Common Purlin Vs. Common Rafter

[Zeke]

What do you see as the advantages and disadvantages of these two roof framing systems?

[witterbound]

I believe the purlin design is more common these days because such a system can be raised quickly with a crane.  Bents can be assembled on the deck, raised as a whole, then the purlins can be flown in to tie the bents together.  Minimizes site time for the crew and crane time.  It also uses heavier timbers in the roof, which many people like.  On the other hand, the common rafter system generally uses smaller timbers for the rafters, and often must be raised one timber at a time - a post, another post, braces, the plate, the ties ... then the other side... then the rafters one or one set at a time.
 

[Jim_Rogers]

One thing to consider in a frame design is the roofing system.
If you're going to do a common rafter system you're going to need to rest the foot of the rafter onto a plate at the eave.
In traditional timber framing the plates were continuous. Sometimes now a days this isn't always possible. So we use a scarf to connect short pieces together to make a longer piece.
The advantage to having a plate above the post tops is that the plate supports the rafters and the posts support the plate. The tie beams that "tie" the eave walls together go one direction and the plates usually go 90° to the ties. Looking at this frame in plan view it creates a grid of timbers. This grid makes the frame strong. It is hard for the frame to rack one way or the other, as each direction has proper bracing to prevent the racking and long strong timbers to make it sturdy.

In a common rafter roof system the decking, if used, runs from gable to gable. If you want a gable overhang it is somewhat easier to just make the decking longer and overhang the last rafter a bit:



In the above photo, which is of an open air pavilion, you can see the plate extending beyond the last bent. And the decking beyond the last rafter.
If it was a house, I wouldn't recommend that the plate extend beyond the external surface of the wall as this invites water and rot into the home. But the decking could extend beyond the last rafter if trimmed out correctly.

And if you want an overhang at the eave the rafter tails can extend beyond the plate to hold up the decking. And if trimmed out correctly they are safe from water which causes rot.
As you can also see in the above photo.

In a purlin roof system the roof decking runs from eave to ridge. This is good if you want to have an overhang at the eave as well; as the decking can run out a bit to create the eave overhang. However it is difficult to create an overhang on the gables, as there is not main timber out there to hold up the decking.

If the bents are assembled, with the rafter attached to the top of the posts then there is no main continuous plate. There could be an interrupted plate or a lowered connecting girt.

The next thing to consider is the connection of the purlin to the principal rafter. There are many ways to do this.

Cutting the top surface of the rafter to create a "drop in" purlin may not be the best joint. Building codes may prevent this design from being used.

And a drop in purlin may not be the best to bind the roof system together.

The next choice could be a dovetail joint on the end of the purlin. Again this cuts the top surface of the rafter and what this means is you have to size the rafter larger to account for this cutout.

What happens when the dovetail shrinks?

It can allow the purlin to pull out of the rafter. Some timber framers have moved away from dovetails for this reason. And they have gone to tusk tenons.

Tusk tenons are a stronger joint as they don't cut the top surface of the rafter and they can be pegged. But the assembly of the roof structure is harder.

If you stand up one bent with rafters attached to post tops and secure it, plumb. And then stand up the next adjacent bent, you have to basically balance this bent so that it can be leaned out to insert the purlins between the two. Or you have to support all the purlins as the second bent is raised. And align each purlin to the second bent as it becomes vertical.

Most raisings that I have seen where tusk tenon purlins were used, leaned out the second bent and had the crane fly in the purlins. To do this method you have to have lots of straps and come a longs to hold the second bent from falling over as you use the crane to fly in the purlins.



In the above photo you can see two bents raised and connected with wall girts. There are no purlins in the roof structure yet. There are some straps and come a longs ready to lean the bents apart so that the purlins can be inserted.
The cross boards on the top of the rafter are to hold the purlins from dropping down to low when the crane lowers them onto the rafter. And the are called cleats. These cleats make a nice ladder for the framer to climb to undo the straps to release the crane from the bent, so that it can be used to lift the next timber.



In the above photo you can see the tusk tenon pockets and housings.

It can be done, but it is harder in my opinion.

Being a traditionalist timber framer, I like common rafter roof system with long top plates.

Jim Rogers

[Jim_Rogers]

Recently a nearby timber framer friend of mine raised an oak frame. He generally uses tusk tenons but my not have on this frame.

Here is a picture of a bent ready to be raised:



As you can see it takes a lot of rigging to raise a frame, at times........

[Alexis]

From what I read, an advantage of common purlin roof is the fact it uses shorter timber. The common Rafter uses a lot of long ( sometimes hard to find) timber

[moonhill]

But  if the common rafter long stock was readily available there would be less joinery with a common rafter roof.  Tim