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Author Topic: Does anyone oversize green timber to allow for shrinkage?  (Read 2247 times)

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Offline EaglePine

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Does anyone oversize green timber to allow for shrinkage?
« on: November 19, 2010, 06:52:08 PM »
I may be about to begin accumulating some spruce timbers for a future timber frame project. The trees are still standing, and the milling will be done by portable bandsaw mill.  I'm somewhat familiar with how slow timbers will dry and shrink, so was just pondering the benefits or lack thereof  of  oversizing  say an 8" x 8" timber by 1/8 to 1/4 "  per side. Also this might allow me to plane them if required ( that is if I had a wide surface planer which isn't out of the question).
   Any threads on this? I couldn't find much, at least not in the short time I looked, so apologies for my laziness.
Any advise appreciated, Thanks

Offline LOGDOG

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Re: Does anyone oversize green timber to allow for shrinkage?
« Reply #1 on: November 19, 2010, 09:02:48 PM »
EaglePine,

   I think I'd probably oversize the beams by 1/8" per inch of desired thickness. I guess it depends on how long you intend to dry them, how you stack and weight them, what moisture content you take them down to. The log home we built for my parents up in WI was built from Norway (Red) Pine. We peeled and two sided the logs and then stacked them. We covered the stacks with large tarps, but the sides were open for air flow. I guess we dried those things that way for 4 years or so. When I came back to mill them the shrinkage was definitely visible, plus there was a bit of twist in some of them. Then there was a bit of soft rot that had formed where the wide flat face of the logs touched each other in some of the logs. I milled them back to accurate thickness dimensions, void of twist and degrade. At the top of the last course in the house we were less than an 1/8 " out of whack anywhere in the wall height. That's not bad for log building.

   Given the chance to do it over again I'd peel the logs and not mill them until after they'd dried for several years. Then take the sides off and do it in one shot. Your situation may be a bit different though since you're dealing with timbers that may be substantially smaller than the overall log diameter. You may want to get some of the meat off the sides to get the drying accelerated in the beam itself. But yes, I'd oversize. I think the spruce may shrink less than the Red Pine.

Offline LOGDOG

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Re: Does anyone oversize green timber to allow for shrinkage?
« Reply #2 on: November 19, 2010, 09:21:03 PM »
One other thing I'd do differently is find some 4" pipe and use that between the layers of logs/beams. That way just the crown of the pipe is touch the log/beam surface. It would let the wood breathe better and probably prevent some of the degrade we experienced from having the wide, flat surfaces touching each other.

Offline Jim_Rogers

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Re: Does anyone oversize green timber to allow for shrinkage?
« Reply #3 on: November 20, 2010, 08:17:20 AM »
There are lots of things to consider when you're at this planning stage.

One is what type of joinery are you going to do? Square rule joinery allows you to use off size timbers and if they have shrank/shrunk it doesn't matter. If you're not doing square rule then you may need oversized timbers.

The next thing is are you going to plane them? If you are going to plane them then yes oversize them by some amount.
The planer mill near me wants 1/4" on each face so that's 1/2" oversized. If you're going to plane them yourself then you may be able to get away with less.

Another thing to consider is the type of wood. Spruce is known to twist a lot, at least the one's I've milled have, in my area. If you think yours are going to twist and you intend to cut the twist out of them, then yes oversize.

Stacking and proper sticker size is very important to storing timbers for drying. As mentioned the wide sticker is not the best solution. You need narrow stickers and they should be dry, at least air dried if not kiln dried to prevent sticker stain, shadow or rot from happening.

The only piece of lumber that we normally oversize for planning on shrinkage is the brace stock.

Braces normally are either 4x6 or 3x5 for example. With 4x6 stock the tenon layout is usually 2" off the layout face and then 2" thick. If the 4" piece of brace stock has dried some, since milling, then it may not be a full 4" thick any more. So what do you do with a piece of brace stock that isn't 4" wide any more? Let's say it's only 3 7/8" due to shrinkage.

If you change the offset to 1 7/8" so that you can have a full 2" tenon then the brace will not be flush with the surface of the post or tie beam. And that maybe ok, but it may not be ok as well, it may look like crap. I guess it depends on what you want to see or don't see.

If you change the tenon size to 1 7/8" then the tenon will be sloppy in the mortise. That is not good. The reason that is not good is that "strength/stress goes to stiffness." What that means is that when a tenon is sloppy in a joint; and the stresses of the frame need to be supported this sloppy tenon doesn't do it's job. So some other joint in the frame has to do it's job as well as it's own job. That's double loading of a joint. This could be ok, but many times it is not. And will/could cause some problems.

Bottom line is that every joint should be right.

You can make your mortise less then 2" so that the 1 7/8" tenon can work, but that means that every tenon this is undersized can only go into that mortise. And that means that you have to process your stock in the right order so that you can "map" every tenon and every mortise so that every tenon has a home mortise that matches. It can be done but it's a lot of extra work do to it.

The solution is to make your tenons the right size by having your brace stock milled oversized at the beginning.

By the way, welcome to the forum.

Jim Rogers
 
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Offline EaglePine

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Re: Does anyone oversize green timber to allow for shrinkage?
« Reply #4 on: November 22, 2010, 07:35:29 PM »
 Thank you for the help, to over size or not to?   This will be my second attempt at getting some timbers together for a frame.
 The first batch ended up in someone else's frame and they seemed to work out OK. If you check my posts you may come across some photos.  That bunch was black spruce, and was properly stacked and stickered, and spent almost four years
between sawing and framing.  Even after that time the center MC was still in the 20% range when fresh cut mortises were checked with the meter.
      I'm just blabbing away here,  square rule method would probably be my first choice for joinery, I just want to have as many options as possible. I guess it is a bit of a non issue, but oversizing would allow for the option of planing later.
     I would just wait until I had more detailed plans of the frame, but sometimes some locally grown trees of good quality
and quantity become available, and that  opportunity just doesn't come around when you are ready for it.
   I guess the main focus is getting the logs milled , and stacked and stickered as best as possible, ready for the time to cut joinery.  The large timbers are the priority at this point.  Thanks! Check out my other posts, make a comment

Offline pegs1

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Re: Does anyone oversize green timber to allow for shrinkage?
« Reply #5 on: December 06, 2010, 02:14:47 PM »
Like Jim said.   It depends on whether you are using square rule or mill rule.   I've always been a mill rule guy.   I designed and built a "jig" and modified a 12" Makita planer to ride on teflon glides so I could plane big long timbers true from one end to the other.   It makes for a very precisely accurate frame when everything is EXACTLY the size its supposed to be  or more importantly that you "took for granted" it was.   That being the case.... the longer the timber is the more you need to "leave" on it so you can plane any twists or "torqueing" out of the timber.   If you are going to stack and dry them... you just need to be sure they are straight and level or you'll just be creating problems mother nature never had when she gave them to you.


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