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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 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 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...
 
pollinator
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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
pollinator
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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.
 
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