the ceiling joists are not directly connected to the rafters, but to the top plate instead. I see two possible problems here - 1. if the rafters are not connected to the plate, they will try to slide outward. 2. If the rafters are connected to the plate, then the horizontal thrust is resisted by the top plates in bending between ceiling joists. Depending on the spacing of the ceiling joists and the wall thickness, the top plate could easily be overstressed.
RE: International Residential Code - Collar Tie Conflict?
In the second case (evaluation of a failure), the rafters were only attached to the top plate and there was significant lateral thrust that could not be resisted by the connection of the rafters to the top plate and by the lateral restraint offered by the joist attachment to the wall studs. In some places the rafter connection failed, while in others the wall pushed out. The ridge beam was a 1x8, but because of its size and the loading placed on it, had significant vertical deflection, thus not absorbing the load as would be desired to mitigate the thrust.
RE: International Residential Code - Collar Tie Conflict?
1st subject: Yes, the ridge must have posts or solid bearing that transfers 1/4 the area of the roof the ridge beam is supporting at each end. The rafters must be attached to the ridge beam to account for the vertical (I prefer sloping hangers) and horizontal force (usually a strap tie above the ridge beam). There are many ways to make this connection.
2nd subject: I do not consider a 1x8 a structural ridge. I could almost bet the rafters were toe-nailed to the 1x8 and nothing else. Essentially, the rafters were "leaning" against each other and creating a very large horizontal thrust on the wall. They are lucky it did not collapse and hurt someone.
I helped review a gym in the 90's made of timber trusses from the 1920's that had a vertical deflection of 6"-10" at the ridge among many other issues. Needless to say, the recommendation was replacement (which the school did). Moral of the story: Even trusses create this thrust and one side should be connected with a metal connector (Like Simposon Strong-Tie's TC24) that allows horizontal movement on one of the walls.
RE: International Residential Code - Collar Tie Conflict?
This was a house built in the 1930's.
RE: International Residential Code - Collar Tie Conflict?
RE: International Residential Code - Collar Tie Conflict?