Are braces that are 12/12 better than braces that might be 10/12 (for instance, if you wanted to have the braces parallel to the rafters?

what a good question.
I would say a 10/12 brace would have similar strength to a 12/12 brace based on the shortest side of your 10/12 brace triangle.
For example, a brace based on the 3/4/5 triangle would count as a 45deg brace with 3/3/4.242 sides. I am not an engineer and would be curious to hear an engineer speak on this.

I'm not an engineer.
That being said, here's my 2 cents worth. I think the 12/12 brace is used because it's balanced. Imagine, as an extreme example, a 2/12 brace. A lateral force which puts the brace in compression would have a mechanical advantage in pushing the post away rather than directing some of the load down the post. That is, the force would be directed more horizontally. If the brace were set at 12/2,the brace would do almost nothing for horizontal forces.
Another place to look at members which work to maintain an angle would be rafter ties. Of course you'd want a collar tie to be level and it's intuitive that the angles are equal at each end thus creating mechanical balance. I would argue that if the roof had dissimilar pitches, the most effective collar tie would be something other than level. Dissimilar rafter lengths or anything that makes the roof geometry unbalanced would have a similar effect.
It's all 'bout balance.
None of this is to say that a brace HAS to be 12/12. Variants are seen all the time. As with any frame design question, let an engineer check it if there's any doubt.

Hi Mo & Mark,

This is an interesting question and as you would expect from me no straightforward answer.

It would be important to look at the degrees of freedom possible for the components that are attached to both ends of the brace and to consider what might happen if excessive thrust or tension was developed in the brace.

For example, in a classic crossframe (bent) where a pegged jowl is present then any upward levering force developed by the brace due to the application of wind forces would tend to try and lift the tie beam off its seat. To keep the joint together would require that the upward component of the force developed in the brace would need to be resisted by the peg. Where no jowl or peg is employed then the tie beam could possibly be lifted off its (dovetail) seat and would instead need to rely on the downward application of dead load as applied by the roof to help keep the tie beam firmly on its seat. The possibility of levering up an unsecured tie beam is very real.

Early English and later American timber frame design attempted to get round this problem by using dropped (i.e. enters the post under the plate) tie beams that employ housed joints and this is quite effective at resisting vertical forces generated by braces.

Medieval houses rarely employ 45 degree braces and instead attempt to align the forces generated in the brace more closley to the component to which the brace joins thus typically one might see curved braces being introduced which would also introduce an element of springiness into the equation due to the bending moments that would now be generated in a curved brace.

In reality we can observe that people do not tend to live in open frames and instead employ wall infill or cladding systems that take over much of the work thought to be done by the braces.

Many old houses are seen to have lost a significant portion of their original braces or have them cut through to make door openings and no great damage seems to result provided the wall that remains is still sufficiently stiff to resist racking.

Braces don't just come in the Jack Sobon garden shed variety - they can be highly decorative and include such standards as ogee, passing, foot, arch type and decorative quatrefoil panel braces all of which will introduce their own design peculiarities when employed in combination with any particular type of frame design.

Finally some structures don't have any overt form of bracing. Check out the Minka farmhouse currently being dismantled in Japan which is an earthquake zone.

Go figure !

Regards

Ken Hume P.Eng.
http://www.kfhume.freeserve.co.uk

Mark, Dan, Ken, thanks for the replies. I was originally thinking of this for braces running from a king post to a rafter that is not 12/12. I didn't even think about the collar tie....



So would this unlevel collar tie become more structurally sound (all other things equal) because the angles are the same between each rafter?

Also, if a 12/2 brace would only serve support in just one direction couldn't sticks be put at different angles depending on the forces they would be subject to?

To amend what I said previously: Mechanical balance is not necessarily achieved by making the angles of your triangle similar/symmetric. There are other factors to be considered such as roof load distribution, rafter length, and rafter pitch.
And perhaps I emphasized balance more than I should have. Sometimes it's necessary from a structural perspective, sometimes its necessary from an aesthetic perspective, and sometimes it's not necessary for either. Sometimes there is enough structural integrity to allow compromise in favor of aesthetics and sometimes it's the other way around.

In your photo example it looks like there would be little need to place a collar on an angle unless you like the look. Would the stresses be identical at each end of the collar? Probably not. But it's not likely to make enough difference to worry about.

If I'm not being vague enough or wishy-washy enough I could get my lawyer to elaborate on this!

Cheers

I like the idea of braces at a sharper angle . The Dutch barns around my area almost exclusively frame their anchorbeam braces at what I would guess is like a 14/12 or even 16/12 pitch, they all have faired extremely well. It seems that perhaps anything from 8/12 to 16/12 would perform mostly equal with 12/12 being the most balanced , but I am just speculating as I am no engineer as well. tb