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Diameter for roundwood timber framing

 
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Hi, this is my first post on this forum.
I am searching for a way to use small diameter trees for timber framing because it is what I have directly on site and I found Ben Law and is great book named Roundwood Timber Framing
which is a good overview of the thing but I can't find good information about how to choose the right diameter in relation to the supporting weight and the tree species.
Has seen in a previous post Whole Trees by Joel Hollingsworth there is a pdf available named Round small-diameter timber for construction which was published by the Technical Research Center of Finland, but it looks too technical to me to understand.
I am just starting in timber framing and I do have a book on building with timber frame that explain the formulas for milled timber and I tought maybe those techniques may be converted in a way to fit roundwood timber framing.
My location is in the region of Gaspésie, Québec, Canada. The main tree species are mainly, Douglas Fir and Spruce, some maples, white birchs and yellow birchs.
If anyone have any clue on this, please let me know!
 
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Lots of questions:

What are you building..barn, she'd, house ?

Are you using the wood for posts, beams, rafters, floor joists?

What unsupported span do you need for them to cover?


 
pollinator
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It is tough to find anything on material smaller than 6 inch diameter.  These might be helpful as a basic reference to give you "some" idea...




On these charts they have the span in feet to the right for each diameter.  For 6 inch diameter Hemlock at 6 foot span you have 618 pounds per foot allowable load .. 3,708 total pounds.

For a 6 inch diameter Hemlock @12 feet span you have a rating of 87 pounds per foot...  1,044 total pounds supported weight.

One thing to keep in mind is that this is for properly graded and dried wood.  Green wood will support only a fraction of this amount of weight.  Each defect and excessive amounts of checking in the drying process also reduce the amount of load strength as well.

This also assumes a "minimum" diameter of 6 inches, if the pole is 6 inches at one end and 5 at the other you have a good 20% reduction in potential strength, use the smallest diameter to determine span and load rating.  Without being a trained grader I would not expect to get the equivalent ratings you are seeing here.  

Not to worry, just do what I do and overbuild everything, my little brother nicknamed me "Maximum Overkill" many years ago because my form of engineering is to make certain there is no chance of failure...  What I lack in engineering know how I make up for in zeal...  lol  If 4 poles should likely do it I put in 6 to 8 and so on and so forth, but I have yet to ever have anything that I have ever built fail...

Another trick that you can use pretty easily is to clear the branches off two like sized poles and screw a strip of 4 inch or 6 inch plywood on the sides of two small poles one above the other making a rectangular box shape with the poles attached inside the plywood top and bottom.  This way more than doubles the strength of a small diameter pole.  If you are talking about exceptionally small poles in the 4 inch diameter this is something that you might consider.  The other trick is putting roof supports close together and putting plenty of them in.

You can also use a beam spanning the middle of your roof support to cut the span in half without cluttering up the inside of your work area, just support the beam well at the ends.
 
Roy Edward Long
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This a youtube video showing the building of a "small" round pole structure out of green undried small diameter round poles.  Pretty simple and nothing special but it gives a visual idea of what you could expect for strength and potentially some basic design ideas...

 
pollinator
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I usually reference span tables for dimensional lumber, treating a pole as whatever #×# could be milled from it. The extra bits that would be wasted slabs if milled are bonus material. This should lead to a pleasing amount of overkill.


I tend to use things in the round for posts and braces more than other uses, and it is pretty hard to end up with poles as the weak part.. With beams and rafters, a taller narrower piece of wood has such a huge strength advantage that I really prefer to use milled material.

I have a very limited amount of timber available, though. If I had lots, the math would be different.

If I had lots and it was largely smaller stuff, I would be thinking about building trusses. Really that stacked pole thing is a very very simple truss..
 
Roy Edward Long
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Myself I own a sawmill and have a large forest of good sized trees so I don't generally find myself using poles to build with.  I have in the past done a lot of pole building though I did not use "very small" diameter poles, I am not sure what the OP is defining as small but I could envision 4 to 6 inch sapling poles.  This being the case it is going it may take a bit of over engineering to get a good strong lasting roof frame.

At four inch to six inch poles your "dimensional" lumber equivalent comes out at about a 3 x 3, with "spruce" that isn't a whole lot to work with for span strength.  If the OP is talking about 6 to 8 inches then that isn't too bad to work with but at this point we really don't have any idea on actual pole size or even shop size or design yet.  With some better definition we can zero in on better advice...
 
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treating a pole as whatever #×# could be milled from it. The extra bits that would be wasted slabs if milled are bonus material. This should lead to a pleasing amount of overkill.  



Yes and no. It’s a good general rule of thumb that should mostly work, but not a hard and fast rule that would substitute for graded lumber. I mill my own lumber on a woodmizer, and I know the guidelines of NELMA for grading, so that helps some, but according to the engineers, kiln dried lumber has more strength than equal sized rough sawn air dried lumber, owing to how the fibers interlock differently during kiln drying. On the other hand, there’s a lot of crap at the lumber yards that amazes me how it could pass grade.
One thing I would definitely do with round timbers is peel the logs when green, to speed drying and to observe defects both existing and as they form during drying (cracks, splits). I’m not a huge fan of framing with roundwood, since you can’t see the inside at ALL, and have the potential to end up with something like a section with heart rot that is weak but hidden.
 
D Nikolls
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Julie Reed wrote:

treating a pole as whatever #×# could be milled from it. The extra bits that would be wasted slabs if milled are bonus material. This should lead to a pleasing amount of overkill.  



Yes and no. It’s a good general rule of thumb that should mostly work, but not a hard and fast rule that would substitute for graded lumber. I mill my own lumber on a woodmizer, and I know the guidelines of NELMA for grading, so that helps some, but according to the engineers, kiln dried lumber has more strength than equal sized rough sawn air dried lumber, owing to how the fibers interlock differently during kiln drying. On the other hand, there’s a lot of crap at the lumber yards that amazes me how it could pass grade.
One thing I would definitely do with round timbers is peel the logs when green, to speed drying and to observe defects both existing and as they form during drying (cracks, splits). I’m not a huge fan of framing with roundwood, since you can’t see the inside at ALL, and have the potential to end up with something like a section with heart rot that is weak but hidden.



Excellent points. I try hard to peel everything that will be used in the round immediately. Among other things it really cuts down on insect issues.

I firmly believe I am ending up with a better product(for posts and braces!) than the average #2 material in your local yard. As you note, there is some astonishing junk out there. Even SS graded can be quite hit or miss. I feel much better about how slow milling can be, in light of the crazy amount of wood I pick through seeking decent stuff at the lumberyard.

True, you can't see the inside, but I saw the tree standing and the end-grain of each piece can be informative. I have yet to find heart-rot in a tree without any sign of it from either the outside, top, or bottom.

I have a (very small, elderly, battered) bandsaw mill, and love having it... yet have no enthusiam for turning out a 4" post out of a log 7.5" in diameter at the base and 5.25" at the top...

It is my belief that structurally significant cracking is made more likely by this milling, too.

If a cedar log is iffy, as most of cedar is, I mill it into 1x siding; even 2x or 4x material can hide too much if the log is patchy...

Please, nobody take this as even vaguely definitive, my volume is tiny and my trees aren't your trees!
 
Julie Reed
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 I have yet to find heart-rot in a tree without any sign of it from either the outside, top, or bottom.  



I haven’t seen it very often, but do encounter it here and there, mostly in hardwood. Not sure if it’s insect or fungus related. The other thing that can stay hidden is shake, and it can also occur in the drying process, so may not be initially observed on either end, and of course can’t be seen in unsawn logs except on the ends. As was previously mentioned, using smaller roundwood for vertical applications or bracing (or in a truss system) is far better than the risk of horizontal load bearing.

It is my belief that structurally significant cracking is made more likely by this milling, too.  



That’s correct, as logs- especially from trees in high wind areas- can have internal tension which may be released during milling, leading to twisting, bending or cracking/splitting (sometimes all 3). Additionally a lot of cracking also occurs due to improper drying of the lumber or even improper storage of logs prior to milling.
 
Gabriel Babin
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Hi there,
I am sorry for the delay. I missed my email alert because I unsubscribe to it by accident.
My researched continued about round wood timber framing. As well as my construction plan.
The idea changed a bit.
I want to build a house with a small shop and firewood storage added to the North.
I chose a salt box desing. The main house would be a two story 20” x 30” with a 10x30 added to the north. The roof as a simple 45 degrees angle.

I have choices of wood on my 33 acres of forest between spruce and fir. Many 6 and 8 inches diameter, some 10 (always talking about smaller end) going to max 18” long for some of them. I could buy some pine to get bigger trees.

Thanks for the advised, very appreciate
 
Gabriel Babin
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(I forgot to talk about span length...and a lot more, I have a lot to talk about this subject)

About span lenght:
On the ground floor, I have a unsupported span of 15 feet. I found an exemple of what my house could look like but with round wood instead.

Also, I want to do, if possible, the entire structure out of debarked round wood. I want to discover ways of using smaller diameter trees and use less wood, even at the cost of great effort. Another reason is that, to me, the natural shapes of trees are very pleasant to the eyes and appeal me beyond traditional timber framing). There are some exemples of interior of houses that integrate roundwood and strawbale wall construction.  The wood I would use  would be green, harvest from last winter OR the few last ones and stored hidden from the sun and well ventilate.
The floor support beams would be sawn on one side to make a level floor upstairs.
I like the idea of multiples small poles put next to each other to use has rafters (exemple in the video below).


I discovered Ben Law from UK. A green woodworker and a round wood timber framer with his famous cruck frame, or A frame design made from roundwood. In his book ''Round Wood Timber Framing'', he describes well the different joints he uses.  Their is a video that shows the auto-construction of his house and this video was really a inspiring to me because it was the closest to how I see my construction, its also very instructive.


The cruck frame could be a solution for me (although I don't know where I could find such long and straight trees).
It's mainly pieces that goes from the corners of the bottom floor to the higher point of the roof. My only fear is that those timbers would make quite big obstacles to the sun trying to enter the house. In fact, the sun that heat the concrete slab of my house is an important source of heat in my design. The other is wood burning from a batch box rocket stove that would also heat water to go trough the concrete slab.

I agree with the idea of Roy Long to jut overkill the minimum strength  requirement wit bigger diameter then what is asked or adding more bents to the frame.

Also, I can't just follow a chart and compare my wood with it. Checkings appearing overtime, important knots (because spruce as a lot of them as I know), tapering of wood trough the end. These are all affecting the strength of the timber. Ben Law have his general rule of thumb for his wood. He says that round timber is 50% stronger then the square milled wood you could get from it. I know that round wood has a few advantages over milled wood :

First, the round shape is stronger than the squared one.

Second, if you leave the shape of the tree as it was made, you keep the entire fiber structure and keep the tree has strong has it was standing in the forest, facing wind and bad waether. I also heard about japanese temple builders who places the round post in the same cardinal points alignement has it had standing in the woods. But that is just next level.

That said, I would not compare my wood withkiln dried, knotless, perfect wood. I would probably compare my round timbers to his equivalent once sawn.

I know that has Canadians and Americans, we have experiences with roundwood framing because of our log homes construction history. I did not find information about round wood structures and knowledge coming from log homes but I am sure there is some.
The Japanese have been using round wood also but I know only a littre about that. I am still very curious.

If you have any advise or knowleges to share, please do.
 
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Love those roundwood structures! I am using round hardwood logs for the framing of my current addition (oak, ash, black cherry and hickory).

The A-frames in those structures are different from traditional cruck framing; bent logs are typically used, sometimes bent enough that they form most of both wall and roof framing. Straight timbers definitely obstruct more floor space than bent ones. Ideally you find trees with one pronounced bend yet straight in the other plane, and if they are big enough saw them in half so they form a symmetrical arch-like frame. Smaller cruck members ("blades") can just be paired so that they balance each other visually. For a 15' clear span cruck and two story height, I would be comfortable using hardwood at least 6" diameter at the small end. That would likely be around 10" diameter at the base. Including a second floor beam connecting the cruck blades, this would be seriously strong and rigid.

Beams of roundwood are likely to be limited more by deflection than ultimate strength. You don't want a bouncy floor or one that bends as you put loads on it, and in most cases if it does not bend noticeably under full load it will be plenty strong. A short, heavily loaded beam could fail by breaking before it bends noticeably. Joints are important, as done wrong they can introduce weak points that will fail prematurely.
 
Julie Reed
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Ben Law have his general rule of thumb for his wood. He says that round timber is 50% stronger then the square milled wood you could get from it.

I have heard this before from other sources, but never any data to confirm it. Does he mention a source for that claim, or is it merely his opinion based on experience? 50% is a lot. It may be true, but I think it depends a lot on the species of wood, as well as how it is prepared and in what position it is used in the construction. As a quick example, I could mill a 4x4 from a straight 5.5” log. Was the 5.5” log 50% stronger? How does one determine that? Is the use vertical, sloped or horizontal? The end use matters the most.
It’s also not typical to use much square milled wood, other than vertical posts. Most construction is done with lumber that has a rectangular profile (2x4, 2x6, 3x12, etc). Horizontal members such as a header or joist bear the most loads at right angles to the fibers of the wood. So in a position where you may want, say, a 4x8 support beam, the 10” log it would take to mill that beam will easily be twice as strong, but that’s also because you could get 2 of those beams from that log (nominal lumber sizes. To get full 8” would need a 12” log for 2 beams). So it’s overkill for sure, but also a lot of extra resources and weight. On the other hand, it’s simpler to peel logs than to mill them.
I think you’ll be fine with round wood, especially with a 12/12 roof pitch.
 
Glenn Herbert
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A 30' long timber frame would require no more than three bays of 10' each, which means only two cruck frames in the middle of the length. That is not more than you would have to have no matter what kind of framing you use.

Depending on the wood used and spacing and other details, I could see cruck blades of 5" tapering from 8" diameter.
 
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I am in the process of building something that's very much on the lines you want to build. I took my drawings to an engineering firm that does timber frame designs to get them reviewed, formalized and certified by an engineer (which translated here into passing code for our building permits).
Our posts are a minimum ten inches in diameter, per specs. In practice, they'll mostly be larger. The engineer's drawings spec Red pine No. 2 or better - we're using maple and oak. Beams are specified at eleven inches diameter and I'm working to stay on that rather than going much larger. Rafters, which will span approximately 14', are specified at either 7 or 8 inches in diameter, with the spacing between them significantly smaller for the 7 inch rafters. This may be our hardest element to fill, as the growth patterns of our trees taper  enough that finding something thirty five feet in length that's only 7 or 8 inches in diameter is hard ;) We may need to do many more shorter poles, rather than spanning the entire roof in one length.

We're in an area with a 50 inch snow load, so that strongly influenced the specifications for our roof.

We are not doing a cruck frame as Ben Law generally uses, but rather adapted a conventional milled lumber framing plan to our specific design goals.
 
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Howdy,

Heres a book that covers round wood building really well.    The Short Log and Timber Building Book,  by James Mitchell

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Glenn Herbert
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Looks like a good book. Currently available on Abebooks: Short Log & Timber Building for as low as $18...
 
Roy Edward Long
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So did you ever build anything yet Gabriel Babin?

I am getting geared up to build a secondary house on the upper half of our farm, it won't be round timber framed or anything but I will have some pretty good sized milled beams in it.

Would love to see any pictures of what you may have done at this point...
 
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