Jack Rafter Joinery

[Ariverrunsthru]

I'm building a 40x40 pavilion with a (6/12 pitch) square-hipped roof. There are eight jack-rafters per wall. I was hoping to hear some opinions on joinery for the rafter-hip truss joint. Is a mortise & tenon the preferred joint, or is a shouldered dovetail preferred?

The hip trusses are 8x12 & the jack rafters are 4x6. All the timbers are Eastern White Pine.

[witterbound]

Don't know how it was done in traditional joinery.  On my frame, we just cut a pocket and then put a timber screw into the two.  A dovetail or mortise would sure be a Lot of work with the tricky angles, for joinery that won't show once you put the roof on.

[bigshow]

Don't know how it was done in traditional joinery.  On my frame, we just cut a pocket and then put a timber screw into the two.  A dovetail or mortise would sure be a Lot of work with the tricky angles, for joinery that won't show once you put the roof on.

+1 

[Heartwood]

Nowadays most timber framers just house the jacks and secure with a timber screw or strap over the top. This does much less damage to the hip than an mortise and tenon, which is not needed except perhaps as additional security when lifting the whole roof assembly by crane. Dovetails are rarely used anymore in joist or rafter/purlin joinery because shrinkage negates the dovetails effectiveness, and the drop-in pocket also weakens the receiving tmber. Soffit/tusk tenons are used instead (if needed at all) but make assembly more difficult. Again, cranes and pre-assembly on the ground make this possible.
A one inch housing should be sufficient. Here's a drawing from the "When Roofs Collide " series in Timber Framing that show the housing; it shows an open housing on the bottom but yours would be closed since the hip is so much deeper. The labels refer the Hawkindale angles for the various cuts, but if it's a regular pitch/regular plan roof you can lay it all out with the rafter square tables.


  

[Ariverrunsthru]

Thanks for the info! I guess I need to fight my idealist tendencies and accept that there might need to be some metal hardware in this structure...

Again, cranes and pre-assembly on the ground make this possible.

Due to the location of this structure and the design, there will be no cranes or ground assembly. I stood the walls up with a gin pole, and the roof pieces will be assembled piece-by-piece (I know, not the easy way). Since the structure is in the woods, I'll hang block & tackle from a sheave, which will be riding on steel cables attached the surrounding trees (Tulip Poplars & Sweetgum).

I understand your explanation of the damage that mortises can cause the hip truss, so why would a soffit/tusk tenon be used? Is it simply because it's a shorter tenon- thus less destructive to the hip truss?

[Jim_Rogers]

I understand your explanation of the damage that mortises can cause the hip truss, so why would a soffit/tusk tenon be used? Is it simply because it's a shorter tenon- thus less destructive to the hip truss?

A soffit/tusk tenon mortise leaves the top edge of the hip in tact. Thereby making it stronger. And "drop in" jack will require a cut along the top surface of the hip making it smaller in a cross section. Normally "drop in jack" would only have a 1/2" or 3/4" housing in the side of the hip thereby not weakening it very much. If the jack requires more of a housing due to load weight and then there is on of these on both sides the cross section of the hip is smaller. It may require the hip to be up-sized to account for these housings. These are the questions that an engineer would be able to answer once he/she has all the facts about the frame design and loads.
Jim Rogers

[Magicman]

Welcome to the Forestry Forum, Ariverrunsthru.   8)

[submarinesailor]

Ariverrunsthru,

Pay attention to what Heartwood (Will) and Jim are saying.  They are 2 of the best around.  Both have taught many, many classes on Timber Framing.
On a side note...Will, I was planning to attend your Sketch Up on the 7th, but life got in the way.........AGAIN.  Maybe next year!

Bruce