Purlin w/ split rafter?

[Blueforester]

First let me say I'm new to this site and first time log/timber home builder and designer.  Can't believe I've been working on this plan and researching for over a year and just now found this site!  Great website!!

  Getting close to finishing a plan and now starting to have concerns on what I have drawn up.  It is a basic 36'x42' log home w/ loft, gable roof w/ shed dormers over loft.  Using vertical timbers in prow front, timbers in loft system, purlins and supports.  Having researched and read a number of post here this evening I'm humbled in saying I'm not planning to use timber frame joinery.  I plan to use lags and Timberlinx connectors, which I will ask about in another thread.

  My planned roof system is open 4"x10" rafters on a 12:12 pitch spanning 16' either side of ridge beam.  I plan to run purlins under rafters close to mid-span  between prowl and loft supported by timbers. Working on re-sizing purlin now as the right side is 33' and I'm going to change from timber to glu-lam.

Issue:  I really would like to have the 12:12 pitch and 36" overhang.  However, this will call for a 26' 4" 4x10 rafter which probably means purchasing a 28' timber.  Is it common, or a good practice, or insane to reduce the span of the rafter by installing two rafters between exterior wall and purlin, and then purlin to ridge beam?  If you are able to do this, what type of connection would work best to connect both rafters to the purlin?  I'm guessing from what I read it would fairly simple if I was going to use timber frame joinery!

Thanks!

Blueforester

  If not, plan B is to change pitch to 10:12 and overhang to 24".

[Carpenter]

It works just fine.  I helped with a Timber framers guild project last summer where the rafters were two piece interupted by the purlin plate.  We attatched the rafters to both the eve plate and the purlin plate with structural torx screws, but I see no reason why lag screws or even heavy spikes wouldn't work.  Good luck with your project!

[jander3]

When I built my cabin foundation, to make use of the materials on hand, I had to split a couple of the joists.  I secured the joists with all-thread, nuts, and washers.  I bolted the joists together and then bolted them to the main support beam.

[Jim_Rogers]

Blueforester:
First of all, let me say, welcome to the forestry forum and the timber framing/log construction section.

  Is it common, or a good practice, or insane to reduce the span of the rafter by installing two rafters between exterior wall and purlin, and then purlin to ridge beam?

This can be done, it is not insane, but each connection of the rafter has to be done carefully and correctly so that it can carry the load of the roof.

I'm not sure if you're aware of it or not, but there is a site where a lot of historic joinery of frames has been illustrated. And you can download these for free.
Go to the Timber Framers Guild website (tfguild.org) and under publications, select historic and on that page select part five and you'll download a pdf file that has lots of drawing of how rafters are done. Some of them show the rafters joining over a principal purlin and how that is done.

If you are able to do this, what type of connection would work best to connect both rafters to the purlin?

As each timber frame is different, and as each frame needs to be understood as a whole; it is tough to say what joint, or which joint would be best for your situation.
You may need to have your frame design reviewed by someone who can give you a good answer to your question. This may take some research on your part to understand all the loads these rafters and the joints will have to hold up.

  If not, plan B is to change pitch to 10:12 and overhang to 24".

It's good to have a plan B; but plan B will have to be evaluated as well.
That can stay as plan B for a while until you research all your options and figure out which is best, and easiest to do.

Jim Rogers

[witterbound]

Are you planning to support that purlin mid span?

[Blueforester]

  Thanks for the replies.  I was beginning to think I had got a little crazy w/ my plan.  I do have the pdf of the the historical joinery and have been studying it.  So, if I understand correctly these connections are not specific to timber frame joinery, traditional that is, and that you can use mechanical fasteners such as screwloks to secure the rafters to the purlin?  If I remember correctly the connection I was considering was one where each rafter end was notched around the purlin such that they meet flush at the top.

  I have acquired quite a bit of data and information on designing and evaluating loads, beam sizes, spans etc. One of the best sources I have found is TFE publishing.  Excellent guides on span tables, deflection and a great CD on using them.  A guy there named Patrick has been extremely helpful in assisting me w/ verifying beams, rafters etc.  And I have to say an early post I read by Jim_Rogers was very helpful and clarified a lot of what I had already been reading about.  Still, this is my first time doing this and I plan to get as much assistance as I can from someone who is in the profession before I do anything.

  I do not plan to use mid-span supports under purlin.  It would put a post in middle of dinning and great room and my wife said no.  Because of the overhang and prow on the front the purlin in the great room will be 33'.  The others are more reasonable in length.  Thus, I'm looking to size a glu-lam for this purlin and will have to do the other purlin glu-lam as well so they will match.

  Not sure on this website how to attach a picture or drawing.  I would be glad to post an image or drawing to illustrate what I have designed to get some suggestion or ideas on them.

Thanks!

Blue

[Jim_Rogers]

You'll have to search this forum for the instruction of how to post a picture. They are here somewhere.
But basically you have to create an album and then put your pictures in that album then you can insert your picture to the message you are posting.

Your design should be reviewed by a professional engineer to get the ok for a 33' purlin with no other supports, other then the ends....

Jim Rogers

[Blueforester]

Yes, Mr. Rogers, you think you are right.  This roof, purlins and couple other items have got me nervous.  I have to say I was a little more at ease w/ the purlin w/  solid single rafters.  After looking at and drawing up what it would like like w/ a split rafter it looks to me like it creates a "hinge" point at this connection.  But then again, I do not know much on timber framing.

  I'm close to finishing plan.  I think I will look into getting a professional opinion on the plan before I do anything.

  Is it ok to ask for referrals on this site?

Blue

[Jim_Rogers]

  Is it ok to ask for referrals on this site?

Blue

Yes, it is. There are many ways to find a engineer who has experience with timber framing designs. One way is to go to the Timber Framers Business Council site and look at the "resource guide." You can download that as well.
But you can do a search online. Enter engineer and your home state, and it should tell you all the timber framing engineer's located or licensed in your state. If you don't put in your state, it will give you a list of all the engineers listed with them. And you can sort through the list and find someone who is licensed in your state. Then you can send them an email or give them a call. You may find more than one listed in your state.

Good luck with your search.
If you need a link to the TFBC site let me know. I think you can access the resource guide from the Guild home page as well.

Jim Rogers

[witterbound]

I'm not sure you need a timber frame engineer.  Seems to me that any regular old structural engineer located in your state would probably be able to analyze your plans. 

[Blueforester]

Thanks for the information.  I will check it out.  I would also welcome referrals from anyone here as well.

  The house plan before this was a regular stick frame w/ the help of a "home designer" that came off a list, and yes, I did find a recommendation on him.  However, things never really went well from the start and after about 6-7 months, may-be longer, and $3K,  we still were not feeling any better so we walked away.  We went back to our original plan of a log home.  And, I decided this time I would work through this design and plan myself.  So, that's my story.

And, I have not told the spouse that, by the way honey we need to get a engineer involved w/ these plans!  Ouch!  She does not know the difference b/w an engineer and a home designer!  Or if she does she will not hear the engineer part.

Blue

[Jim_Rogers]

Blue,
I do hear what you're saying about informing the wife.

However a 33' span is nothing to take lightly. That is for sure.

Being that you're in Alabama, you probably don't have any snow load to consider for your roof, but I'd be surprised if you don't have a large wind load to consider from high winds of hurricanes. 

Whether or not you use a timber frame experienced engineer or not, you could/should have your final plans reviewed before you order your stock.

Good luck with your project.

Jim Rogers

[Blueforester]

Thanks Mr. Rogers, but I really need the in luck in telling the wife.

  No snow load here.  I used 16 psf live load on the roof.  I'm in central Alabama.  We get a tropical storm every now and then.  According to IRC we are in the 90 mph range.

  I've been studing plan and and have came up w/ an idea to lessen the span of this purlin.  First,  5' of this purlin passes through the front at the corner of the of prow and supports the overhang over the prow.  (Cantilever)  The other end will pass through a framed fireplace/ chimney chase then about 5' and supported in the wall of the shed dormer.  I can place a post on the inside of the chimney chase, and one below to the foundation as the basement fireplace is directly below.  The main purlin would then be 22'2" supporting edge to supporting edge.  I could then put a second shorter purlin from the post to the shed dormer wall.  This would actually work better as it was going to be difficult to get a 33' timber.  That is why I was thinking glu-lam.  Is 22'2" extreme for a purlin?  Seems like I should be able to go back w/ a timber.

  I need to figure out how to post a drawing or image.  I sure would like to get some thoughts and ideas on what I have.

  I think I'm going to try sell the idea of a review of the plan to my wife as insurance!

Blue

[Jim_Rogers]

Is 22'2" extreme for a purlin? 

Well, 22' 2" is still kind of long, but it maybe doable. Another way to reduce the span is to add braces from the posts up to the purlin. These would carry some load, however most engineer's don't figure these do carry load, just add to stiffness.

Again, there are instructions here on this site as to how to post drawings or pictures. You just need to find them and then follow them. It isn't hard once you've done it a few times, it will be easy.

[witterbound]

Are you planning to use a ridge beam?

[Blueforester]

Yes, I will have a ridge beam.  I did a preliminary plan a while back to get an idea about what the cost of this home would be.  I priced log the logs timbers and my builder got a take off for the rest of the materials.  We had the ridge beam sized at that time and it called for a 6 3/4"x22" glu-lam.  However, I think at the time that was for a full length ridge beam.  I have since changed my plan to something similar as above w/ the purlin.  I added an intermediate support in an interior wall in the loft at about 1/3 of the span.  Now I have a two ridge beams joined over this post instead of one long ridge beam.  So I think this may reduce the ridge beam size some and maybe cost as well. 

RH

[routestep]

Blueforester if your still interested.... on May 11 2009 Alex had the same type of question; purlin and upper and lower rafter. Go take a look if you have a little time at the discussion.

[Blueforester]

Yes, I'm still planning on a split rafter @ purlin due to the total rafter length of 26' 3".  What I'm trying to figure out now is the best way to make this connection.  Have also decide to go w/ glu-lam purlins due to length of span on one side.

I will check out the other discussion.

Thanks!

BF