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Green wood/Dry wood? #7478 02/16/99 05:14 PM
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PMATULEWICZ Offline OP
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Greetings! Have just begun to delve into timber frames and have formed a question. Since the frames were traditionally built from "green" lumber, what are they built from now? If they are still built from green wood, how do you make allowances for dimensional changes if you enclose the thing in one of those structural panel skins which isn't likely to give much?

Re: Green wood/Dry wood? #7479 02/18/99 02:16 PM
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Joel McCarty Offline
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This is a deceptively simple question.

Historically, buildings were less well insulated and far from airtight, so timbers that were locked together were allowed to dry slowly in situ. Many's the contemporary timberframer who has put up a frame in the Fall, slammed on the stress skins and turned up the furnace so that the rest of the trades could work indoors for the winter (and so that the homeowner could get out from under that bridge loan and into the house and a mortgage as soon as possible.)

This haste has generated all manner of tales about loud and violent checking (especially in low-quality timber) and prodigious amounts on moisture forming on the inside of the windows (especially)and upon every other hard surface. This is less of an immediate problem when an air to air heat exchanger has been installed (as many contemporary timberframers recommend). Still, the timbers do seem to twist and check more in houses that are tight and heated right after the frame goes up.

To combat this, some timberframers have invested heavily in salvage wood (a by-product of the Malling of America). Heavy timber of excellent quality can be found in the framing systems of old industrial buildings. Also various drying technologies are turning up the heat, so to speak, on fresh-sawn timbers. One effective method combines microwaves and a vacuum dehumidifier in large reinforced steel caverns.

None of this comes without an upcharge, and some suggest that there is a political and enviornmental cost, as well. Salvage and kiln-dried timbers are always more expensive than fresh-sawn; harder to find, harder to cut, etc., and there is almost always a much higher waste factor, as well.

Hope this helps.

Re: Green wood/Dry wood? #7480 02/18/99 02:55 PM
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PMATULEWICZ Offline OP
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Soooo....The frames are still often constructed of "green" lumber for purposes of ecomony? Is large size kiln-dried lumber a commonly available item or is it a specialty all its' own? My question stemmed more from the fact that having studied log home construction a bit. With these, it is common construction practice to make various mechanical allowances around wall openings and interior walls and such to allow for a prodigious amount of "settling". Just wondered how much change in dimesion is to be expected from a framed structure and or how it is accounted for.

Re: Green wood/Dry wood? #7481 02/19/99 08:42 AM
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Bill Keir Offline
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Good morning, No activity in my sector so I thought Id stick my nose in here

You should remember that timber to all intents and purposes does not shrink along the grain. (less than one percent, assuming that it is stright grained, - like much of the timber in north America seems to be - unlike our English Oak)

The fact that we build our imberframes with the posts vertical, (thats the theory when we start out anyway), means that unlike the log builders, we do not need to allow for any mechanical settlement.

It is possible to predict the behavior of timber in terms of how it will change shape (kind version), distort / deform (unkind version), but a lot less easy to predict the extent/degree/ammount of movment.
Or rather I should say I dont know a fool-proof method of doing this, but I would suspect there are people who do, so lets here (sic) those ideas -

[This message has been edited by Bill Keir (edited 02-19-99).]


Bill

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The Optimist sees Opportunity in every difficulty
Re: Green wood/Dry wood? #7482 02/19/99 11:46 AM
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Joel McCarty Offline
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The USDA puts out a great fat thing called The Wood Handbook. It contains an astonishing amount of information about the mechanical properties of wood, including a chart by species that expresses shrinking as a percentage of size. You could look it up.

Dry wood almost always costs more than wet wood, and is usually more difficult to work. Someone had to own the stuff and hang onto for a while for it to dry naturally, your low-tech option, or someone else had to buy one of those great whacking whiz-bang kilns, pour megawatts of electricity through it and ship the resulting 'product' around the world.

More than a few quite servicable and lovely timber frames have been assembled from green or air-dried wood over the last thousand years or so.

Re: Green wood/Dry wood? #7483 02/19/99 02:24 PM
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Grigg Mullen Offline
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It seems like my colleagues have pretty well covered the bases.
Bill Kier is correct in that timber shrinks much less in the longitudinal direction. Another difference in timber frames vs log homes is that the window and door openings are not cut in the timberframe itself. The openings are in the (usually more stable) enclosure system such as stress skin panels. So whne the frame moves, it does not immediately and directly effect the openings as would happen in a log building.
Joel's point is well taken on the difficulty of both obtaining and working dry timbers. I have discussed the kiln dried oak with one timberframe shop that did a major project uaing that wood. There were numerous tales of burnt drill bits and rolled chisel edges. The same shop also does a lot of work in recycled timber. The cost is higher than green timber, and again, it is more difficult to work.
The trade off being that the end result is more stable, and, at least with recycled timber, more socially responsible.
Another good book on the movement of wood is Bruce Hoadley's "understanding Wood". Included in the text is a method for calculating the shrinkage of wood.

Re: Green wood/Dry wood? #7484 02/19/99 02:44 PM
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PMATULEWICZ Offline OP
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Ah yes. Thank you gentlemen. Points well taken. The fact that most of the openings are in the "stable" skin would eliminate most of the settling problems. I assume that if the interior is drywall finished an owner could expect some cracking as things settle into place but that must be relatively minor compared to having a substantially "live" shell like you would in a log home. As for antique oak....with that I am well aquainted. I currently live in an 1840's vintage Greek Revival with an oak "balloon" frame. Broke many tools and fasteners while trying to tame a 12x12 corner post while replacing part of the sill. One other question if I may. I've seen decriptions of different surface treatments for timber frames ranging from hand hewn to planed. I have little experience dealing with green lumber so forgive my ignorance. Can you plane green lumber without making a mess of it? Or is that treatment reserved for more seasoned wood?

Re: Green wood/Dry wood? #7485 02/19/99 03:26 PM
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Grigg Mullen Offline
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Green lumber can be planed very well. It's considerably easier that dry lumber. Fairly common practice is to work with planed green timber and beltsand off any dings or marks when fabrication is complete.

[This message has been edited by Grigg Mullen (edited 02-19-99).]

Re: Green wood/Dry wood? #7486 02/22/99 08:13 PM
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Rudy R Christian Offline
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Hello all,

Just some more input. The treatment you speak of is the result of the process used to convert logs into timbers. Hand hewn refers to the result of squaring up logs with an ax, and finishing them with a broadax. Rough sawn finish is the result of making timbers on a sawmill.

Planing timbers is generally done with a hand plane, a portable power planer or a large stationary planer. The largest of these can plane all for sides of a timeber to dimension and square in one pass. The timber has already been squared up on a sawmill before it is planed. In all cases it is easier to plane green wood, but as it continues to dry the grain will raise some, depending on species.

For the most part, planing is done to faciltate applying a finish to the timbers. It is more predictable, and requires less oil (or whatever you are using). It should be noted that apllying finish to green would require an understanding of how the finish will react to the drying. Oil finishes are less likely to cause a problem.

Good luck with your research.


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