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WOOD STRUCTURE ASSESSMENT
This article series explains the causes of cupping in wood boards & wood board right side up advice for steps, decks, ramps: this article explains the causes of wood board cupping and gives advice concluding that unless a board is already badly cupped or has a "bad" side, when construction of outdoor decks and wood stairs you should place boards with "bark side up" - that is pith side (tree center side) of the board facing down.
We include a warning about "shelling" damage on decks and steps - a phenomenon that occurs more on the pith side of boards. There has been plenty of argument about bark up vs bark down. Field experience, FPL experts, and further discussion here all agree that "bark side down" advice in some sources is not the best practice when building wood decks, steps, ramps, platforms, especially outdoors.
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Cause of Outdoor Deck Board & Stair Tread or Wood Trim Cupping & Shelling - Advice for Which Side Should go "Up" When Placing Wood Deck Boards, Stair Treads
The two fundamental sources of wood board cupping are moisture differences across the thickness of the board and inherent properties of the wood cells and cell distribution patterns that comprise the board.
[Click to enlarge any image]
Our photo shows a pre-cut treated wood deck stair tread board. Despite the pith-side grooves cut by the manufacturer the board has cupped its outer edges upwards towards the bark side of the tree.
Article series contents
This cupping pattern shown above is consistent with the prediction of wood experts (wood tends to cup towards the "bark side" of the tree and incidentally, shows that this stair tread board is going to tend to hold water, grow algae, form ice, or otherwise be less safe than a board whose center had arched upwards. In use, if the upper surface of this board is usually wetter than its underside it may tend to flatten out.
The extent to which an individual board will cup depends on those two sources of cupping movement combined with the wood grain pattern of the individual board: flatsawn boards will cup more than quarter sawn boards - we illustrate these differences below.
Although conditions are different outdoors and indoors, still we find that deck boards as well as wood floor boards and stair treads tend to cup so that the concave side of the board is towards the floor's more dry side, and the convex side of the cupped or curved boards tend to be facing the floor's more wet side.
If you're not sure about cupping direction or the definitions of concave and convex, this U is concave and this n is convex. A concave board has its outer edges curled upwards while a convex-cupped board has its outer edges curled downwards.
Wood Cupping & Uneven Moisture Content
The moisture variation contribution to wood board cupping occurs because when a wooden board has uneven moisture content across its thickness it will tend to expand more on its more wet side, causing that side to "arch" or become convex while the opposing side becomes concave or "cupped".
Outdoors where despite temporary wetting from rain, deck boards and wood step boards open to air on both sides, some including chemists at the US FPL are of the view that these boards will ultimately have a moisture content within 2% of uniform across the board's thickness.  My view is that moisture content will vary across an outdoor deck board's thickness over time as a function of rain, snow-melt, or construction over wet, poorly-ventilated crawl areas.
Wood Cupping & Tangential. Wood Shrinkage
This moisture difference seems to dominate wood board cupping outdoors or inside even though wood experts often describe a different effect: tangential. shrinkage and movement that leads the outer edges of flatsawn boards to cup towards the outside of the tree (towards the bark) and away from the heart, adding that the smaller the original tree and the closer to the tree's heart from which a flatsawn board was cut the more extreme will be its tendency to cup.
That wood board cupping is an old and long discussed topic is evident from US FPL citations such Clarke (1930) in the U.S. alone. Finally, Cloutier et als (1997) noted that during wood drying, water conductivity was generally higher in the radial direction than in the tangential direction. which, along with the presence and structure of ray cells in wood fibers may help explain the actual mechanism wood board cupping that is either absent or confusing in most articles on this topic.
A restatement of tangential. wood shrinkage that may be more clear is to pose that wood shrinks more along the circle of its growth rings than across the thickness of the board. By examining the end-cut of a flatsawn board you will see that one side of the board will show longer arcs of growth rings (the side closer to the bark side of the tree) while the other side of the board (closer to the tree heart) will sport shorter arcs (growth rings).
Location & Length of Growth Rings May also Help Explain Board Cupping - re-stating Tangential. Shrinkage Movement
Our end grain view of a flat-sawn board shown above suggests a third possible source of wood board cupping: the pattern of growth rings through a cross section of the board.
In this photo (above) the long arches of growth rings that would also roughly map the round tree shape from which the board was cut. The winter wood (more dense) is marked by the darker growth rings in the board. The wider gaps of light colored wood are summer wood, a less dense growth of cellulose cells.
Cross section cut of lumber shows growth rings & maps the bark side of the tree, predicting the cupping direction
The long arcing red lines of winter wood occur in the center of the board and arch upwards towards the bark side of the tree. The blue lines in our illustration are also marking winter wood growth rings but notice that in a flat-sawn board these lines occur nearest to the edges of the board and that they also represent the typical end-grain of a quarter-sawn board illustrated below.
As we know that more shrinkage occurs across the width of a board through the flat-sawn long-growth-ring arcs (red lines) than will occur through the quarter-sawn end grain portion of the same board (blue lines) we thus have a third source of board cupping: more shrinkage wants to occur in the center of the board than in its edges. But how would this pattern explain the direction of cupping?
Notice that the longest red arcs are towards the upper or bark side of this board? More shrinkage around the growth ring or arc line means more shrinkage where these lines are longest in the board - the upper center board surface in our photo will shrink more than both the board edges and more than the lower board center surface, thus tending to curl the board's outer ends upwards.
Wood Cupping Develops a "Set" in Boards
Ultimately boards that have cupped due to these moisture differences develop a "set" and remain cupped even when the boards have dried. You can observe this dramatically in an interior wood floor that has been flooded, then dried. Boards will remain cupped.
Next: see ANSWER to BARK SIDE UP or DOWN
Poor drainage of upwards-cupped wood boards
Cupped ramp boards (or deck and platform boards) hold water and form algae or ice more quickly than boards that drain properly. They sometimes rot faster too.
Look at the end-grain of any deck, ramp, or wooden walkway board and notice the curved lines that mark the winter wood layers of the tree from which the board was cut.
If the visible curves (the darker colored latewood or winter-wood growth rings) are positioned such that the "cup" formed by these arcing lines is facing upwards - that is the convex side of the board and arcs faces up (opposite of the board in our page top photo) then these boards that are cupped upwards and will drain more poorly, stay wet longer, rot faster, and in freezing climates are going to stay icy longer. Wetter boards also collect more algae and are a slipping hazard, especially on decks and stairs.
Watch out: Use common sense. Especially with pressure-treated wood that may have been soaked unevenly, boards may be quite warped when you are using them in construction. Before nailing a deck or ramp board in place, look at the board surface itself - sometimes the boards don't follow these "cupping rules".
Also see DECK & PORCH CONSTRUCTION.
Watch out: As we introduced above in our US FPL quotation, on wooden decks and stair treads a defect referred to shelling can occur with the bark side down method as this position tends to leave more poorly-penetrated (by preservative) heartwood exposed to sun and weather.
Shelling is a term used by some builders to refer to the loss of portions of a board surface as late wood growth (the outer surface of the tree and thus the "bark side" of the board) separates from early wood growth (the inner portion of the tree or the tree-center side of the board).
Shelling is reported to occur more often in lumber made from Douglas Fir and Southern Pine.
When building an outdoor deck wooden stair tread, it is a good idea to avoid using boards with questionable, likely to separate, portions of their upper surface. Shelling may be less of a problem with wood trim that is to be placed vertically and kept painted or sealed.
For another safety tip when building wooden stair treads using 5/4 deck boards or 2x lumber, avoid using boards whose edge that will form the stair tread nose is, by inclusion of knots, chips, or other defects, likely to break away or form an uneven stair tread nose.
Uneven or irregular or chipped stair tread noses are another trip hazard.
Bark Side Up or Down for Firewood?
The New York Times reported (February 2013) that the bark-side up or down debate rages unresolved in a related venue, the stacking of firewood by Norwegians. People appear to be split 50:50 about whether it's better to stack split log firewood bark side up or down. Having heated my home for years on firewood alone, and split and stacked a lot of wood, it's easy to understand the argument. 
Bark side up: sheds water better, so we fantasize that the wood will dry out better stacked in that position. But bark side down exposes the two split sides of a roughly triangular split firewood log or a semi-circular split half-log to having its freshly cut and split side up and possibly exposed to more airflow.
Which did I find was better? Neither. What was more important was to cover the top of the woodpile to keep the whole thing dry. Stacl the firewood so that it has good air circulation, and cover the top of the woodpile. That made more difference than anything else ... except ... I found putting bark side down on the very first course of logs laid on the dirt seemed to invite bugs into my woodpile about 8% faster than if I stacked wood in the other orientation - for the first course.
Watch out: the slipperiness of a ramp varies enormously depending on what is on its surface, including algae, sand, dust, dirt, water, snow, ice, and even some add-on walking surfaces and paints. At SLIPPERY STAIRS, WALKS we discuss the SCOF for wet algae-covered surfaces.
Also see WOOD FLOOR DAMAGE.
Continue reading at ANSWER to BARK SIDE UP or DOWN or select a topic from the More Reading links shown below.
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