Thanks UK4x4!


John I am getting it!!! The picture helps alot!!
What about if the beam runs the other way? How would that affect how the posts are placed on the walls to the beam? Would it change the load bearing to just the walls or would it continue to the foundation?

Run the beam the other way and you still need to have solid bearing to the foundation. Notice that the post under the lower end of the beam might want to go right through the middle of the fireplace - not a good idea.

Hint - better would be to have a deep header spanning the fireplace opening and then posts on each side down to the foundation wall.

Or remove fireplace   

is it necessary/is there some way to calculate if a rafter tie is necessary on every rafter for a ridgeboard?  What about every third one on 24" spacing? Also, does the pitch affect this...seems that lower pitch would be more tolerant of this.

Am I correct that the rafter ties can be higher than the wall? On a 10 foot wall and 15 foot ridge ht., how far up the rafter can they be?

Rafter ties can be as high as one third the way to the peak. That is the bottom tie. The top tie can be a collar tie in the top 1/3 of the rafters or a metal strap over the top of the ridge board that ties both rafters together. Check with your inspector to confirm.

Thanks...

And one on every rafter?

I cannot ask the inspector.....we do not have one. Not having one does not encourage me to "cheat"  as it does no good if the wall falls down, but are their options?

The spreading force that a rafter tie is resisting is a function of the roof pitch. Your thinking is backwards though. The horizontal spreading force created by the vertical load becomes greater the shallower the pitch.

Draw half of a gable roof with a 12/12 pitch, a right triangle with the vertical and horizontal legs of equal length and a 45 degree rafter angle. The horizontal component of the vertical load is the ratio of the legs to one another. In other words if the vertical load on that side of the ridge is 1000 lbs the horizontal thrust that the tie must resist is also 1000 lbs. If the pitch were 4/12 the horizontal component would be 3000 lbs. If the pitch were 1/12 the thrust would be 12,000 lbs. Look at the ratio of the lengths of the vertical and horizontal legs of the triangle, they are describing the forces.

If you raise the tie above where the rafter is bearing on the wall it begins to create a concentrated bending load where the tie is connected to the rafter. The force being resisted by the connections also increases as the tie is raised. As the tie is raised the rafter needs to become deeper and the tie connection needs to be made stronger. skip a rafter, the forces double, skip 2 and they triple.

Skip a rafter tie and take care of the above and the roof will probably not sag. Skip another and the ridge should probably be sized as a beam capable of carrying the untied rafters without undue deflection.

Got it. My logic started with a flat roof, rather than the other way...I figured at flat there is zero pushing out, but that is not really relevant and a whole other story.

By my calcs.....ties on the rafter/wall plane and 6/12 pitch is about 2,000 lbs (using your calc example) lets me double the load by spacing every other one and have load about like a 3/12 pitch.

In real life,the roof side is about 24,000 lbs at 20/20 loading, so that is 2,400 lbs per tie, skipping a rafter at 24 in spacing and 40 feet length.

Since the tension is outward, the assumption is there is not a lot (any) bending/twisting force on the tie, so it becomes a task of fastening them on the  ends to resist that "pulling" force and enough dimension to keep from sagging in the middle.  Sounds like 2x10 will be OK. This is all figured on SYP. Still working on getting the figures that compare for the species I will use. Currently the wood i am considering is about 30% stronger in bending/breaking than SYP...but that remains to be verified.

What a great thread!  I'm amazed at the generosity of the Dons and Johns of the world in sharing their knowledge with us.