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Author Topic: Rafters for cabin  (Read 190 times)

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Offline Nebraska

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Rafters for cabin
« on: November 09, 2019, 12:00:23 PM »
My question is, if I saw 3 1/2 by 8 or 10" rafters free of heart center, either green ash or Ponderosa pine (it's what  readily available)  should I cut a relief groove on the heart side of the beam to lesson the checking as it dries,  if so  how deep and how wide?? Circular saw or router and jig seems the best options to me.. The what for is below.

In pursuing my second childhood, I have been dreaming scheming and planning a cabin, ever since I got my pond put in. I've gotten electricity to the site and a water line in.  The  cabin is going to be 14x24, 12/12 roof pitch,  3 ft eves and over hangs,  8ft porch on the front.  Rafters are going to be 3 1/2x8 spaced  at whatever width works for snow load, trying to leave 4 " exposed  beam to below. 
Front wall under the roof will be log, thinking of trying vertical logs (different look) with splines glued and pinned together with  timber screws or rebar. Logs will be attached to a bottom plate and a top plate,  Cathedral ceiling with a loft  to 12 ft,  beams  from front to back to give the vertical log wall rigidity. 8 x8 corner posts with conventionally framed walls on the other three sides, (easier to run wiring and insulate) covered with (board and batten, live edge, or log siding) I haven't fully decided. I have  big I beams to sit on poured columns  for a foundation. Most of the work I will do myself with contractor oversight.  When this  gets closer I will start a build thread.   Still planning on approx summer after next.


Offline Don P

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Re: Rafters for cabin
« Reply #1 on: November 09, 2019, 09:37:44 PM »
No, you increase the risk of a horizontal shear if you kerf the wide face. With FOHC checking is reduced. You'll be ahead of the game if you saw and sticker the materials now and let things dry and move before assembly. Then resaw or plane the cupping and warp out.

Your description of vertical log construction brought to mind the oldest church known, Greensted in England, C.A. Hewett's "English Historic Carpentry" has very good sketches of the construction but I can't find those sketches online. The stavkirks of Scandanavia are similar, this wiki although not in English has some good sketches showing the plates and the wall bracing in a tall, large church;
https://ca.wikipedia.org/wiki/Stavkirke. If you hit the English tab on the left column there it takes you to the English page which has other pics. Google "stabbur" for some more vertical log/plank work. Anyway just grist for the mill.

On your foundation, the weak link there is the hinge between the beam and piers, think about welding up a rigid connection that connects the two to resist pier overturning.

Sounds like a fun project :)
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline Nebraska

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Re: Rafters for cabin
« Reply #2 on: November 10, 2019, 09:44:26 PM »

@Don P 
Thank you for the reply.

Yes it was a picture of an old Norwegian church picture that turned wheels, also the fact that logs check radially towards their center,  and when laid horizontally some of the checks point the wrong way and hold water.  Since this cabin has absolutely no practical purpose but entertaining, it can be what I vision (with approval from the boss ;)). I was reading regarding pine tar and the old church came up figured it's worth a try. Water goes down  and away from the wood.

The kerf I was contemplating was on the narrow face centered on the inner growth rings, and  that would be the invisible  face that would be on  the top of the rafter. I am milling them a little over sized and they will dry for over a year(most likely 2 or 3).  Plan is to set them on the mill to true them up before final use. With the he kerf cut  I was hoping to lesson the checking on the other three visible faces as they dry.  I thought may be 3/4 to 1 inch depth.

My attachment of the I/H beams to the concrete piers was going to be heavy j bolts anchored to the rebar in the forms. With a lapped welded flat iron holding the beam flat. Then crossed steel rods welded between the two beams ...long skinny sideways xs  1 every 8ft,  and maybe diagonal bracing. Should be pretty bullet proof.

Offline Don P

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Re: Rafters for cabin
« Reply #3 on: November 10, 2019, 10:21:01 PM »
A check typically follows the shortest (weakest) path from heart to bark, so a shallow kerf on the narrow face probably won't do much. As you get away from the heart usually the timber cups more than it checks.

I built a few log homes that had log "siding" that was basically half logs split down the heart and applied to a 2x6 frame wall. A few of those had full logs up to 8' and then the half log above. The full logs would of course all check. Most of the half logs had minor checking and a convex back face. The saw cut had provided the "kerf" and allowed the differential tangential/radial drying stress to simply cup that face. So most of the checking potential is relieved when you saw free of heart. The downside is that since the sawing is unbalanced around the heart you may release growth stress that causes the timber to bow. You'll see that as you are sawing if that is going to be a problem.
A laborer works with his hands
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An artist uses his brain, his hands, and his heart

Offline Nebraska

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Re: Rafters for cabin
« Reply #4 on: Yesterday at 07:23:20 AM »
Ok that makes sense.  I will remove minor drying issues when I true them up before use, if not they become, dunnage,  a resawn 2x6 or firewood. Thanks.


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