Notice to all readers: I am not a structural engineer. The statements made below is based on extensive research online and not made by an engineer professional. I am an architect with some basic understanding of structural engineering. Do not make design decisions based on this post without first doing your own research and speaking with an engineer.
With that being said, let's talk about collar ties.
The above picture shows one of the "collar ties" that was original to the construction of our home. These were installed every 4 or so rafters. My original intention was to keep these and wrap them in trim but i decided to remove them. Here is why.
First off, let me clarify what collar ties are exactly. Sometimes engineers design collar ties to prevent separation of the roof at the ridge due to wind uplift. Collar ties should only be required when there are no ceiling joists or structural ridge beam to serve the function. When collar ties are properly designed to resist wind uplift, the engineer usually will specify through bolts and spike grids anchored to the roof rafters. The hardware is required in order to properly connect the collar tie to the rafter; a few nails, as was the case in our house, won't do the job. Plus the "collar ties" were 2x4's. In order for it to be substantial to function as a true collar tie it would need to be a much larger member.
Most of the codebooks and construction textbooks today dont require collar ties or show them in details anymore. However, building codes do require that the rafters themselves 1.) need to be properly sized to accommodate roof load requirements and 2.) that the ceiling joists be fastened at lap joints to resist outward thrust from the weight of the roof.
If the design of roof framing does not allow for the roof rafters to be connected to ceiling joists, then collar ties and/or rafter ties would be required to resist the forces caused by gravity loads that otherwise cause the roof to pancake and push the exterior walls outwards. Rafter ties are required by code, unless the engineer designed the house so that there is a ridge beam that carries the load of the roof.
The sketch above shows the condition of our house. Typically, older homes had 2x8 or 2x6 roof rafters spaced 12"-24" o.c. Most newer homes nowadays have pre-manufactured roof trusses that don't allow for a vaulted ceiling.
The ceiling joists that run between the exterior walls alongside each rafter are being utilized to resist tension from the vertical forces, not collar ties. They are much better at resisting thrust provided they are fastened to the rafters as well as to the top plates of the exterior walls.
There should not be collar ties between rafters above the ceiling joists.... period!
The blue circles show the important connections between rafters and ceiling joists. The existing rafters were only being anchored to each other with long nails. I decided to buy 3" x 9" simpson gusset plates to nail to each rafter ridge connection. I used about 12 simpson nails at each pair of rafters. This was a small investment I decided to make for added strength / security. Ultimately, I don't know if this was actually required but it gave me piece of mind. You can also make the gusset plates out of plywood.
In addition, I fastened the rafters to top plates with additional nails. Any ceiling joists that were sistered together received additional nails and blocking. Again... piece of mind.
There's also the question about the actual weight of the roof from snow/wind/etc. causing the roof to sag. I've given this a lot of thought, but then I realized that our roof has been this way for 120 years. In addition, we were adding intermediate walls in the design that would be fastened to the rafters.
The next step was to insulate the roof... Stay tuned!
Brandon E. Young