Purlins, how many of you dovetail them and how many don't?

[Thehardway]

I have heard a number of opinions out there on securing purlins.  Traditional method would be to cut dovetails and wedge to fit.  Seems most frames today merely house and strap with steel strap.  Others even leave them proud on the rafter or truss.  I see advantages and disadvantages to all of these methods.  Dovetails are certainly the most time consuming and technical.   They also remove the most meat from the rafter.  Any opinions or advice.

[beenthere]

If any wood is removed from a rafter, one has to calculate the residual depth to determine if the design strength requirements are still met.

[Raphael]

I've got them jointed three ways in my house.
In the shed addition where they don't need to tie anything they are just simple drop ins.
In the office the purlins dovetail into the outer rafters (which only have purlins on one side) and tusk tennon into the inner rafter pair (which have rafters coming in from both sides).  No wedges needed in the dovetails as the tusks pull everything together via draw-bored pegs.

[maineframer]

One way to cut them is a half dovetail and use oak wedges to draw them up snug. These are very straight forward to cut and install. I believe that not cutting the dovetail on both sides reduces the tendency of the purlin to split. Cutting a straight housing for the purlin is also a perfectly acceptable and rugged joint.

[Thehardway]

Would this be an accurrate representation of the four primary methods we have discussed so far (minus the tusk tenon)?

[Jim_Rogers]

I have posted these before, but here are some examples of tusk tenons:



And:



Tusk tenons require that the pieces be leaned out to insert, if assembling bents with rafters attached.
If you're using continuous plates then you begin at one end of the building with a pair of rafters installed and braced off and then insert the purlins, hold them up and then insert the next set of rafters into them. An assembly plan is very important when using tusk tenons. Or you lean out the second pair of rafters while you insert the tusk tenons into their mortises.

Leaning out rafter pairs requires a lot of rigging such as straps and come-a-longs so that the rafter pairs don't lean out very far, as the crane flies in the purlins.

Jim Rogers

[Joel Eisner]

Bob,

You were at the frame raising and we did a housed approch with a 1x12 oak peg holding them in.  They were easy to cut and fast .... which was nice since we cut the other end of the purlin once the frame was up and the bents were plumb.

Joel

P.S.  Hows the project? 

[Thehardway]

Jim,

Thanks for the pictures and clarification on the tusk tenon.  Knew what it was but was familiar only with use in summer beams rather than roof purlin.  Do you view the tusk as a better joint for purlins than those above? Sounds like difficulty of assembly would make it undesirable for my application. Would take a lot of hands to line up all the purlins.

Joel,

Have been laid up with a bum knee this week.  Things are going sloooo...   Hope to make some good progress over Christmas Holiday.  I don't recall any detail setting the purlins on your frame, I think I left just after the bents were all raised but the purlins weren't yet set.  What was the depth of the housing you used and the size of the purlins?

[Jim_Rogers]

Understanding the raising sequence is important. Writing a plan of doing it is called a "script" just like a movie script. It tells what timber will be raised in what order.
When you're going to do a bent raising, (bent raisings are when the rafters are attached to the top of the posts, and the frame has interrupted plates connecting the bents) and use tusk tenon purlins to hold the rafters together and support the roof whatever it is (sips or boards), you can use other things to help you accomplish this.
One such thing is called a cleat.
You'll notice in the picture below:



that there are two bents raised and they are held from falling over by two diagonal sets of straps and come-a-longs.
You can see on top of the rafters some cleats, they look like ladder rungs that I kid would use to climb a tree to make a tree house.
These cleats are aligned to the bottom side of the tusk tenon mortise and housing, as shown in this picture:



Now on top of the purlins is another cleat. It is aligned with the eave side of the purlin. So, the crane can fly the purlin up and set it on top of the rafter and it won't slide off the rafter onto the ground.
Then when the rafter pairs are leaned out the purlin can drop down into position and not fall
through to the deck.
How you do it is you lean out the rafters until the lowest pair of purlins, one on each roof plane drop their tenons down and the align with the mortise. Then you tighten up the come-a-long at this purlins' location just enough to hold it there.
Then you insert the next one up the rafter as you stitch the rafter pairs together by tightening up the come-a-longs you only have to have a person align the tusk tenon to it's mortise.
Once the tenons are all seated you can peg them off.
The cleats hold the weight and after the frame is done you remove the cleats from the top sides of the rafters and purlins. 

Link this:



It does take a lot of straps and come-a=longs to do it.....

Jim Rogers

[Thehardway]

Wow, the cleats really simplify things.  I was envisioning a person to align each purlin as the bent was leaned in. Very nice tutorial.  If I were to do a bent style raising I might use the tusk tenons as I can see how it would really lock things together well.

My trusses and rafters will be dropped into beam pockets in the ICF walls shown in the below picture. Once in the pockets it will be impossible to lean them out far enough to drop in the purlins so I am thinking the top down placement methods are going to be my only real options.



The gable ends are also fixed in place so they can not lean one way or another as you can see below.




I was planning to drop all of the trusses into their pockets shim them to height, get them plumb and brace to gable ends and each other as I get them plumb.  Purlins will then be cut to appropriate length and dropped into the pre-cut housings in the trusses.  Braces would then be removed and panels screwed in place.

[Don P]

Going back to what Beenthere said. Testing on full sized timbers has shown that drop in notches should be figured using the section remaining between the purlins. Previously most people have used the remaining upside down T section to figure, it doesn't appear that the wood below the purlin contributes very much and that is probably unconservative. The same professor suggested the tusk tennon as it removes the most wood from the neutral axis of the timber and leaves more wood on the upper and lower edges where the bending stresses are the greatest.

On the TF we worked on the frame is now about 2 years old. The drop in dovetails had a ~1" long tail. With skrinkage several are now down to 1/2" bearing and no way to get back in there to wedge them  :-\.