Code evaluation of rafter, joist, and collar tie geometry
Code evaluation of rafter, joist, and collar tie geometry
(OP)
Hi all,
I have been asked to do a code evaluation for a conventionally framed residential roof (4:12) in Alberta Canada that has failed under snow load. The snow load was most likely at or just past the design load.
I am using the 1965 NBCC in the review. The code gives maximum rafter spans, say for example 11'10" for a 2x6. The rafters used were 2x6 spanning 15' to an unsupported ridge plate. The code also requires all structural members to safely support the design loads and prescribes the number/length of fasteners for each joint (rafter, ridge, ceiling joist, collar tie).
I have some general concerns with doing a code evaluation for a structure this old...but my specific question is whether it is appropriate to identify the rafter span as a code deficiency when the accepted practice of the day may have been to use collar ties to increase the effective span of the rafters. The collar ties where 24" below the ridge so that the 11' span could have been technically met if the collar ties were viewed as "supports".
It is clear from a modern analysis and literature review that the rafter set does not work (connection forces, unbalanced load at the ridge intersection, no fasteners at the rafter-to-1x6 ridge plate, etc) and this is also why the roof is failing and why the construction practice was abandoned. But is it inappropriate to use modern methods and knowledge to show that the structure did not meet code in this particular case?
Thanks to all in advance.
I have been asked to do a code evaluation for a conventionally framed residential roof (4:12) in Alberta Canada that has failed under snow load. The snow load was most likely at or just past the design load.
I am using the 1965 NBCC in the review. The code gives maximum rafter spans, say for example 11'10" for a 2x6. The rafters used were 2x6 spanning 15' to an unsupported ridge plate. The code also requires all structural members to safely support the design loads and prescribes the number/length of fasteners for each joint (rafter, ridge, ceiling joist, collar tie).
I have some general concerns with doing a code evaluation for a structure this old...but my specific question is whether it is appropriate to identify the rafter span as a code deficiency when the accepted practice of the day may have been to use collar ties to increase the effective span of the rafters. The collar ties where 24" below the ridge so that the 11' span could have been technically met if the collar ties were viewed as "supports".
It is clear from a modern analysis and literature review that the rafter set does not work (connection forces, unbalanced load at the ridge intersection, no fasteners at the rafter-to-1x6 ridge plate, etc) and this is also why the roof is failing and why the construction practice was abandoned. But is it inappropriate to use modern methods and knowledge to show that the structure did not meet code in this particular case?
Thanks to all in advance.






RE: Code evaluation of rafter, joist, and collar tie geometry
If the thing has been in service for the last 40 years, why has it failed?
Dik
RE: Code evaluation of rafter, joist, and collar tie geometry
The roof has not collapsed but the ridgeline is sagging up to 5 or 6 inches, particularly where the two gable ridges intersect. There is little or no support at this location except for short posts or struts under the 1x6 valley rafters. I don't think these systems works on paper but they seem to survive the years on redundancy, friction, opposing forces, and reserve strength.
RE: Code evaluation of rafter, joist, and collar tie geometry
Dik
RE: Code evaluation of rafter, joist, and collar tie geometry
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Code evaluation of rafter, joist, and collar tie geometry
RE: Code evaluation of rafter, joist, and collar tie geometry
That being said, I am further surprised that 2X4's would work for rafters considering the edge spacing requirements and the tension force generated with raised collar ties rather than resting on the exterior wall plates.
Could be that this was just a failure waiting to happen.
Mike McCann
MMC Engineering
RE: Code evaluation of rafter, joist, and collar tie geometry
The span tables were based on actual clear span, not a collar tie arrangement, but today I discovered a section in the same code that explicitly defined collar ties as a support for the purposes of determining rafter spans. In other words, it looks like the rafter spans in my case technically meet code so my dilemma is solved for the moment.