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kiln dry vs air dried strength

4/1/21       
steve barth

Is there an appreciable difference in strength between kiln dried and air dried wood? I am working with someone on a timber frame building, and he insists that the pegs need to be air dried as that is stronger.
The pegs would be made out of white oak.

4/1/21       #2: kiln dry vs air dried strength ...
John Member

No difference once they get to the same MC.

John

https://www.fpl.fs.fed.us/documnts/fplrn/fplrn055.pdf

4/1/21       #3: kiln dry vs air dried strength ...
rich c

I believe they want a dry peg so when driven in, the beam holes will shrink and lock in the pin. A common situation with green chair building. Damp hole with a dry rung. The kiln dried peg may have a bit more driving strength at the end so it doesn't mushroom as badly.

4/1/21       #4: kiln dry vs air dried strength ...
Matt

Rich, could you describe what you mean by driving strength?

4/1/21       #5: kiln dry vs air dried strength ...
rich c

The ability to hold it's shape when being driven in with a mallet. On my Japanese chisels I soak the handle so it softens and I can mushroom it over the steel ring. When dry, the oak won't mushroom. Same for the timber frame pins. Any bit of high moisture and the head mushrooms over. Not a pretty look for timber frame pins.

4/6/21       #6: kiln dry vs air dried strength ...
Gene wengert-wood Doctor

There is no difference between the two if they are at the same moisture content AND if the kiln dried wood was not dried over 160 F and was not over-dried.

4/6/21       #7: kiln dry vs air dried strength ...
Mark B Member

Rich hit the nail on the head. The pegs need to be dry so when the timbers shrink they will tighten around the peg. Otherwise the peg will dry and fall out or be loose. Additionally if you make the pegs green, while it will be very slight, when they dry they will be oval not round.

4/6/21       #8: kiln dry vs air dried strength ...
steve barth

thanks all for the feedback. The Wood Doctor gave the info I was specifically interested in. It would be interesting to know at what point high drying temps (over 160) really affect strength drastically, but I imagine that's probably species specific...

4/7/21       #9: kiln dry vs air dried strength ...
Gene wengert-wood Doctor

It is species specific. It depends on the temperature. It depends on the length of time at elevated temperatures. It depends on the wood's moisture content. It depends on what strength value is of interest...for example, toughness is affected before ultimate strength.

The USDA's Wood Handbook has more detailed discussion.

4/7/21       #10: kiln dry vs air dried strength ...
Gene wengert-wood Doctor

Mark B's response bring to mind two questions, If you have a 1” diameter hole drilled in the face of lumber or a timber, what happens to the hole when the wood dries a little more, or loses posture? The answer is that the hole shrinks across the grain, but not along the grain. So, the hole has an oval shape, with one dimension being the same and the other dimension being less. Note that the hole does not shrink, but the wood around it does.

Now, what happens to a 1” diameter wooden pin put in the hole. The pin will shrink is diameter and will shrink somewhat more than the change in the hole diameter, which means it will loosen. For this reason, it is suggested that pins be kept in a very dry environment prior to use, so they will not dry or lose moisture but will gain moisture and swell, making a tight joint even without adhesive. I have seen pegs stored in hot sand.


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