Framing question

[fcpnorman]

I am wondering if  collar ties( I think that is what they are) can be moved higher. The cabin is a single story with stick built walls with a A framed roof. The cabin is 24 x40. The roof is framed with 4x beams that are joined at the peak with gussets. Where the roof framing members meet the exterior walls, sit the collar ties. These carry across the entire building and are the structural framing for the loft. This creates a very low ceiling. The collar ties are two 2x8's sandwhiched on either side of beams. I would like to move the ties that are over the open portion of the cabin higher, but I am concerned that they were engineered to be that specific height to deal with snow and wind loading??? I am still trying to figure out how to post pics- I have them of the beams and loft area.
Thanks for any insight.

[MountainDon]

terminology in an image

[MountainDon]

What you are calling collar ties are the rafter ties. Both are important parts of the structure and as illustrated there is a certain leeway as to their position. Not shown though, is the size increase that must be made to the rafter as the rafter tie is raised. That is a part of the IRC.  The upper ties, the collar ties, help hold the roof together so high winds are less likely to peel off the roof. The rafter ties keep the wall tops from pushing outwards.


Posting images help...


Are the rafter ties one piece or are they spliced over any interior load bearing walls?  Rafter and rafter tie spacing?

[fcpnorman]

Here is a photo from the loft down to the open area of the cabin. And one photo of the loft and roof framing.

[fcpnorman]

There are no splices that I could see. The rafter ties are at the wall height- 8 feet, and the gussets are at the peak of the intersection of the beams.

[PEG688]

IMO there is no practical way to move those ceiling joist up. You'd screw up the triangulations and weaken the wall tying together strength that they provide. 

They are the floor joist to the loft area as well right?   

I'd say you're at edge of the building envelope as is with only gussets up at the peak , there more than likely should have been a collar tie say 3 or 4 feet down from the ridge.
   

 

[Don_P]

I'd say you're at edge of the building envelope as is with only gussets up at the peak , there more than likely should have been a collar tie say 3 or 4 feet down from the ridge.

The gusset is doing the same job as a top third collar tie, firmly holding the pair of rafters together. Just as in a truss. Remember a typical ridgeboard carries no load it is simply an assembly aid. A strap or tie between the tops of the rafters keeps them from opening up if wind ever inflates the building... an end wall failing in a big wind.

The pairs of ties down low keep the rafter feet from spreading the walls. If the rafters are on 4' ctrs, then look at the 2' spacing requirements in the rafter table that is appropriate for your snow load. Your rafter should be at least as deep as called for and at least twice as thick... which they appear to be. If that checks out you may raise the ties as long as they stay in the lower third of roof height, the lower the better. The connections will need to be at least double what the heeljoint table at the end of the span tables specifies.

At that point, if it all checks, I'd call my engineer.

[PEG688]

At that point, if it all checks, I'd call my engineer.

Then get out your check book, engineers are rarely cheap or practical. 

[rick91351]

Then get out your check book, engineers are rarely cheap or practical. 

Peg I found one that works fairly cheap on personal residential (owner builder stuff).  But never practical............  I think its a turf thing. 

[Don_P]

There was a disagreement of opinion, the solution is to check with the recognized authority. This is a few hundred dollar job engineeringwise. We didn't get into this by asking the engineer to do something that is practical .

This calc can show you what the design thrust to be resisted is,
http://theownerbuiltcabin.com/calculators/TT/raisedtiethrust.htm
With that, a connection capable of resisting the design force is found. The awc connections calc is a good resource there.

[PEG688]

Maybe it's due to the area I live and work in ,  high seismic zone , high wind zone , nothing we've had a engineer look at cost a "couple of hundred bucks" to design,  let alone build to the requirement's they impose.  They along with many "standard" building codes are at least part of the reason buildings cost so much in the PNW.

Have at it , it ain't my money. Good luck.
   

[Don_P]

I've seen so much impractical it would be hard to hang it all on just one group. But I do know some more deserving. Knowing which battles are worth fighting is a big factor, I try not to create a tough job for the engineer. PEG feels it is impractical, it may be. To me this is a minor skirmish.  This is so close to textbook I would walk the inspector through the logic of what I explained above. I've had young inspectors jaws drop when they come in to a roof like this. Walk through it... It has rafters of the same depth the code would have specified for 2x at half the spacing. These are (more than) twice as thick as normal and are spaced twice as far apart as normal... that part structurally is the same as a conventionally framed roof in 2x rafters. It is double tied, plenty of opportunity to resist the thrust. As long as the sheathing is rated for the span of the rafter spacing the tieing rules are really the same.

But, the bottom line is when you don't know the answer and you don't know who to listen to, you won't be wrong to consult an engineer. This was the reason for that line in my first post.

The horse of another color is a structural ridge, the ties then become just for show.

[PEG688]

These are (more than) twice as thick as normal and are spaced twice as far apart as normal... that part structurally is the same as a conventionally framed roof in 2x rafters. It is double tied, plenty of opportunity to resist the thrust. As long as the sheathing is rated for the span of the rafter spacing the tieing rules are really the same.

Maybe I'm missing something , but I THINK he wants to do away with the current,  what I'd call ceiling joist , or loft floor joists.   Or what in Mtn . D's posts are called joists or rafter ties.

What I think he want to do is move all those up to create more head room / higher ceiling height in the lower level.

  Maybe he only wants to raise the one up in the open area he shows, which seems odd as they are spaced out and it's open above so I wouldn't think that area would seem tight / low ceiling height.

The area under the loft would be the space that would seem to have a low ceiling.

So maybe I have the whole situation wrong   

What do you think he wants to move up Don P. ??


ETA:  I re-read the first post , so it's only the 6 in the open area.  So I'll change my opinion to a,  sure maybe it's do able , I guess I had it in my mind that the open area wasn't a low ceiling area seeing it is mostly open.  I thought it was the whole place he wanted to move the ceiling up .

So it's about 16 feet from the exterior wall to the loft?  If so sure move them up , maybe thru bolt the ties  with 5/8" bolts thru both sets of ties and the beam ,  say 3 or four bolts at each rafter , staggered , one up , one down spaced 4 to 6 inches as you move inboard ,  instead of just nailing or lag screws .   It looks like there's a painted  black 2 x on the exterior wall already, or is that some thing else painted black?  If it's a 2 by I'd say leave it in place and lag it to the rafters.   

[alex trent]

The original question was "can i raise the rafter ties in the open area". Note he calls them collar ties in the first post, but it has been ascertained they are rafter ties, by nomenclature and function.

The simple answer is yes, as long as they stay in the lower third of the rafter length.

They also need to be of sufficient size, which Dob P thinks they are...and there are charts to check that.

The last thing is to ensure the rafter ties are secured properly to the rafters  My suggestion is to use the right nails and plenty of them rather than bolts.  Looks better (avoids the "this should have been a battleship look) and hold just as well or better.

In any case, not a complex problem.

I have 8 foot rafter ties in my cabin..6 feet apart......I thought about raising them, but I actually like the low look.

[fcpnorman]

Thanks for the input. We have been considering building a cabin, while also looking at several here in California that are for sale. We have looked at this one, but one of our concerns was the height of the rafter tie/ceiling joist. I would assume that the loft floor members would be considered joists, and the ones that are not supporting a floor over the open area are to be considered rafter ties.
Walking up the staircase you must make an awkward transition through the rafter ties, and whdn you are sitting in the open area you feel an odd feeling with the low elevation of the rafter ties. I would like to lift them, even a foot or two and then wrap them and create a faux beam.
Thanks again for all your input, this site is awesome.

[Don_P]

here in California

There is the horse of another color! I suspect PEG is right on the cost and engineering may well be a requirement there. Before making an offer I would take the pictures and have a quick consult with the building dept or an engineer.

[PEG688]

There is the horse of another color! I suspect PEG is right on the cost and engineering may well be a requirement there. Before making an offer I would take the pictures and have a quick consult with the building dept or an engineer.

  Why thank you

   Maybe just go for it , who's to know?? I would highly recommended the thru bolts over nails , they could be let in a 2" wide 3/4" deep hole , or IF he's going to wrap the joists , the wrap could cover the bolt heads and nuts as well.