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Wood expert arguments explain the bark side down advice for deck and stair construction. Wood experts explain the three types of wood shrinkage and movement, define the most significant type of wood shrinkage pattern and direction, and explain why all else being equal a wood board will often curl with its edges towards the bark side as it dries.
This article series explains the causes of cupping in wood boards & wood board right side up advice for steps, decks, ramps, concluding which side of boards should face up or down (bark side down or bark side up in some cases) when building a deck or exterior wood stairs. Also see WOOD FLOOR DAMAGE.
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When the bark side up or down, pith side up or down argument seemed closed, or at least were sick of it, our friend Barry Lam lent us a wood joinery text by a Terrie Noll, a wood expert whose wood joinery advice reiterated the argument supporting placing deck boards bark side down..
Illustration at left adapted from The Joint Book Terrie Noll cited below.
Cabinet makers and other wood joinery experts pay great attention to the properties of wood, particularly its shrinkage, or wood's cyclic shrinkage and expansion in response to variations in ambient humidity. These details become critical in fine joinery such as furniture that might otherwise become damaged or even not usable if wood joints open up, separate, warp or jam.
The amount and direction of wood shrinkage as well as any tendency of a board to cup or bend depends significantly on the wood species and more, on how the board was cut and from what portion of the original tree or log it was cut. But most framing lumber such as shown in our photos above is plain sawn or flat sawn - the illustration shown here.
As we cite from Mr. Noll's excellent wood joint techniques book, he argues that a flat sawn or plainsawn board tends to cup such that the bark side of the board will be concave - that is, if we want our wood deck or step boards to best shed water and we want the cup side down then we'd place the bark side down - opposite of my experience and that of Bernie.
Here is Noll's description of the directions of wood shrinkage.
While this text explains why flatsawn boards shrink more across their width than quatersawn boards it does not itself explain the direction of board cupping as shown in Noll's illustration presented earlier. To explain wood cupping as a feature of wood shrinkage we need to look for reasons that a board experiences different amounts of shrinkage on one of its flat sides than the other.
Individual board growth ring differences viewed from the end grain across the board's width
Notice that in our photos except for a rare "perfect quartersawn" board, looking at end grain reveals that the cross grains in the board are by no means uniform across the board's width. In the flatsawn board to which our black arrow points the wood closer to the board's edges, especially its right hand edge, is more vertical - closer to the quartersawn pattern, extending vertically across the thickness of the board.
In contrast, towards the same board's center the growth rings are more flat or longitudinal - that is, tending to extend horizontally across the width of the board.
Individual board growth ring differences viewed from the end grain across the board's thickness
But it is more likely to be variations in growth ring patterns across the board's thickness that will ultimately explain the direction of board cupping. In our photos of cupped boards, ask your self why one side of the board expanded (as it got wet or more moist) more than the other side of the same board. These variations continue through a board's thickness, but most likely you'll see one of two conditions that explain the direction of board cupping:
Ok so we've got three kinds of movement in wood, of which tangential movement is the most significant. But none of these explains cupping. Rather cupping occurs when one face of a board shrinks (or expands) more than the other. Just above we looked at direct variations in moisture content to explain cupping in wood floor boards.
But cupping may also occur due to variations in the wood grain pattern, the position and shape and location in the board of the growth rings, and the presence or absence of large areas of homogenous wood cells or summer wood that are present in the width of the board and that are not the same between the two board faces.
Or re-stating this point, a wet or damp (18% or above moisture content) board that is more or less uniform in moisture throughout its thickness will still tend to become concave as it dries due to differences in the wood grain pattern across the board's thickness and due to the location and position of the growth rings in the board's cross-sectional thickness.
So picture trying to straighten out a growth ring. If I put the bark side is "up" when nailing a deck board, the "arch" formed by the growth rings points "up" - that is, the concave face of the arch is down and the convex side of the arch is "up". Now as the board shrinks and tries to straighten the arch, it never will reach perfect straightness but it'll get straighter; The convex side remains arched "up". For a board with bark side "up" to straighten its growth ring arches enough to become concave side up the arches would have to pass through dead flat and continue to bend in the opposite direction. That just does not happen. So I'm not sure what "boards cup in the opposite direction" means in the quotation above.
If you take a look at our photos of flatsawn board end grain growth ring patterns throughout this article you'll see that the bark side of the board of the board has more growth rings intact while the pith side or heart side of the board has larger open areas of summer wood that is not divided by growth rings; this is especially true for flatsawn boards on which the center of the board is approximately in line with what was the center of the tree (our flatsawn board marked by the arrow in our photo).
Next: WOOD CUPPING vs WET SIDE
 The moisture content in wood varies depending on not only green un-dried lumber versus dried or kiln dried lumber, but also wood species, the ambient environment, and more. Green lumber that has not been soaked by rain or floating down a river may still have moisture at 30% or above; and wet wood that has been soaked may have 2 1/2 times as much moisture as that same wood species when it has been dried or kiln dried. Free water on or in wood dries quickly but bound water within wood cells takes much longer to dry or requires kiln drying or other measures for its removal.
 The fiber saturation point of wood or wood's FSP is defined as the moisture content of that wood when all of the free water has been removed. Picture the clothes in your clothes washer at the end of a spin cycle. The wet clothing has been squeezed until you couldn't get more water out of it - that clothing is at its fiber saturation point. And just as FSP varies among wood species, if you've ever done laundry you've noticed that some fabrics retain less water at the end of the washer's spin cycle than others.
 Glen D. Huey, "Why Wood Warps", Popular Woodworking Magazine, 12 July 2012, retrieved 7/17/2013 original source http://www.popularwoodworking.com/article/why-wood-warps, reprinting from Woodworking Magazine, Summer 2009.
 Terrie Noll, The Joint Book, Popular Woodworking Books, Cincinnati OH, www.popularwoodworking.com Quarto Publishing, , Inc., 2002, ISBN 1-55870-633-x
 R. Bruce Hoadley, Understanding Wood, Taunton Press
 U.S. D.A. Forest Products Laboratory, "The Wood Handbook",
 Cloutier, Alain, and Yves Fortin. "A model of moisture movement in wood based on water potential and the determination of the effective water conductivity." Wood Science and Technology 27, no. 2 (1993): 95-114. - Abstract:
 Clarke, S. H. "The differential shrinkage of wood." Forestry 4, no. 2 (1930): 93-104. .oxfordjournals.org
 Boyd, J. D. "Relationship between fibre morphology and shrinkage of wood." Wood Science and Technology 11, no. 1 (1977): 3-22. Abstract:
 Gu, H., A. Zink-Sharp, and J. Sell. "Hypothesis on the role of cell wall structure in differential transverse shrinkage of wood." European Journal of Wood and Wood Products 59, no. 6 (2001): 436-442.
 Barkas, W. W. "Wood water relationships, VI. The influence of ray cells on the shrinkage of wood." Transactions of the Faraday Society 37 (1941): 535-547. Excerpting:
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