Can boards shrink in length as they dry

[warren46]

I sawed some 16 ft long 1X10 out of yellow pine for a customer to use as lap siding on a car port.  These boards were put up green.  When I say green the trees were happily growing in the morning, sawed in the afternoon and put up on the building the next day.

He came in to get some more lumber yesterday and said that the boards had shrunk so that there now was a one half to three quarter inch cap between the ends of the siding on the building.  I have always said that a board will shrink in width and thickness but not in length for all practical purposes.  Is my thinking incorrect?

[WDH]

Juvenile wood cut from the center of the tree will shrink a little bit along the length. 

[OldMasterTech]

Definitely shrinks lengthwise. I installed EWP 2 x 4s end to end last year now there is a 1/2" gap at every joint.

[teakwood]

I think length shrinking is around 0.2-0.3%

[Don P]

In normal wood those numbers are true but not in juvenile and reaction wood (compression wood in softwoods). I've had to go back on several decks and replace boards and broken screws that shear off as the wood shrinks in length. The microfibrils that make up the thickest portion of the cell wall, the "S2" layer, have the most influence on mechanical properties and shrinkage. In a juvenile tree or in reaction wood those microfibrils lay on the bias, at an angle to the grain direction. In "normal" wood they are aligned more with the axis of the tree. Shrinkage happens as the moisture between those microfibrils leaves the cell wall and they move closer together.

This is a deck board I like for showing this. Right after a rain it will lay flat, in extreme dry it stands up a foot off the deck. It has sheared the screws to the first 2 joists. The juvenile wood on this face shrinks lengthwise, the wood on the backside is normal and doesn't shrink appreciably in length, so it bows. It has done this several thousand times, or more correctly it is always doing this.


You can often spot compression wood as a dull "peanut butter" looking grain in the pine. It does occur naturally but we created a lot of that when we thought making fiber as fast as possible was the goal.

[WDH]

It is better to leave the juvenile wood corralled between two outside layers of mature wood.  You can do this best by always making sure that the pith is in the middle as you look across the end of a board.  If a board is all juvenile wood, like some 2x4's sawn from the dead center of a tree, every bad thing in the book can happen, and usually does when dried super fast like it is done in the big southern pine mills.  Dimension pine is dried from green to 19% in 24 hours. 

[GeneWengert-WoodDoc]

Compression wood can shrink upwards of 3%.  Most compression wood is in the juvenile core.  Juvenile wood in softwoods is often compounded with compression wood, so we can see 3%, green to oven-dry.  Of course, wood in use will probably not range so much (green to oven dry), so figure about half of this for green to 12% MC.  If the wood is put up dry, it might swell some in the rain, but will not dry much under 12% MC, so lengthwise shrinkage is minimal.  Normal shrinkage is 0.1% for many species without juvenile or compression wood.

Another factor, away from the juvenile core, which is 15 to 20 rings out from the center, are knots.  The knot shrinks normally, but because the knot is 90 degrees in grain angle to the grain in lumber, knot shrinkage adds to longitudinal shrinkage or tries too.

Another factor is is that the standard shrinkage values are for straight grain, which might not be true to softwoods.  In fact spiral grain in Softwoods is the norm in the juvenile wood, which is likely the main reason why we see this lengthwise shrinkage.  The sawyer can also create wood with a grain angle which means lengthwise shrinkage.  This is why we encourage sawing parallel to the bark...to keep grain angles close to zero.  Of course this is not true in spiral grain or around knots...the natural grain is not achieved with parallel to the bark sawing.

Note that 1/2" shrinkage on an 8' piece is 1/2% or 0.5% shrinkage.  This is 5 times more than the typical.

[Don P]

Good points. When framing a house a carpenter frames with the crown up, but we are also grading the lumber. You don't want to put a large knot on the bottom tension edge of a large horizontal member like a joist or rafter if at all possible, that zone contains short grain. I really like to see a strap of nice straight grain on that bottom tension edge if at all possible. With the grain distortion around a knot if there is one near an edge that edge will shrink lengthwise causing the crown to show on the other edge. One of those can't win situations.

I remembered I had a few really old pics in the gallery
This is some severe compression wood, there's the "peanut butter" grain I was talking about. Compression wood is also quite stiff but less strong than normal. The tree is buttressing under the lean and using more lignin to support the compression side of the lean.


This milled log has several zones of compression wood.

More reaction wood in the juvenile core and a wicked bow.


Checks follow the grain, this is some of the spiral grain Doc is talking about, that one had enough twist and weathering to become dimensional firewood.

[GeneWengert-WoodDoc]

To confirm, splits and cracks will follow the grain, so the cracks in the previous picture show grain direction.  Good example of spiral grain indeed.