# Rafter ties



## Par (Jan 25, 2010)

2006 IBC 2308.10.4.1 - when rafter ties are used where ceiling joists are not parallel to rafters, how far up from the wall top plate (or at the exterior supports) your jurisdiction will allow without requiring engineering.


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## rktect 1 (Jan 25, 2010)

Re: Rafter ties

It is a commercial building.

It requires engineering.


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## vegas paul (Jan 25, 2010)

Re: Rafter ties

Commentary says (yeah, I know it's not code...) that anytime rafter ties/ceiling joists are raised above the top plate, engineering is required to determine if it is equivalent to the prescriptive requirements.


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## fatboy (Jan 25, 2010)

Re: Rafter ties

I agree with the gents above.


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## TJacobs (Jan 25, 2010)

Re: Rafter ties

Ditto.

See also 2308.10.4.1:

...Where ceiling joists or rafter ties are not provided at the top of the rafter support walls, the ridge formed by these rafters shall also be supported by a girder conforming to Section 2308.4...

2308.4 Design of elements.

Combining of engineered elements or systems and conventionally specified elements or systems is permitted subject to the following limits:

2308.4.1 Elements exceeding limitations of conventional construction.

When a building of otherwise conventional construction contains structural elements exceeding the limits of Section 2308.2, these elements and the supporting load path shall be designed in accordance with accepted engineering practice and the provisions of this code.

2308.4.2 Structural elements or systems not described herein.

When a building of otherwise conventional construction contains structural elements or systems not described in Section 2308, these elements or systems shall be designed in accordance with accepted engineering practice and the provisions of this code. The extent of such design need only demonstrate compliance of the nonconventional elements with other applicable provisions of this code and shall be compatible with the performance of the conventionally framed system.


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## cboboggs (Jan 25, 2010)

Re: Rafter ties

I concur. Engineering required.


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## Plans Approver (Jan 25, 2010)

Re: Rafter ties

I agree also.  But why is this in Residential Framing?  Is it because it is the same, but different?


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## TJacobs (Jan 25, 2010)

Re: Rafter ties



			
				Plans Approver said:
			
		

> I agree also.  But why is this in Residential Framing?  Is it because it is the same, but different?


They could be building where the IRC is not adopted, or it's an R building under the IBC.


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## Plans Approver (Jan 25, 2010)

Re: Rafter ties



			
				TJacobs said:
			
		

> They could be building where the IRC is not adopted, or it's an R building under the IBC.


OK, we have state-wide adoption, I'm not used to that type of situation. sorry.


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## JBI (Jan 25, 2010)

Re: Rafter ties

Although 'Bay Area' could mean any one of a number of places, I've heard it most in reference to the San Francisco Bay area of California... You know, the State that did not adopt a Res Code... That might explain the Residential framing question using IBC sections.

FWIW, I agree with the others... Engineer required.


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## Par (Jan 25, 2010)

Re: Rafter ties

Thank you for all the inputs. Here (San Jose area) we allow up to 12" above the top plate without engineering (for R3 only). It is not a written policy, just a consent among the plan checkers.


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## brudgers (Jan 25, 2010)

Re: Rafter ties

Table 3.9 in AF&PA WFCM 2001 gives the factors needed for calculating the additional strength required at the rafter to plate connection based on how high up the tie is.


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## jar546 (Jan 26, 2010)

Re: Rafter ties



			
				brudgers said:
			
		

> Table 3.9 in AF&PA WFCM 2001 gives the factors needed for calculating the additional strength required at the rafter to plate connection based on how high up the tie is.


After reading the Note: at the bottom of Table 3.9 and 3.9A, I would only be comfortable with an engineer's stamp too.  Or maybe you can increase the overhang to reduce thrust.  

How about a ridge beam instead?


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## brudgers (Jan 26, 2010)

Re: Rafter ties



			
				jar546 said:
			
		

> brudgers said:
> 
> 
> 
> > Table 3.9 in AF&PA WFCM 2001 gives the factors needed for calculating the additional strength required at the rafter to plate connection based on how high up the tie is.


After reading the Note: at the bottom of Table 3.9 and 3.9A, I would only be comfortable with an engineer's stamp too.  Or maybe you can increase the overhang to reduce thrust.  

How about a ridge beam instead?

L/32 deflection?  I don't think so.

If you have lateral deflection of the rafter after sheathing has been installed, it's not due to the location of the collar tie.

I doubt that the $50/sheet engineer is going to look at the issue either.


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## rktect 1 (Jan 26, 2010)

Re: Rafter ties

Someone has to stamp the plans regardless.  That someone is going to be a licensed DP (architect or engineer).  That is technically engineered at that point.  Whether or not he/she specifically gives you calcs for the rafter above the truss plates isn't really an/the issue.  Which is not to say it should be blown off.  If the plans examiner does catch it specifically, he certainly can/should ask for it.

edit: :lol: That's funny I typed in arkitect (spelled correctly) and the BB rewrote it "man with pencil who draws".


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## brudgers (Jan 26, 2010)

Re: Rafter ties



			
				rktect 1 said:
			
		

> Someone has to stamp the plans regardless.  That someone is going to be a licensed DP (man with pencil who draw or engineer).  That is technically engineered at that point.  Whether or not he/she specifically gives you calcs for the rafter above the truss plates isn't really an/the issue.  Which is not to say it should be blown off.  If the plans examiner does catch it specifically, he certainly can/should ask for it.edit: :lol: That's funny I typed in arkitect (spelled correctly) and the BB rewrote it "man with pencil who draws".


Is the building any safer because an engineer ignored something that is outside of typical engineering practice, than if a non design professional ignored it?

What's the goal, safe buildings or code administration?


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## rktect 1 (Jan 26, 2010)

Re: Rafter ties



			
				brudgers said:
			
		

> rktect 1 said:
> 
> 
> 
> > Someone has to stamp the plans regardless.  That someone is going to be a licensed DP (man with pencil who draw or engineer).  That is technically engineered at that point.  Whether or not he/she specifically gives you calcs for the rafter above the truss plates isn't really an/the issue.  Which is not to say it should be blown off.  If the plans examiner does catch it specifically, he certainly can/should ask for it.edit: :lol: That's funny I typed in arkitect (spelled correctly) and the BB rewrote it "man with pencil who draws".


Is the building any safer because an engineer ignored something that is outside of typical engineering practice, than if a non design professional ignored it?

What's the goal, safe buildings or code administration?

We all have our strengths and weakneses.  I said if the plans examiner catches it, great.  Good for him.  But we all do not know everything nor do we all know the correct path to correct it.  Hopefully between all of us 98% of it gets caught.  That may be a stretch though so whether it was an engineers miss or arkitects miss or plans examiners miss is irrelevant.  The plans still reguire being signed/sealed.


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## kilitact (Jan 26, 2010)

Re: Rafter ties

Use the Tables in IRC Chapter 8


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## jar546 (Jan 26, 2010)

Re: Rafter ties



			
				brudgers said:
			
		

> Is the building any safer because an engineer ignored something that is outside of typical engineering practice, than if a non design professional ignored it?What's the goal, safe buildings or code administration?


Safe buildings is the answer.  Of course this has caused me some headaches in the past when I rejected an engineer's stamped repair spec because it was outside the design criteria of Ch 16 of the IBC.  There was nothing he could have "engineered" that would have worked.  Only a system that has been through a certified testing procedure could have been used.  He filed a compliant with PA L&I because I would not accept his rubber stamp with no calculations.

I have been tempted in the past to just accept an engineer's stamp but realize that I am not looking to cover my butt, I am here to ensure safety.  If I know it is wrong at my level an engineer or architect certainly should too.


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## RJJ (Jan 26, 2010)

Re: Rafter ties

I agree with Jeff & those that feel it must be designed.


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## brudgers (Jan 26, 2010)

Re: Rafter ties



			
				RJJ said:
			
		

> I agree with Jeff & those that feel it must be designed.


"Design" doesn't necessarily require a pencil or a slide rule (hint, Jeff).

Raising the the tie up half the rafter means you need six nails instead of three, because the load has doubled.

It's not rocket science.


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## GHRoberts (Jan 26, 2010)

Re: Rafter ties



			
				brudgers said:
			
		

> Raising the the tie up half the rafter means you need six nails instead of three, because the load has doubled.


Perhaps only 5 nails. Perhaps the tie needs to have a larger cross section.

Not quite rocket science, but not trivial either.


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## brudgers (Jan 27, 2010)

Re: Rafter ties



			
				GHRoberts said:
			
		

> brudgers said:
> 
> 
> 
> > Raising the the tie up half the rafter means you need six nails instead of three, because the load has doubled.


Perhaps only 5 nails. Perhaps the tie needs to have a larger cross section.

Not quite rocket science, but not trivial either.

Expect double the load.

As is the case with other simple dowel connections, double the nails means double the resistance unless the wood splits.

Tie would not require additional cross section as tension is reduced the further up the rafter it goes...that's why the additional forces pass to the the heel joint.


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## GHRoberts (Jan 27, 2010)

Re: Rafter ties



			
				brudgers said:
			
		

> GHRoberts said:
> 
> 
> 
> ...


Expect double the load.

As is the case with other simple dowel connections, double the nails means double the resistance unless the wood splits.

Tie would not require additional cross section as tension is reduced the further up the rafter it goes...that's why the additional forces pass to the the heel joint.

I don't know if your analysis is correct or not. Iam not interested enough to care but

If one needs to double the nails, one generally needs to change the cross section. (The problem does change from no bending forces to having bending forces. But the details can be worked out by an engineer from a prescription.)


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## FredK (Jan 27, 2010)

Re: Rafter ties

Engineer it.


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## TJacobs (Jan 28, 2010)

Re: Rafter ties



			
				brudgers said:
			
		

> Table 3.9 in AF&PA WFCM 2001 gives the factors needed for calculating the additional strength required at the rafter to plate connection based on how high up the tie is.


I like the WFCM, but remember the limitations of the WFCM.  Chapter 1.1 Scope presents many limitations on WFCM's usage.  But it does fill in a lot of blanks left by the IRC with plenty of good illustrations.  Every building department and every homebuilder should have one.


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## brudgers (Jan 28, 2010)

Re: Rafter ties



			
				TJacobs said:
			
		

> brudgers said:
> 
> 
> 
> > Table 3.9 in AF&PA WFCM 2001 gives the factors needed for calculating the additional strength required at the rafter to plate connection based on how high up the tie is.


I like the WFCM, but remember the limitations of the WFCM.  Chapter 1.1 Scope presents many limitations on WFCM's usage.  But it does fill in a lot of blanks left by the IRC with plenty of good illustrations.  Every building department and every homebuilder should have one.

From a design standpoint, some of the information is specific to the geometry limitations...generally, components of the MWFRS.

Other parts like Table 3.9 are valid beyond the geometry limitations...the distribution of forces in a rafter with a tie is the same regardless of the length of the rafter...only the magnitude changes.

My point is raising the tie a foot should not automatcially require engineering.

If you are checking the adequacy of the rafter to plate connection (which in my opinion, you should be doing particularly if whe your worried enough about it to require engineering for a slight deviation), then it's another 30 seconds to determine if it is adequate for the additional thrust.

It's just alternate means and methods.


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## TimNY (Jan 28, 2010)

Re: Rafter ties



			
				jar546 said:
			
		

> I am not looking to cover my butt, I am here to ensure safety.


First, the "man who draw with pencil" thing has my whole office laughing.

Second, well said.


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## Paul Sweet (Jan 28, 2010)

Re: Rafter ties

If the tie is halfway up the rafter, the bending moment in the rafter is tripled.  You may have to resize the rafter in addition to providing more nails.


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## brudgers (Jan 29, 2010)

Re: Rafter ties



			
				Paul Sweet said:
			
		

> If the tie is halfway up the rafter, the bending moment in the rafter is tripled.  You may have to resize the rafter in addition to providing more nails.


As I'm certain you know, for a rafter of 7 feet or more, moving the horizontal member up a foot will reduce the maximum bending.

That was the original question.

The one to which I suggested that requiring engineering might be unnecessary and for which I used the table a tool for assisting in making such a determination.


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## RickAstoria (Jan 29, 2010)

Re: Rafter ties

Be careful about the number of nails, you could run out of wood. Options may shift from a cluster of nails to a couple of 1/8" thick steel plates sandwiching the rafter tie and a 1" diameter lag bolt.minimally a 2x6 rafter tie and rafter. 2x4 (I would say 3/8" lag bolt max size. The plates are for reinforcement.

Just some loose thoughts. Sizing would need to be determined with prescribed loads.... now where is that International Performance Code.


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## texasbo (Jan 29, 2010)

Re: Rafter ties



			
				RickAstoria said:
			
		

> Be careful about the number of nails, you could run out of wood. Options may shift from a cluster of nails to a couple of 1/8" thick steel plates sandwiching the rafter tie and a 1" diameter lag bolt.minimally a 2x6 rafter tie and rafter. 2x4 (I would say 3/8" lag bolt max size. The plates are for reinforcement.Just some loose thoughts. Sizing would need to be determined with prescribed loads.... now where is that International Performance Code.


Rick, would you object if I used "Be careful about the number of nails, you could run out of wood" in my signature? Thanks in advance.


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## brudgers (Jan 29, 2010)

Re: Rafter ties

Nails are allowed until the wood splits by NDS.


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## Mule (Jan 29, 2010)

Re: Rafter ties

Speaking of nails....There's a section in the IRC that states you can substitute a 40 penny nail for 16d's when nailing down rafters..........just imagine how that would work????


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## RickAstoria (Jan 29, 2010)

Re: Rafter ties



			
				texasbo said:
			
		

> RickAstoria said:
> 
> 
> 
> > Be careful about the number of nails, you could run out of wood. Options may shift from a cluster of nails to a couple of 1/8" thick steel plates sandwiching the rafter tie and a 1" diameter lag bolt.minimally a 2x6 rafter tie and rafter. 2x4 (I would say 3/8" lag bolt max size. The plates are for reinforcement.Just some loose thoughts. Sizing would need to be determined with prescribed loads.... now where is that International Performance Code.


Rick, would you object if I used "Be careful about the number of nails, you could run out of wood" in my signature? Thanks in advance.

Sure, it is a good and funny line.


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