2x8 roof rafters, adding collar ties
2x8 roof rafters, adding collar ties
(OP)
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.






RE: 2x8 roof rafters, adding collar ties
1) those with collar ties?
2) those without collar ties?
3) both?
Tips for collar tying:
1) the rafter tails below the tie work very hard in bending. Position your collar ties low enough to deal with this.
2) the collar tie to rafter connection places a lot of tension perpendicular to grain stress on the rafter. Watch fastener end distances and try to engage most of the rafter cross section with the fasteners.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: 2x8 roof rafters, adding collar ties
I'm assuming you are confusing collar ties with rafter tie. Collar ties occur at the top 1/3 of the rafter and are in place to resist wind uplift to keep rafter from seperarting from the ridge.
See attached for good explanation from fine homebuilding.
Rafter ties allow you to form a field built truss and remove the necessity for a structural support at the ridge. If your rafters are over spanned, adding additional rafter ties won't help you increase the capacity of the rafters but it will help reduce the demand on the rafter tie connection.
If you are referring to adding additional collar ties at the top, fhey won't really help you too much. They will help with some ability to resist unbalanced snow loads and wind uplift. I think the requirement based on conventional construction is to have collar ties @ 48" oc max so adding in additional infill ties will meet that requirement. Aside from large wind events, I don't think you will see noticeable roof framing improvement as a result.
RE: 2x8 roof rafters, adding collar ties
RE: 2x8 roof rafters, adding collar ties
Agree with KootK's comments and warnings!
RE: 2x8 roof rafters, adding collar ties
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: 2x8 roof rafters, adding collar ties
It's a common confusion, but the two elements are different and serve different purposes. The article I posted I believe explains it we'll.
RE: 2x8 roof rafters, adding collar ties
RE: 2x8 roof rafters, adding collar ties
RE: 2x8 roof rafters, adding collar ties
Dik
RE: 2x8 roof rafters, adding collar ties
Dik that makes some sense to me. If rafter ties are properly in place the collar ties I believe are actually in compression for standard gravity loading.
RE: 2x8 roof rafters, adding collar ties
Always kinda had you figured for a leggy brunette.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: 2x8 roof rafters, adding collar ties
Interesting. Most roofs here are conventionally framed unless vaulted or have big dormers or other crazy stuff.
Hey, I resemble that remark!
RE: 2x8 roof rafters, adding collar ties
Most homes built earlier than 1980 in our area are conventionally framed. To this day I have never seen one of these roofs without a "flaw". Typically, at least one (usually more) is present:
1) ceiling joists perpendocular to rafters in some area without additional rafter ties
2) rafter tie to ceiling joist occurs only every 3rd rafter and connection only has a few nails
3) one side of the ridge is partially vaulted but no consideration to the fact that the rafter tie no longer runs plate to plate
4) purlins are overspanned and only have braces at perpendicular partition walls rather than closer spacing required.
5) ceiling joists are not properly connected at splice location
6) no structural support at hip / ridge intersection.
7) collar ties are spaced too far apart.
8) at prulin brace locations no consideration given to impact on floor loading below for concentrated load.
Because these are so common I'm nevervous about contractors properly installing conventional framing. Less opportunities for error with structural ridge. As we don't have snow loads these roof defects tend to perform "OK". Areas with higher snow loads I assume can't get away with these errors and may have them less frequently (I hope at least)
RE: 2x8 roof rafters, adding collar ties
RE: 2x8 roof rafters, adding collar ties
RE: 2x8 roof rafters, adding collar ties
I hear ya. I see that all the time on older houses - less so on newer ones. In my experience, I rarely see a problem except where the roof pitch is really shallow or the entire roof is framed with the ceiling joists perp. or the 1/2 vaulted situation or long shed dormer.
Folder plate, diaphragm, sheetrock, helium - who knows. Same as above, they seem to work in our area unless the roof is a shallow pitch.
FWIW, I always support the hip-ridge-valley joint unless the roof is small. Homeowners love it when they want to finish off their attic and there are posts everywhere.
RE: 2x8 roof rafters, adding collar ties
All I can tell you is that the builder is a client of mine.
RE: 2x8 roof rafters, adding collar ties
I'd also be a little nervous of the slip between the strap and the rafter. Although maybe not an issue if the deflection is large enough in the rafter.
RE: 2x8 roof rafters, adding collar ties
Now, for the OP - can you tell us what is the nature of the problem?
I'm guessing that since you want to "strengthen" the 2x8 rafter, it might be because the rafter is exhibiting bowing.
Collar ties (in the upper third) go into compression under gravity loading (as dik and jdg mention).
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.
RE: 2x8 roof rafters, adding collar ties
RE: 2x8 roof rafters, adding collar ties
Yup, that is why it is best to install them tight to the underside of the ridge.
RE: 2x8 roof rafters, adding collar ties
DC - so, what is the problem and what solution do you like for it?
RE: 2x8 roof rafters, adding collar ties
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.
RE: 2x8 roof rafters, adding collar ties
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.
RE: 2x8 roof rafters, adding collar ties