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Wood board cupping caused by moisture differences, wet basements, crawl space moisture, or indoor flooding: here we explain how to look at which way a board has cupped to determine where moisture is or has come from.
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.
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Wood Cupping In Response to Which Side of Board is More Wet
For decks constructed high enough to allow air circulation below the deck flooring, the moisture level of the bottom of the deck or step boards is less changeable than the deck's upper surface.
While both sides may become quite wet in rainy or melting-snow conditions, the upper side of the deck or stairs in many locations gets direct sunlight more variations in moisture level than its under-side.
In the ramp and entry platform shown at above left, there is no sun exposure on the under-side of this wood ramp but nonetheless the underside of the entry deck and ramp are exposed to air, drying, and perhaps more stable moisture levels than the upper surface that is further exposed to rain and snow.
In contrast, in the case of the ground-level deck shown below, the under-side of the deck boards may remain more wet than the upper-side, thus reversing the level of moisture exposure and risking that the deck boards will cup in the concave pattern on the walking surface- DF.
The ground level entry deck shown above, at a green lot cabin built in Two Harbors, MN, in 2006, using treated wood for both framing and the deck boards is shown in-use in 2013. Below we see details of the condition of this deck in 2022, sixteen years after its original construction.
The lack of ventilation under this deck is a factor in the rot developing on several of its boards (yellow arrows)..
FPL experts agree that on an exterior wood deck, ramp or stair the under-side of wood boards tends to vary less in moisture level than the upper or exposed side. In addition to cupping problems the exposed deck side is more prone to checking and of course photo oxidation wear. - DF
Below we show another board in this same entry deck, one with notable cupping that holds water, inviting greater algae growth and possibly rot.
So I [SB] stand corrected and will install future decking bark side up – ... or down ... according to the wood experts, or more likely avoid the issue altogether and use bark-less composite materials.- Steve Bliss
Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, materials & components, & project management through complete construction.
Interior Wood Floor Cupping In Response to Which Side of Board is More Wet
When one side of a board contains more moisture than the other, the wetter side tends to expand and "arch" forming a convex shape while the opposite side of the board, the more dry side, tends to form the under-side of the arch or the concave shape.
Convex-Cupped Interior Wood Floors
In our photo at above left we are using a non-invasive electronic signal type Tramex moisture meter to check the moisture level in an interior wood floor.
At 18% the moisture is a bit high in this region; It would be useful to also check conditions below this floor. At above right you can see convex-cupped finish wood flooring by shining a flashlight along the floor surface.
Similarly, in our page top photograph of cupped wood flooring the boards are arcing upwards at the board center and down at the board edges: we think that there is more humidity in this room-side of the floor in all of these convex-cupped flooring photos, or that the boards may have been exposed to flooding or a very wet floor upper surface.
Our photo at left provides a close-up image of significant convex wood floor cupping following upper surface soaking of the floor.
Since documentation of this condition was required we used a combination of light and a straight-edged object (a pencil) whose shadow pattern makes clear the degree of convex cupping of the wood flooring.
Concave-Cupped Interior Wood Floors
Conversely, when we see a wood floor installed over a wet or damp basement or crawl space, we may see the opposite pattern: the boards will be cupped with their concave side facing upwards towards the room.
This situation is illustrated in our photo at left. There are two common reasons we find concave cupped wood flooring indoors:
The the wood floor is constructed over a wet basement or crawl space
The wood floor was exposed to flooding that trapped water remained for some time between the finish floor and the subfloor below.
These conditions can cause extreme wood floor damage, even to the extent that the floor expands and explodes upwards as we demonstrate
The US FPL document discussed at BARK SIDE UP ARGUMENT continues with explanation of other reasons for placing wood with bark side up and pith side (tree center side) down, of which the second notes variations in moisture content across deck boards:
... The
bottom [under-side] of the deck is not prone to
checking because its moisture content is less changeable than
that of the surface.
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In addition to any citations in the article above, a full list is available on request.
[1] 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.
[2] 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.
[3] 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.
[4] Terrie Noll, The Joint Book, Popular Woodworking Books, Cincinnati OH, www.popularwoodworking.com Quarto Publishing, , Inc., 2002, ISBN 1-55870-633-x
[5] R. Bruce Hoadley, Understanding Wood, Taunton Press
[6] U.S. D.A. Forest Products Laboratory, "The Wood Handbook",
[7] 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.
[8] Clarke, S. H. "The differential shrinkage of wood." Forestry 4, no. 2 (1930): 93-104. .oxfordjournals.org
[9] 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.
[10] 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.
[11] 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."
[1]Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
[2] Decks and Porches, the JLC Guide to, Best Practices for Outdoor Spaces, Steve Bliss (Editor), The Journal of Light Construction, Williston VT, 2010 ISBN 10: 1-928580-42-4, ISBN 13: 978-1-928580-42-3, available from Amazon.com
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: info@buildingadvisor.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
[3] America's Favorite Homes, mail-order catalogues as a guide to popular early 20th-century houses, Robert Schweitzer, Michael W.R. Davis, 1990, Wayne State University Press ISBN 0814320066 (may be available from Wayne State University Press)
[4] 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
[5] "Are Functional Handrails Within Our Grasp" Jake Pauls, Building Standards, January-February 1991
Access Ramp building codes:
[6]UBC 1003.3.4.3
[7]BOCA 1016.3
[8] ADA 4.8.2
[9] IBC 1010.2
Access Ramp Standards:
[10] 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.
[11] ANSI A117.4 Accessible and Usable buildings and Facilities (earlier version was incorporated into the ADA)
[12] ASTM F 1637, Standard Practice for Safe Walking Surfaces, (Similar to the above standards)
[13] The Circular Staircase, Mary Roberts Rinehart
[14] Construction Drawings and Details, Rosemary Kilmer
[16] 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. "
[18] 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 sales@lawyersandjudges.com
[19] 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 ] -
[21] How to Clean Moldy Wood Framing & Sheathing How to clean/seal mold from/on exposed lumber or plywood subfloor or roof sheathing indoors - some suggestions based on our field and laboratory research
[22] Lighting, proper use of: proper aiming of a good flashlight can disclose hard to see but toxic light or white mold colonies on walls.
[23] Slips, Trips, Missteps and Their Consequences, Gary M. Bakken, H. Harvey Cohen, Jon R. Abele, Alvin S. Hyde, Cindy A. LaRue, Lawyers and Judges Publishing; ISBN-10: 1933264012 ISBN-13: 978-1933264011
[24] 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 sales@lawyersandjudges.com
[37] "Are Functional Handrails Within Our Grasp" Jake Pauls, Building Standards, January-February 1991
[38] 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
[39] 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: ericgalow@gmail.com or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions.
Paul Galow [Website galowconsulting.com ] - technical consultant on networking, LAN design, applications support. Galow Consulting Services [Website galowconsulting.com ] , 914-204-1749, email: paulgalow@galowconsulting.com
In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: info@carsondunlop.com. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Carson Dunlop Associates provides extensive home inspection education and report writing material. In gratitude we provide links to tsome Carson Dunlop Associates products and services.