Kneewall Questions

[Dberry]

I've attached a drawing and a picture of a roof (Micky's?).  I'm inclined to platform frame instead of balloon, and am concerned about the "hinge" effect of the kneewalls discussed briefly in another thread.  I've drawn a diagonal brace as an idea to solve the hinge effect.  I was thinking a diagonal 2x4 brace every rafter would solve the problem, but it seems like a klutzy approach.  The kneewalls are just under 3' high and the rafters are 2x12.

What do you guys think?  

[manhattan42]

The concern isn't so much because there is a kneewall, the problem is that at the moment you have no horizontal cross bracing to stabilize these bearing kneewalls.

Gable roofs, like yours, have a potential to push bearing walls out as the roof comes under load from wind or snow or even from its own weight.

Typically, ceiling or floor joists rest on top of bearing walls and these keep the bearing walls from spreading when the roof comes under load.

In a cathedral ceiling like the one shown,  the horizonatl cross bracing gets moved up off the top of the bearing walls or removed altogether.

This makes for a much weaker roof system by decreasing the load bearing capacity relative to the height of the cross braces.

Raising the horizontal cross members requires the rafters to be upsized to compensate for their lessened ability to withstand loads.

The higher the lateral cross braces from the top of the bearing wall, the weaker the roof system.

Your diagram shows horizontal cross braces in the form of ceiling joists/collar ties at what appear to be the 8 foot level above the floor. If these are to be part of your design, it may be all you need.

Otherwise, The diagonal bracing you propose will have little if any effect upon strengthening the roof system.

[FrankInWI(Guest)]

I'll tell my story again.  My first home was a nice 1930's bungalow.  It was a VA loan and the inspector was pickier than any of us wanted him to be.  Well, he missed a whooper!  
The attic was kind of prepared for some day making an apartment out of it.  Well, in looking at it closely I was shocked to note that the walls were pulling away from the floor joists!  The house was much wider in the middle than at the ends.  It was balloon construction.   The collar ties were few, and pretty high...again, presuming expansion some day.  Well....the roof pushed out the sidewalls.  
I was an auto body man then.  I got hydrolic floor jacks in and the pushed 4X4s up to chains hung from rafter to rafter.  Meanwhile, I got about 4 come-alongs and pulled the side walls together while I jacked up the roof.  If worked !  THEN, being body man and used to BOLTS, I bolted each balloon 2 x 4 into the floor joists a few times.  
I then proceeded to put a big dormer in, walls up, and made a great little apartment.  I'm still the type of carpenter that uses a lot of non traditional tools in my rough work.

[glenn kangiser]

Cool story, Frank.  I used to help the body man out after work when I was a Dodge Mechanic ---- or maybe I just hung out to learn things.  He taught me body work and car painting.  Another one taught me radiator repair.  I never took over the body mans job there but did do re-cores, rodding and repair of radiators for a while.  Good knowledge to have.  Like you, I find that knowledge from one trade is easily borrowed from, transferred to and used in other trades.