Scribing Timbers to Localize Checking

[Andries]

A friend has a few hundred red pine timbers stacked in a hay barn.
The dried timbers were amazing to me because there were only a few that showed checks or cracks. . . . except for on one face.
He explained that they had been scribed. "Pick the worst face and run a 3/4" deep kerf with a thin blade down the length of the timber."
So, his week I figured to give it a try on some bur oak.



Yeah, that metal stain is the worst face.



I know that a few of you mill timber frames frequently (@Jim_Rogers, @Brad_bb  @Peter Drouin and others) - have you done this as well, or is it one of those hit and miss techniques?
Also, does this compromise the strength of a 6x6 or 8x8 timber very much?

[Brad_bb]

No I don't do that.  I mill hardwood only.  Most of my timbers have been for my buildings- my shop, barn, mini-barns, my mom's house, and my upcoming house.  Typically a timber only checks on one side.  I presume that by kerfing like that you are trying to control what face it checks on.  That is fine for a post, but I don't think I'd want that on a horizontal beam. Checking is dis-continuous as it goes down the timber.  Meaning it stop and starts, stops and starts.  It is not considered a defect. I'd also hazard to say that in a boxed heart beam, the checking will occur mostly near the neutral plane.  Will the kerf make the check continuous?  In a horizontal beam, I don't think I'd want that.

I would also assume that you don't like the look of checking so you would face those kerfs to the outside so they are on faces you won't see when finished.  If you did see those faces, you'd see more of a straight line versus a natural looking check.  The kerf will now stop checking, it will just encourage it to be on that face.  You are creating a weakness in the face so as drying stress occurs, it will have a tendency to relieve the stress by checking on that face.  You may find though, that once in awhile, it may have a mind of it's own and check on another face, especially in hardwood.

You should also consider whether kerfing will limit your choice of faces for layout.  For layout you want the two faces that are the most perpendicular to each other.  I'm not sure if this will limit you?  

One thing I've done a lot is cut my timbers 1/2 inch oversize, put them in my pole barn for 1-4 years, then put them back on the mill and use my mill mounted beam planer to square them up again.  By then Most of the movement is finished.  The benefit is that between cutting joinery and raising, nothing should really change.  It's a side benefit to making timbers for yourself and while you're still cutting others, some are air drying naturally.

[Andries]

Thanks Brad, this is for 6x6 and 8x8 timbers, which right now is bur oak, but can also be ash or elm.
The dimensions of these timbers are oversized for re-sizing later, once dried.



I'm trying to choose the "worst" side or face to scribe, and minimize the checking on the best three faces. Hopefully.  : 



I've been scribing the face closest to the pith because that face usually sees the worst checking.

[Don P]

I've done it and it does work in a squarish timber. Checking normally follows the shortest path from heart to outside, the weakest path, as you mentioned. You are hopefully creating that weakness and choosing the checking face. In a beam orient that up or down and there is no strength penalty. It is done in wall logs commonly enough that it is mentioned in the log home standards in this country. I've seen it mentioned by English green oak builders as well.

I had a surprise order show up for one of my wooden buckets last night, there was only one completed one left hanging from the ceiling down in the shop this morning, oops, there's about 10 partials on a shelf I need to finish up. It was oak bottomed and I must have swept some kind of iron dust through the floor above down into it at some point, it looked bad, there were iron speckle marks in the bottom of the bucket and I needed to clean it up and shoot some finish on it and a tray today. I scrounged and found the jar of oxalic acid and scooped a teaspoon or so into a cup and added a little warm water then dipped a rag into that and worked it into the bottom. About 10 minutes later the metal stains were gone. I mopped it out with some clean water, let it dry, light sand, sprayed and life is good.

[Andries]

Thanks Don. There's a lot of metal in my arborist sourced trees, so the oxalic acid tip is a good one. 
There's got to be some lyrics about heavy metal, bands and sawmills in some Country song. 

[Peter Drouin]

I have never had anyone ask me to do it or seen anyone do that, I do think it will compromise the beam.

[Don P]

Hang on, let's look at the heavy timber grading rules on checks...
Here it is,
Select Struc;

Checks – Seasoning checks, single or opposite each other with a sum total equal to 1/4 the thickness.

#1, same language. #2;

Characteristics and limiting provisions are: Checks – Seasoning checks.

Wall log allows kerfing up to 1/2 depth but that is not an unsupported beam.
Applying a little logic here. The concern in a beam is horizontal shear. If the kerf is oriented up or down you are encouraging a vertical separation and reducing the chances of horizontal shear. This is a positive rather than a negative. Think about a built up beam, those are all essentially vertical breaks, there is no problem there. I can saw vertically completely through the beam, stick it back together without a problem. Occasionally we will see someone try to build up a beam of horizontal members, no go, that is viewed as the sum of the section moduli of the flat timbers laid side by side rather than stacked... the square root of the vertical section strengthwise. If oriented vertically you are helping control rather than taking your chances, a win-win.

[Andries]

The thinking behind scribing the timber is the same as placing joints or seam lines in concrete.
The seam or joint is an intentional weak spot, so that when frost heaves the pad, it will flex or crack at the joint. 
With timbers, it's the same idea. 
Lets say that I have a 6x6 hardwood timber which has the heart centred at one end but close to a face at the other. Experience shows that most, but not all, of the checking will occur where the pith comes close to the surface. The saw kerf is a minor weakness (like the seam in the concrete pad)and all (or most) of the checking should happen there.
That's the timber face that I'll run the 3/4" kerf, down the centre of the face.
The scribing that I'm doing isn't on all the timbers, just on the ones that will be used as posts.

[Don P]

On posts and timbers the allowable checking is less restrictive than on beams and stringers, for SS and #1;

Checks – Seasoning checks, single or opposite each other with a sum total equal to 1/2 the thickness of the piece. 

[firefighter ontheside]

All of the logs in my red pine log home have been kerfed with a chain saw.  The wall logs are kerfed where the logs come together so that it cannot be seen.  The log purlins are kerfed on top.  These logs were all green when the home was built.  There is very little checking visible anywhere in the home.  From looking at other companies homes, before I settled on this guy, checking in other homes is a big problem.  It works.  Kind of like cutting a kerf in poured concrete slabs.

[JRWoodchuck]

I've seen it done a lot in Japanese timber framing/joinery. Those guys are pretty good at building things. Like Don pointed out it all depends on orientation for strength. 

[kantuckid]

To localize seasons cracks:
Based on Don P having made the suggestion, my EWP "D" wall logs, used in stack wall construction, will get a double planer run, down the center of the underneath side on each log as I cut the dovetail corners.
 Will be done with the Dewalt 680 hand power planer I have coming in the mail. The plan/hope is keeps some season cracks off the interior face which shows or the bark edges out in the weather.
If I make it another 7 years or so past the walls going up, I'll let you know how it well it works. :D  For now I'm using his experience as my teacher. 

[Andries]

I'm scribing timbers, as weather allows.
I'm hoping that it helps localize the checking, and that I really did pick the worst face to scribe.
Time will tell, and I'm with you KKID,  others' experience will be my guide.
Let's compare results in a few years ( maybe not seven). 😆

[kantuckid]

My seven years notion comes from stuff I've read on how long for a log wall built green to settle. My log home (the walls) was born in 1979.