Author Topic: Collar ties  (Read 6192 times)

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Offline rakuz66

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Collar ties
« on: February 12, 2008, 12:22:38 PM »
Someone please tell me if I understand this correctly. I've been doing a lot of research on collar ties and this is my understanding.  Collar ties (by themselves) place in the upper 1/3 of a roof will do little to prevent sag or wall thrust.  However, collar ties used in the upper 1/3 in conjunction with ceiling joist at the wall plate level, collar ties will then help reduce rafter span/sag.

Offline FrankInWI

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Re: Collar ties
« Reply #1 on: February 12, 2008, 05:57:55 PM »
rakuz66
this is an important topic to me.... I don't understand "ceiling joist at top wall plate level".  what do you mean by that. I have to add in some colar ties, and want to keep them high.... so am interested in this other factor you mention. 
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Offline PEG688

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Re: Collar ties
« Reply #2 on: February 12, 2008, 06:25:38 PM »


 This question can not be answered correctly without knowing many factors not given in the org. question.

 Width of building , # of storeys , rafter size , ridge size , what ceiling height is desired , etc.

 I believe the "joist at top plate level " has to do with the normal placement of ceiling joist is on top of the top plates of the top walls. Althought that isn't where ALL ceiling joist ALWAYS go . But that would be my guess as to what  RAk was saying , I could be wrong , I sometimes am , not often though  ;)
When in doubt , build it stout with something you know about .

Offline rakuz66

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Re: Collar ties
« Reply #3 on: February 13, 2008, 05:55:20 AM »
Frank, it seems the words rafter ties and ceiling joists are sometimes used interchangeably. ***This is my understanding*** I call them ceiling joists when they sit on top of the walls plate and run to opposite wall to keep walls from spreading, ie: thrust.  However, some people call them rafter ties.  Collar ties vs. rafter tie and/or ceiiling joist are two opposite animals.  Collar ties are usually placed in the upper 1/3 of a roof to stop rafters from spreading at peak due to wind, however some people state that they can also stop rafter sag, etc.  Rafter ties seem to be placed in the lower 2/3 of a roof to keep walls from spreading and prevent rafter sag.  Ceiling joists sit on top of wall plate and run as described above.  Strongest by far, but limits ceiling height.  Again, I am only learning about this stuff just recently as I have been rehabbing an old camp that was way out of code. 

Offline rakuz66

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Re: Collar ties
« Reply #4 on: February 13, 2008, 06:06:30 AM »
Peg, I posted before about my 9ft beam??  Remember me? "overkill boy"  Anyway, onto my next issue reagrding this old camp.  I believe that you have seen the pics in the old posts.  Well, the existing 2x8 rafters fall short by 1' 6" according to code.  Here's the scenario, 1 story camp, ceiling will hopefully be attached to rafters.  Camp width is 22 ft, so rafter span would be 11 ft correct??  60 PSF live load.  New York State code says 2x8's can only span 9'6" w/ ceiling attached.  So, I placed 2x8 collar ties at the ridge on every rafter pair, they span 5ft across from rafter to opposite rafter.  Also, there were old 2x4 rafters that the new 2x8 rafters were sistered to.  Plus, I installed those 2x10 ceiling joists

John_C

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Re: Collar ties
« Reply #5 on: February 13, 2008, 07:01:26 AM »
I hope PEG rides in to the rescue here 'cause I'm confused.

I've never seen a roof spread at the peak except in a total failure, think hurricane or tornado.

Rafter ties, collar ties , whatever stiffen the roof structure a bit and help resist the outward forces that the roof exerts on the walls.  Ceiling joists that attach to the top plates & rafters & span the building do the best job of tying  the walls together.  Neither would substantially change the ability of the rafters to bear a 60 psf load.

If the existing rafters are too small and you need to satisfy an inspector I can only see two solutions. One would be to "sister" the existing rafters.  A doubled 2x8 might satisfy them or a 2x10. They might offer a suggestion.  The other would be to use the existing rafter as the top chord of some built in place truss. Almost certainly would need a structural engineer to sign off on that one.  If the powers that be want this to meet current code you will need to work with them.

If there is no code and it has held up for some years, but you just want to reinforce it some I'd be inclined to do some kind of scissors rafter ties. Top plate to 2/3 of the way up the opposing rafter, joined in the middle. That would triangulate the structure very nicely.  An inspector probably wouldn't care for it but it ought to be strong enough.

As always YMMV

Offline PEG688

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Re: Collar ties
« Reply #6 on: February 13, 2008, 07:54:13 AM »


 Rak it is sort of hard to keep every ones "job" sorted out. With all the weird screen names we use it sometimes gets blurry as to who's who and whats what.

  If you have a building dept. involved you need to ask them what they want you to do , or if they being public servants WHO you are in a way paying , think taxes , and they will not help you will have to hire a engineer ,  or maybe a architect / designer in your area, to design what your looking for / need to do.

 Generally if it's existing and your not doing "to much" ( ya whats to much , ever building dept / inspector will have a different opinion on that) existing slight over spans in older buildings are allowed , not always , but the local inspector going to be the only one who can say , "Ya it's OK."  Or  his boss the local building offical.

  For what it's worth a 1' 6" over span on a other wise well built building as in 16 " OC framing , 1/2" or better sheet good  or 3/4 " real boards , etc . is no big deal . Span charts have been changed over the years as new wood isn't as good . Could be those rafters where code when the place was built . Maybe thats a angle to look into ?

 Good luck.

 BTW collar ties connect to rafters , joist sit on wall plates and /or ledgers . The two are similar but a collar tie is IMO aways up higher and helps some with wall spread  but mostly with stopping ridge sag. YMMV , there are always exceptions , never say never . etc etc .     
When in doubt , build it stout with something you know about .

Offline rakuz66

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Re: Collar ties
« Reply #7 on: February 13, 2008, 09:12:38 AM »
Thanks guys, I'm sure it's fine, it lasted for years before I even put ceiling joists in to stop wall spread or put in collar ties.  I'm just being my usual "overkill" self.  Sorry, always looking for opinions on how to build better.

Offline MountainDon

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Re: Collar ties
« Reply #8 on: February 13, 2008, 09:29:27 PM »
Collar ties and rafter ties are confused a lot. Basically a collar tie holds the peak of the roof together. A rafter tie keeps the bottom ends of the rafter pairs from spreading; since they are nailed to the wall tops that also prevents the wall tops from spreading. When placed on the wall top plates the rafter tie can be the ceiling joist, and vice versa.

A collar tie only needs to be a 1x4 and at every other rafter. A rafter tie has to be a 2x4 at a minimum and with newer codes at every rafter. When used as a ceiling tie it must be sized to support the ceiling span load.

The diagram below illustrates placement limits in the rafter triangle.

Rafter ties can only be left out when some other method is used to build the roof. A ridge BEAM, supported at each end clear down to the foundation is one method. Ridge beams eliminate the horizontal force at the wall but need to carry one half the weight of the roof load. An engineer is usually required.

click on image for larger size
« Last Edit: March 01, 2011, 08:54:42 AM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

 

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