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Answer to the question: should boards be placed bark side up or bark side down. Here we summarize advice from both wood experts and deck builders about which way to place deck boards and wood stair treads outdoors.
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. Our page top photo shows a stack of 2x lumber. From the topmost board in the photo the 5 boards are facing 1-bark up, 2-bark down, 3-bark down, 4-bark up, 5-bark down. These are all flat-sawn boards.
If we prefer to follow the opinion of many deck builders whose opinion arises from years of field observation, we'd put the bark side up unless the board is already cupped - that is, we like to place deck boards such that the arch shown by the board's growth rings (visible at the board end cuts) tends to be "up".
In sum it seems that carpenters like bark side up and most people of either camp will agree that moisture differences in practice tend to dominate cupping effects.
Some reasons that builders like bark side up, a less technically-supported position and one that has been my [DF] own for a long time, are at BARK SIDE UP ARGUMENT.
Why do deck builders disagree with the science of board cupping? It may be that what deck builders are actually observing is a deck board flattening effect as two opposing forces are at work: tangential shrinkage wants to make the deck board edges curl upwards towards the bark side (thus arguing for bark side down) while frequent soaking of the upwards facing surface of a deck board wants to make the upper surface of the board expand more than its bottom surface, causing the board edges to curl downwards toward the tree heart. The ultimate effect may be flatter deck boards or less cupping in actual use.
Look at each board. Don't install a deck board with a visibly concave surface facing up. In the case of the stair tread shown just above, it has a concave upper surface. If possible, I'd have left this tread in the lumber yard and picked a different one. Apparently the manufacturer who pre-grooved tread walking surface patterns on the intended "upper surface" of the tread was not paying attention to the end grain pattern of the boards used.
For quartersawn boards or boards whose end grain shows that the quartersawn board pattern dominates the board, look at the board by eye and place its most arched side with the arch up.
The wetter side of boards will tend to arch upwards as that side of the board expands more than its more dry side.
A result is that boards used on wood decks whose upper surface is regularly more wet than under surface may tend to develop concave cupping in the deck surface. Our field experience is that this effect is less in boards placed as we described for flatsawn just above.
Assuring that the deck surface drains adequately (don't but boards too tightly) reduces this cupping effect as well as other slip trip fall hazards such as slippery algae growth (SLIPPERY STAIRS, WALKS)
[Click to enlarge any image]
The rest of this article illustrates and explains wood cupping, wood warping, summarizes the arguments made by various wood experts as well as experienced builders and carpenters, and cites authoritative sources.
R.W. Barkas, back in 1941, addressed the question of the directions of wood grain shrinkage, stating:
"The shrinkage of wood is not the same in the three directions of the grain. It is greatest in the tangential (t) direction where the shrinkage per unit change in moisture content dr/dm lies for most woods between 0.2 and 0.4.
In the radial (p) direction dp/dm is usually about half this value,
while in the longitudinal direction ... [shrinkage] is much smaller, amounting to about 1/50th of the tangential. It is difficult to measure d(lambda) / dm accurately, and in standard tests it is not usuallyi attempted. These differences in shrinkage may be accounted for by the presence of ray cells in the wood ... " - Barkas (1941) 
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 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:
A model of isothermal moisture movement in wood during drying using the gradient in water potential as the driving force is proposed. The moisture transport coefficient used in this model is the effective water conductivity. It is a function of moisture content, temperature, and direction of flow. The boundary desorption curve of the effective water conductivity function is established in the radial and tangential directions of aspen sapwood from nearly saturated to dry conditions at 20, 35, and 50 °C using the instantaneous profile method. The results show that the effective water conductivity increases exponentially with moisture content and temperature. The effect of temperature cannot be solely explained by the variation of the viscosity of water. The variation of the moisture content-water potential relationship with temperature would explain a large part of this effect. The effective water conductivity was generally higher in the radial direction than in the tangential direction in a ratio varying from 1/1 to 25/1 depending on moisture content and temperature. The flux-gradient relationship obtained at given moisture contents were found to be linear, confirming the validity of the model for the experimental conditions considered in the present work.
 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:
This is a study on the shrinkage of wood representing the wide range of morphology variation in leaning trees. It involved 13 trees of Eucalyptus regnans, one of Eucalyptus sieberi and four of Pinus radiata, and specimens taken at close intervals around the circumference of each. Data indicated a systematic modulation, between extremes at upper and lower sides of each stem, in longitudinal growth strains, relative proportions of thin, medium and thick-walled fibres, microfibril angle in the S2 layer of these, and both Klason and acid-soluble lignin content. Analyses indicated that the microfibril angle in S2 was a prime factor in influencing both longitudinal and volumetric shrinkage reactions; proportion of thick-walled fibres in the tissue, thickness of S2 relative to S1, and variations in lignification also were involved. Unusually thick-walled fibres were associated with visco-elastic strain recovery effects, which could form a substantial part of dimensional changes apparently attributable to shrinkage.
 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:
"The shrinkage of wood is not the same in the three directions of the grain. It is greatest in the tangential (7) direction where the shrinkage per unit change in moisture content dr/dm lies for most woods between 0-2 and 0.4. In the radial (p) direction dp/dm is usually about half this value, while in the longitudinal direction ... [shrinkage] is much smaller, amounting to about 1/50th of the tangential"
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: email@example.com
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
 American Plywood Association, APA, "Portland Manufacturing Company, No. 1, a series of monographs on the history of plywood manufacturing",Plywood Pioneers Association, 31 March, 1967, www.apawood.org
& "The Elimination of Unsafe Guardrails, a Progress Report," Elliott O. Stephenson, Building Standards, March-April 1993
 "Are Functional Handrails Within Our Grasp" Jake Pauls, Building Standards, January-February 1991
Access Ramp building codes:
 ADA 4.8.2
 IBC 1010.2
Access Ramp Standards:
 ADA (Americans with Disabilities Act), Public Law 101-336. 7/26/90 is very often cited by other sources for good design of stairs and ramps etc. even where disabled individuals are not the design target.
 ANSI A117.4 Accessible and Usable buildings and Facilities (earlier version was incorporated into the ADA)
 ASTM F 1637, Standard Practice for Safe Walking Surfaces, (Similar to the above standards)
 "An Example of Colonial Paneling", Norman Morrison Isham, The Metropolitan Museum of Art Bulletin, Vol. 6, No. 5 (May, 1911), pp. 112-116, available by JSTOR.
 Falls and Related Injuries: Slips, Trips, Missteps, and Their Consequences, Lawyers & Judges Publishing, (June 2002), ISBN-10: 0913875430 ISBN-13: 978-0913875438 "Falls in the home and public places are the second leading cause of unintentional injury deaths in the United States, but are overlooked in most literature. This book is unique in that it is entirely devoted to falls. Of use to primary care physicians, nurses, insurance adjusters, architects, writers of building codes, attorneys, or anyone who cares for the elderly, this book will tell you how, why, and when people will likely fall, what most likely will be injured, and how such injuries come about. "
 Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
 Slips, Trips, Missteps and Their Consequences, Second Edition, Gary M. Bakken, H. Harvey Cohen,A. S. Hyde, Jon R. Abele, ISBN-13: 978-1-933264-01-1 or
ISBN 10: 1-933264-01-2,
available from the publisher, Lawyers ^ Judges Publishing Company,Inc., www.lawyersandjudges.com firstname.lastname@example.org and also from the InspectAPedia Bookstore (Amazon.com)
 The Stairway Manufacturers' Association, (877) 500-5759, provides a pictorial guide to the stair and railing portion of the International Residential Code. [copy on file as http://www.stairways.org/pdf/2006%20Stair%20IRC%20SCREEN.pdf ] -
 Slips, Trips, Missteps and Their Consequences, Second Edition, Gary M. Bakken, H. Harvey Cohen,A. S. Hyde, Jon R. Abele, ISBN-13: 978-1-933264-01-1 or ISBN 10: 1-933264-01-2, available from the publisher, Lawyers & Judges Publishing Company,Inc., www.lawyersandjudges.com email@example.com and also from the InspectAPedia Bookstore (Amazon.com)
 "The Dimensions of Stairs", J. M. Fitch et al., Scientific American, October 1974.
 "The Elimination of Unsafe Guardrails, a Progress Report," Elliott O. Stephenson, Building Standards, March-April 1993
 "Are Functional Handrails Within Our Grasp" Jake Pauls, Building Standards, January-February 1991
 Sam Williams and Mark Knaebe, "The Bark-Side/Pith-Side Debate", The Finish Line, (A Forest Products Laboratory finishing factsheet), December 1995, U.S. Forest Products Laboratory, retrieved 9/13/12, original source http://www.fpl.fs.fed.us/documnts/finlines/willi95b.pdf [copy on file as Bark_Side_Wood_FPL.pdf] R. Sam Williams and Mark Knaebe
are researchers in Wood Surface
Chemistry at the USDA Forest
Service, Forest Products Laboratory,
One Gifford Pinchot Drive,
Madison, WI 53705–2398
 Sarah Lyall, "Bark Up or Down? Firewood Splits Norwegians", The New York Times, 20 February 2013, p. A4.
Eric Galow, Galow Homes, Lagrangeville, NY. Mr. Galow can be reached by email: firstname.lastname@example.org or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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