2x8 roof rafters, adding collar ties 2

[DCStructures]

Any tips on strengthening 2x8 roof rafters? Collar ties currently exist on every 4th rafter. I'm thinking that adding collar ties at every rafter could easily increase capacity. Any tips? Suggestions? Warnings? Cautions? Thanks in advance.

 

[Ron]

jdgengineer....I understand the difference between a collar tie and a rafter tie. In the absence of a rafter tie, such as for vaulted ceilings, the collar tie must suffice and has to serve both purposes. If I can dig it up in my files, I have a photo of a roof framing failure when a roofing supplier overloaded the ridge by stacking shingle bundles too close together. The collar ties failed at their connection to the rafters and the side walls pushed out.

 

[XR250]

Collar ties will help to brace the rafters but I want to note that the addition of collar ties will effectively increase the thrust at the bottom of the rafters and the connection from the rafters to the rafter ties is sometimes overlooked.

Yup, that is why it is best to install them tight to the underside of the ridge.

 

[HouseBoy]

XR makes a good point however...that doesn't solve the problem of bending in the rafters (assuming that is the problem DC is trying to address).

DC - so, what is the problem and what solution do you like for it?

 

[XR250]

XR makes a good point however...that doesn't solve the problem of bending in the rafters (assuming that is the problem DC is trying to address).

Ahh, yes. I got lost in the thread. As long as the rafter-ceiling joist connection is checked (like you stated - and any other joints in the ceiling joist), then adding collars is one way to fix that.
You also have to check the collar for buckling (if this has not been mentioned already). I usually make the collar a built-up "Tee" section if that is an issue or just sister it with another member.

 

[jdgengineer]

Ron I have to say I disagree. I value your input and knowledge, but if the rafter ties are not in place, it's my opinion you should not try to rationalize the collar ties to serve as rafter ties. The tension tie needs to be in the bottom 1/3 to be effective. If it is at the top of the rafters it is extremely ineffective and the lever arm on the rafter is very large.

I would argue that in a vaulted condition without a structural ridge if the collar ties are the only element to stabilize the rafters and prevent the walls from bowing out that it is a framing deficiency. The rafters should be supported near the peak with a bearing wall, vertical struts, or a ridge beam. Perhaps, you could rationalize them if you had continuous rafter spanning over a purlin, but a lot of that roof support would come from the rafter cantilever action not the collar tie.

 

[Ron]

jdgengineer....I don't disagree with you; however, it was done often. Not much anymore because we don't see much conventional framing, but some of the older houses have some strange framing as I'm sure you know1