common rafter span

[Alexis]

Is there a way to have long span ( 30ft) with common rafter or do you need intermediate support?

Is there a reason why we see so few common rafter in new timber frame house?

thanks

Alexis

[moonhill]

Alexis, I am finding I am a bit of an odd ball, I for some reason enjoy common rafters, tapered, with wane.  I think purlins are sought after more due to the look of a more traditional old style, but a lot of that reasoning comes from the Bensonite form of framing, or modern framing, if you will.

I would like to hear more input on this topic and the reasons for purlins.  There may have been a thread on just such a topic if I recall.

30 feet is a stretch but done in the past without a principal purlin at mid point.  I would put a principal purlin in at mid point.  Your rafter stock would not have to be as big, a little more joinery, but that is why we build in timbers, for the joinery. 

Tim

[jfl]

I'm not sure if I understand your question. What is the load on the common rafter?

To get a span that long, you will need a VERY large piece of wood and/or a perfect one (select structural...)  I guess it would be very difficult to find very long, very strong (no knots) timber.  At best, it is un-efficient.

To reach long span like that, you can do 2 things: A) use engineered beam (glued wood, not really timber framing...) B) use trusses.

jf

[Jim_Rogers]

I would have to agree that the load and all circumstances have to be know in order to give an "opinion" about this rafter span.

Spacing and roof pitch are some of the circumstances that should be presented as well.

Having a central supporting principal purlin would probably be best depending on the total design.

Jim Rogers

[Tom]

Perhaps this link might be of some help.
http://www.awc.org/pdf/WCD1-150.pdf

[Clovis]

Don't want to steal this thread but.... My span is 24', 12/12 pitch, total rafter length of about 18', 4' centers and a collar tie every 8'. Low snow loading and with steel roofing. timber size of 4" by 8". I love the look of common rafters, any thoughts? Clovis

[LeeB]

How about a picture or a drawing so us uneducated folks know what you fellow's be talking about?

[Ironmower]

I agree with Jim, pitch and spacin' is crucial. I would think (not for sure :) that the steeper the pitch the smaller the rafter could be(too a point). and collar ties would be a must......I'm sure on of our pro's on here can enlighten you a little more ;) ;D

[witterbound]

The rafter size is only one part of the equation.  You're also going to have to figure out the size and location of your tie beams.  These are the dudes that keep the plates, where your rafters sit, from spreading apart.  When I had my frame engineered, the engineer wasn't at all concerned about the size of my common rafters (I'm using 6x6 principal rafters and 3x6 common rafters).  I've got a low pitch 6:12, and only a 16 ft span.  The engineer was very concerned about the rafters desire to spread the plates apart, and to twist the top of the plates over.  So, I've added some king posts and a ridge beam, and have beefed up my ties (8x12) over the middle of my living room.  I think it's way over engineered, but I also like the look of larger timbers.   

[Alexis]

The building I have in mind is a 29x42 two story house with 12/12 roof
77psf snow load
I like the simple joinery of common rafter, when adding a support at the middle, of do you do the joinery. Is it one long rafter or two shorter one?

thanks for input

Alexis

[Rooster]

On a 29ft width, and 12/12 pitch, without any rafter tail.... rafters would be 20ft-6in (and change).

My thought is that if the tie beam of the bents were within 3ft of the top of the top-plate on the side wall, you might not even need the purlin....any wider (building width), any lower (tie beam), or any shallower (roof pitch), and you would have to do the collar ties or purlin.....I didn't run any numbers, just compairing this with any number of barns in my area.

Good luck!

Rooster

P.S.  I like common rafters also!!