Framing question

[topcoat]

In this shop I am building, I have a question on the framing of the roof. We have a 30x32 footprint, 12 ft walls, 12/12 roof pitch. It is mostly open span on the inside, except for a loft which projects out 10 ft from one of the gable ends, and is carried by a 30 ft lvl. The rest of the ceiling is open, cathedral. I need to put in collar ties. What is the fewest number of collar ties I can install for structural integrity? How far apart can they be spaced? How far up the rafter from the top of wall can they be located? Also, the gable end opposite the one the loft is installed in, has an overhead door and we are doing shear walls for stability. Any advice would be appreciated.

[Eieio]

It is difficult to answer that with out knowing your local building codes, rafter spans or type of lumber being used..

The spacing and placement of the collar ties should be indicated on the plans?


Usually a local structural engineer would run his calc's for you and answer these questions indicating the size of the rafters, type of lumber, location of the collar ties and their spacing


It will be impossible for anyone to give you a definitive answer without some more pertinent information and even then it's a guess as best..

[Blue]

Those numbers have to be run. Any truss company can input those numbers and it will come out based on minimum load req. for your area here. Some lumber yards will have the program to run the engineering as well.

[topcoat]

Ok, thanks guys.

[Silvertree]

Are your rafters sized and engineered to require collar ties?Collar ties could be structural to preventing outward thrust, but most framing techniques don't demand collar ties to prevent that. If you are building a loft in the place that would take care of whether you need collar ties in that area. No in the loft area.

Collar ties are sometimes only ornamental and in that case put them anywhere you want.

The real question is how are your rafters engineered with the ceiling joists. Your only concern should be the building spreading and that won't happen because of the joists.

Is this a trick question?

[topcoat]

The original plans called for a structural ridge, which would have required no collar ties. I changed to non structural because there was a significant cost savings in not having to construct the gable walls first (with no deck) and not having to bring in a crane to hoist it. Thats the background behind the question.

The span of the rafters is around 22'10". The rafters are 2x12 16oc, and the roof will be metal. At 12/12, it wont hold much load.

[topcoat]

Are your rafters sized and engineered to require collar ties?Collar ties could be structural to preventing outward thrust, but most framing techniques don't demand collar ties to prevent that. If you are building a loft in the place that would take care of whether you need collar ties in that area. No in the loft area.

Collar ties are sometimes only ornamental and in that case put them anywhere you want.

The real question is how are your rafters engineered with the ceiling joists. Your only concern should be the building spreading and that won't happen because of the joists.

Is this a trick question?

Paul

There are no ceiling joists. Its an open ceiling. The loft is located at the top of one gable end wall, and carried by the lvl. So, definitely no need for any ties in that area. I am trying to keep the ceiling as visually open as possible, and the collar ties are definitely for structural purpose, as you noted to prevent outward thrust. I listed the rafter specs in my post above, and we have hurricane tied at the ridge and top of wall.

[JasonW]

I have an addition on my house that I did 3 years ago. I ran a double 16 foot, 12" lvl from the house to the outside Gable wall of the addition. Both ends of the lvl were supported by solid blocking from the foundation to the ridge on the second floor loft.

The second floor loft had 5' walls (2 x 6) and the lower floor had 8' ceilings. The walls were balloon framed and a ledger board was "let in" to the wall framing to support the second floor. This also acts as a "collar" for the wall system.

With that being said, I had a roof pitch that could not exceed a 8/12 pitch and I was quite comfortable going with a 2' center with 2 x 12's as rafters.

[naptownCr]

Cabo in our area with a 20# snow load used to call for collar ties placed in the bottom third of the rafter and no more than 48" on center.
As far as snow loads and sizes you will have to check with an engineer that is familiar with the specific loads for your area.

[topcoat]

Will doing shear wall on the opposite gable from the loft allow me to start the collar ties further than 48" out from the gable? 6 ft off the loft gable and opposing gable would create nice symmetry with collar ties at 48"oc.

[Eieio]

I thought collar ties were placed top 1/3 Nap?

[Silvertree]

Well with a nonstructural ridge you probably will need the collar ties, although with 12" rafters you may be able to put them up higher and get that vault effect better.

I'm no engineer, 12/12 is a no snow on the roof pitch when using metal roofing.

I always did my calcs and then got an engineer buddy to check them for me.

If you have solid blocking under the ridge with the 12" rafters I'm guessing you don't need them.

[ChrWright]

I was wondering if this was a trick question as well. :grin:

In any case, the term 'collar tie' is typically misunderstood. A properly engineered collar tie is designed to resist uplift--NOT prevent sag and wall spread. 'Rafter ties' serve that purpose.

http://www.nachi.org/collar-rafter-ties.htm

When you remove the ceiling joists to vault a ceiling, the rafters themselves tie the walls together--provided they are supported by a structural ridge beam, and are tied off properly to the top plates.

In absence of a structural beam, the rough rule of thumb is to install a collar beam every 4' (doubled on either side of the opposing rafters). All of this, as Rory said, depends a lot on what your local codes are--and should be determined by a local engineer.

http://www.finehomebuilding.com/CMS/uploadedImages/Images/Homebuilding/QA/hb077QA01-04_lg.jpg

http://www.finehomebuilding.com/how-to/qa/framing-cathedral-ceiling.aspx

[Eieio]

Collar ties go at the upper 1/3

Rafter ties are bottom 1/3

[naptownCr]

I thought collar ties were placed top 1/3 Nap?

Chris had this correct and the short ones near the ridge are to prevent uplift.

Collar ties go at the top 1/3

Rafter ties are bottom 1/3

Above post being said and adjusting for regional differences in terminology and using the midwestern terminology as the standard.
collar ties would go in the top third.
Rafter ties go in the botom third.

[Silvertree]

Non structural ridge he said. If that's the case its collar ties, but walls can be built to compensate for lateral loads.

Screw this man, now I'm getting confused, I'm just a singer in a rock and roll band.:mad2:

[topcoat]

Thank you all for you help.

[neolitic]

How far along are you?
If you can still resize the
OHD header (maybe trussed?),
and get a post in for the "loft" gable,
going back to the ridge beam
still sounds like your answer.
IMHO

[JasonW]

How far along are you?
If you can still resize the
OHD header (maybe trussed?),
and get a post in for the "loft" gable,
going back to the ridge beam
still sounds like your answer.
IMHO


........................

[framer55]

Will doing shear wall on the opposite gable from the loft allow me to start the collar ties further than 48" out from the gable? 6 ft off the loft gable and opposing gable would create nice symmetry with collar ties at 48"oc.

Proba:)bly.

Just saw Jason's picture. I would be putting in rafter ties if I were you. You run the risk of the building spreading rather quickly. (bottom 1/3 of the rafter 4' on center.)

You must live where codes are hit or miss!