In my continuing quest to understand what holds my house together I have stumbled upon this unanswered question.
While installing my ledger boards for the loft I got to thinking more about rafter ties. On one side of the house I have aligned the top of the ledger board with the top of a window header, utilizing the header as the ledger in that section.
(http://lh4.ggpht.com/_UroNSkE-1kw/TDXvCw2f5HI/AAAAAAAAABU/gzKgavmZheA/s400/ledger.jpg)
I can't see why that would be a problem but let me know if it is.
As I understand it, the loft joists (20 by 30 1 & 1/2 story) function as rafter ties. I have learned here how a rafter tie's integrity is compromised when it is raised above the top plate.
I would assume that it would be compromised when it is lower than the top plate as well, probably in a completely different manner.
How much so?
Would that weakness be compounded when the tie is located directly above a window opening, as one of mine will be?
To build according to the prescriptions given in the codebook the tie is at the plate or is in the lower third of roof height. The codebook then goes on to state that if this cannot be done then a ridge beam should be used.
Once you step outside of that you are into an engineered solution.
When a non prescriptive plan crosses my desk I am supposed to call an engineer and have him check the design. I would not be at all suprised if the engineer came back saying it is fine. I would also not be suprised if he backed off to a structural ridge. That is what I experienced recently.
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To build according to the prescriptions given in the codebook the tie is at the plate or is in the lower third of roof height. The codebook then goes on to state that if this cannot be done then a ridge beam should be used
So would assume that means a tie 1 ft. below the plate is less effective than one 1 ft. above?
It's water under the bridge for me and I'm not the type to lose sleep over what is done but I am curious.
Armed with the information I have now I probably would have built using a ridge beam, especially since I had already decided to use a center beam (pier & beam foundation) and my loft extends nearly to the mid point of the house, providing a likely spot for a vertical supporting beam.