What's the length of your ridge?
Is scarfing really necessary?
From what I understand, continuous grain is a good thing.
If you can get a long enough timber....

Hey TG, total length would be around 34'. I have been so fortunate as to work with a number of those beasts, usually 8x12 D fir, but this project is on my own, with the only help coming from my trusty spaniel named Wallace. I'd love to use a full one, but the material handling is my biggest concern...

Cheers,
Brock

You - dog - 34' ridge -
How big is your dog? :p

Yes, I see...
So, I'll butt out now and continue lurking instead.
Peace

@ Brock,

trying to get anything longer than 20' seems to be pretty tough these days. You could consult with a logger.
But if you are going to scarf it anyway so that you can handle it yourself you should be able to put the scarf right on the king post.
Even a 20 footer is still pretty heavy to be dragging it around without help.

The problem with Tom's second drawing (scarf over post) is that you've reduced the thickness of the beam by one-half at the point of greatest shear stress. As I understand braces, they are designed to prevent racking, not to carry load (though they may contribute to load bearing somewhat). The location of the scarf over the brace isn't so that it is supported by the brace, but rather it is located so that the combination of shear stress and bending stress are as neutral as you can get. It just so happens that it falls about a brace-length's distance off the post...

Here is the drawing that Joe was referring to:



Between the posts the unsupported timber will sag or deflect. Over the center post the timber will crown.
This causes this "exaggerated" wave shape effect in the timber.
At the point where the sag or deflection changes to a crown is called the point of "inflection".
This point is where the timber is neither sagging nor crowning. It has the least amount of both forces.
This, I am told, is where the joint should be made. If I understand all this correctly.

Here is the graph to prove this:



I personally don't recommend putting a scarf over a post because of this graph, but each situation is different, and they all have to be figured out, and built based on this.

Jim Rogers

Yup, a 60" table with a Vemco drafting machine, but generally it only gets used for final drawings. Graph & Hex paper for preliminary design work (and my own projects).

I would like to get proficient with a modern 3D CAD program. They sure come in handy when discussing things online or generating perspective drawings.

@ Jim,

I just purchased a bunch of literature to see which books you guys are actually talking about. One of them was the Sobon book of historic american timber framing which is where I also saw this drawing. ( Another reason was to compare Benson and Sobon since this comes up fairly often and I had no idea what any of this meant. Having looked at both I would definitly fall into the Benson category).

Back to the topic. Your/ Sobons explanation makes sense, at least parts of it. If you applied a load to the beam in the drawing it would indeed sag where it is not supported. But it does not actually go anywhere (neither up or down) above the post. It would crown up once you add the braces, in theory. But since everything is pegged together it won't. So if you have a full lenght beam going across the three posts shown and it sags midway between the posts it would put tension on the beam fibers above the post because it needs to "stretch/ become longer" to make up for the sag. Same would happen when the joint is put above the brace. Because the brace does not actually give. So it really supports the joint and holds it up as the post would. So in my opinion it should not really matter where you put the scarf joint. If in doubt I would add another post.

But Sobons drawing aside. Does anyone know of a lab test where this situation has been tested. There are test for pegged tenons and braces but I did not come across a test for the scarf joint.

If a brace is loaded in compression it can and will lift a plate off of the post if it is not pegged down. I believe that the braces do help carry the load and that the shear and moment charts do not take this in to account. The scarf, placed over the post with braces both sides, is supported in three places not just one. If the scarf is placed over the brace and the brace is helping to carry the load, where are the shear forces now ?
I believe in supporting scarf joints.

Doubting Thomas

@ Tom,

I totally agree.