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Rafter splice / scarf joints?

Started by jeremy86, September 29, 2016, 03:13:00 PM

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jeremy86

Hello,
I am building a timber frame home, and sawing all my own logs; I have a band  sawmill.  I have ran into a bit of a conundrum; my sawmill max sawing length is 16'-0" and the rafters on my timber frame (28'x26' High Post Cape design 7 x 10 rafters) are longer that my sawmill can take.  So, my first thought was to do a scarf joint of some sort for the rafters.  My question is what would be the best type of scarf joint to do?  I had thought about a "Bridal scarf Joint" for the simplicity, but not for sure if this would be the best?  Or, is there a better way?
Thank you for your time!

Dave Shepard

Unless you have a purlin plate, I would find a way to saw longer on your mill. You are talking about a ton of extra work, and I'm not sure there is really a good scarf for a rafter. Most scarf joints are supported.
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ChugiakTinkerer

An extension to the sawmill bed would be the least amount of work, in my mind, and wouldn't pose any potential issues for your rafters.  If you haven't looked yet into the Sawmill forum you'll find lots of threads where people have done just that.  If you were milling just one or two timbers it would probably be easier to just do the saw and scoot method, but for a dozen or two rafters it seems like a much better option to extend the rails.  Most manufacturers sell extension kits, or you could make your own out of angle iron or other appropriate steel stock.  I need an extra 2' on my sawmill in order to cut the timbers that I will need.  My plan is to weld up a parking space for the sawhead that is only 3' long or so and use the factory rail bed for actual cutting.  That will enable me to saw up to 18'.
Woodland Mills HM130

ChugiakTinkerer

By the way, welcome to the forum!  If you update your profile to indicate where you live, it will help folks give you more relevant and helpful advice.  Also, please let us know about your band sawmill.
Woodland Mills HM130

jeremy86

Thank you everyone for the info, I too thought of just adding on to the sawmill. 

Brad_bb

Jeremy86, welcome!

Lessons I learned from my first timberframe build:
1. Design your frame to use shorter rafters, or design the rafters to be broken into two pieces(assuming common rafter design).
2.Long rafters can bow a lot, especially in white oak.  This will force you to do a lot of blocking and ratchet strapping to straighten them as you fix sheathing in place to hold them in place.
3. Split heart Oak Rafters cost a lot more and will not stop much of the bowing and twisting in white oak.
4. Free Of Heart (FOH) long rafters in White Oak will cost up to 6X as much from the mill.

Of course the aforementioned applies to common rafters.  If your frame design is a principle rafter frame, then no you should not scarf two pieces together.  A principle rafter works as a single member of a bent structure.    It would really help to see your frame design.  I know you said your rafters are 7x10 which makes me think it's a principle rafter design.  On the other hand, It also makes me wonder whether it's a rafter or a purlin.  Seeing the frame would help. in a 7x10, I'd use a boxed heart timber.   What material is the frame designed for?
Brad
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

jeremy86

Brad,
It is a principle rafter setup, with 3 bents, so 3 principle rafter sets.  The main material will be white oak, but I have several other logs red oak, great straight solid sycamore, also some black walnut.  I have my own sawmill that I am doing all the milling with, so I am not to worried about price. ;)  As far as the plans I am a mechanical engineer so I am drawing all the plans out myself and haven't got them completely done.  But my design is based off the High Post Cape design in Steve Chappell's book " A Timber Framer's Workshop".  I have modified some of the plans, basically the frame plan is a 24' x 24' I have changed that to 26' x 28' to fit my needs a little bit better; 26' would also be the bent width.
You had mentioned braking the rafters into tow pieces if common rafter design.  Could you elaborate on this? 
Also, I believe that the original frame design from the book was designed for white pine.  But, I figured that since I am making it a little bit bigger and with hardwood I would just stick with all the same timber dimensions.  Thanks for all the help!

Brad_bb

So in this first picture, the common rafters are 30 feet long.  I have the first two pair that overhang the gable end continuous.  In retrospect, they could have been broken at the points indicated into two shorter pieces.  The rest of the rafters are broken into two and still we had a lot of bow that we had to pull out as we did the first sheathing.


 

We could have broken the common rafters in this frame, but we weren't smart enough to do that at the time and had continuous rafters which cost a lot more.


 

In your case of principle rafters, you need continuous pieces.  These are two force members and with any dynamic movement, well, I would want them continuous.  buy some  bed extensions for your mill and mill the correct length.  Hopefully you have a forklift or FEL  to move the logs.  You can mill them with just your mill, but it takes a lot more messing around to cut 16 ft, then cut the slab off to get your mill head free, then reposition the log to finish the cut.  First side is the toughest to reset the log correctly with wedges and blocks.  Last resort is to order these pieces from a mill.  Boxed heart the cost shouldn't be too bad compared the other cuts I mentioned (split heart and FOH).  I'm an ME too.  I would still pay to run the design by an engineer experienced in timberframes -like Firetower.  They'll also spec your foundation specs and connections.
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

S.Hyland

Quote from: Brad_bb on October 03, 2016, 09:56:58 PM
Jeremy86, welcome!

Lessons I learned from my first timberframe build:
1. Design your frame to use shorter rafters, or design the rafters to be broken into two pieces(assuming common rafter design).
2.Long rafters can bow a lot, especially in white oak.  This will force you to do a lot of blocking and ratchet strapping to straighten them as you fix sheathing in place to hold them in place.
3. Split heart Oak Rafters cost a lot more and will not stop much of the bowing and twisting in white oak.
4. Free Of Heart (FOH) long rafters in White Oak will cost up to 6X as much from the mill.



Hi Brad,
   I have a couple observations on the points you made on common rafters. I would actually prefer rafters to be boxed heart. Split heart in hardwood can be exceptionally wild and unstable. I'm not quite sure why one would want FOH Oak rafters either, certainly not worth the upcharge. Can you explain why these were even considered as options?
   I actually like to use softwood rafters many times on an Oak frame. It cuts down on the dead load for the rest of the structure and can be a neat contrast.
   I would also argue that there are good reasons to leave common rafters as long as possible. Material cost is a factor as you mentioned, but one also has to factor in crane time and extra fabrication time for split rafters. One piece is always better structurally than two, as well.  Picking twice the number of common rafters can easily outweigh any material savings gained by splitting the rafters. There are a lot of situations where it is desirable and expedient to split the rafters on the plate, but I wouldn't say that it is preferable every time it is possible.

Nice pictures! Are they some of your timber framed work?
"It may be that when we no longer know which way to go that we have come to our real journey. The mind that is not baffled is not employed. The impeded stream is the one that sings."
― Wendell Berry

Brad_bb

A lot of good points!  When the common rafters are 4x6 x 20ft or 4x8 x 30ft, It's a bit hard to box the heart and keep the heart boxed ;)  Too much chance for defect, grain direction change etc.  If they were a bigger cross section....

When the material cost goes to 6x for FOH, It can definitely be worth splitting even considering the labor and crane.  With these cross sections, and lengths, split heart(Which is actually not split down the very middle but is close) a good number of sticks moved a lot, bow, crown and twist. FOH yielded mostly just a bow, which is much easier to work with.  Had we done more splitting of rafters, it would have reduced the size, length, and quality of log/tree from which it could be cut.  This would have lowered material cost very significantly.  Even if it were a wash with labor, it would be preferable not to have to fight the degree of bowing that we dealt with.  They all look straight now.

Jeremy86's design is much different from what I was dealing with, and he should use continuous principle rafters.  The lesson I learned was a stinging one.  I hope someone else could learn from my experience and not have to learn the hard was as I did-  with a shocking bill from the mill.
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

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