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Old 03-16-2014, 02:03 AM   #1
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My Garage Rafter Tie Idea...What Do You Think?


Here is what I finally decided to do to clean up the rafter ties in my garage. Currently they are a mess, but I won't get into that. I've provided a link to my proposed plan. I did it in Excel and PDF'ed it. It looks like it will open in online Word, so some of the lines aren't as clean as they were when I drew it.

Let me know what you think and if you have any suggestions for improvement, I'm all ears.

Basically I'm leveling up the rafter ties, replacing 2x4 with 2x6, and creating Fink trusses.

http://1drv.ms/1hnUNbX

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Last edited by builditbig; 03-16-2014 at 02:13 AM.
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Old 03-16-2014, 11:37 AM   #2
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My Garage Rafter Tie Idea...What Do You Think?


This is an interesting idea, but there are a few issues you should think about. First off, you say you are building Fink trusses. Well enough, but these are home made trusses, and you are planning to connect the elements using Simpson plates nailed in. You probably should check with Simpson to make sure they feel their plates are suitable for permanent use in a truss, my recollection is that Simpson makes a mending plate designed for repair, not permanent use as part of a rated truss.

Second, if you build a truss that is fully rated, you don't need rafter ties, since a truss exerts no lateral load on the walls. The rafter tie is effectively the bottom chord of the truss. As to whether it needs to be 2x6 rather than 2x4, you would need to run the calcs for the truss to size the individual elements.

Last point is that your truss is asymmetrical, in the sense that the left side includes an extended truss member (the rafter) which presumably you plan to nail to the left side wall. This is unusual, I have never seen it done this way. In order to determine if this is structurally acceptable, you would need to compute the bending and shear forces on the extended top chord of the truss (the rafter). Normally the bottom chord of the truss is attached to the wall, and any truss extension is used to carry a small roof load only. Good luck with the project.

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Old 03-16-2014, 11:53 AM   #3
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My Garage Rafter Tie Idea...What Do You Think?


Quote:
Originally Posted by Daniel Holzman View Post
This is an interesting idea, but there are a few issues you should think about. First off, you say you are building Fink trusses. Well enough, but these are home made trusses, and you are planning to connect the elements using Simpson plates nailed in. You probably should check with Simpson to make sure they feel their plates are suitable for permanent use in a truss, my recollection is that Simpson makes a mending plate designed for repair, not permanent use as part of a rated truss.

Second, if you build a truss that is fully rated, you don't need rafter ties, since a truss exerts no lateral load on the walls. The rafter tie is effectively the bottom chord of the truss. As to whether it needs to be 2x6 rather than 2x4, you would need to run the calcs for the truss to size the individual elements.

Last point is that your truss is asymmetrical, in the sense that the left side includes an extended truss member (the rafter) which presumably you plan to nail to the left side wall. This is unusual, I have never seen it done this way. In order to determine if this is structurally acceptable, you would need to compute the bending and shear forces on the extended top chord of the truss (the rafter). Normally the bottom chord of the truss is attached to the wall, and any truss extension is used to carry a small roof load only. Good luck with the project.
Thanks for the feedback.

I had similar thoughts about the tie plates as well. I have seen USP plates rated for truss use, so I will verify that Simpson's are similarly rated, as well as the appropriate connectors.

The truss bracing is just added support given that the ties will run about 18'. I could run the ties without the bracing and then they'd be called rafter ties or ceiling joists. Currently there are 2x4, 20' ties with one king post to prevent sag. Essentially now I have King Post trusses.

Effectively this will be a raised tie truss system, but with just one side raised. Again, this is because the original construction is one wall is 8', the opposite wall is just 7'.

Last edited by builditbig; 03-16-2014 at 12:12 PM.
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Old 03-16-2014, 12:37 PM   #4
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My Garage Rafter Tie Idea...What Do You Think?


You seem to be trying to build a raised heel truss on the 7 foot wall side. However, the way you have drawn it, this is not a truss, as the extended part of the rafter is not part of a triangle. You may want to look at conventional raised heel truss design to see the difference.
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Old 03-16-2014, 12:51 PM   #5
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My Garage Rafter Tie Idea...What Do You Think?


Quote:
Originally Posted by Daniel Holzman View Post
You seem to be trying to build a raised heel truss on the 7 foot wall side. However, the way you have drawn it, this is not a truss, as the extended part of the rafter is not part of a triangle. You may want to look at conventional raised heel truss design to see the difference.
I see what you're saying. I'm not sure how a conventional heel would work in this case given that the truss on the short wall side won't be above the top plate. Seems in all of the diagrams that the heel sits on the top plate in a raised heel.

So what would be the difference between what I'm doing and simply raising the ties, keeping them within the bottom third of the rafter run?

I'm effectively doing this...

http://www.sydenhamstimberframe.co.u...es/rafter3.gif

...just asymmetrically.
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Old 03-16-2014, 01:26 PM   #6
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My Garage Rafter Tie Idea...What Do You Think?


Here are some pictures of the garage. I've provided some captions to try and describe what I currently have.

http://1drv.ms/1hpcA2I
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Old 03-16-2014, 06:06 PM   #7
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My Garage Rafter Tie Idea...What Do You Think?


Thanks Daniel. You got me to thinking and based on the IRC it looks like I should probably sister up the tied rafters because when I make the raised tie adjustment I am marginally at the span limit.
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Old 03-17-2014, 05:18 PM   #8
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My Garage Rafter Tie Idea...What Do You Think?


For anyone that's interested, I just spoke with another engineer (the first one was a hack), and he basically confirmed that my plan was just fine. The only modification was he said it would be best to sister the rafters on the side where I am raising the ties off the top plate and to use Simpson ties to beef up the rafter/top plate connection on the wall where I'll be raising the ties.

Awesome.

So...looks like I'll be busy this weekend. :-)

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