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Arguments for placing boards bark side up: why do some deck builders and contractors prefer to place boards and stair treads bark side up in outdoor construction? 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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Empirical Evidence for the Bark-Side Up View of wood deck or stair board placement
In Best Practices Guide to Residential Construction author Steven Bliss asserted, "always install trim [and outdoor deck boards and stair treads] “bark-side down,” since the annual rings try to straighten as the wood shrinks." His view agreed with many wood experts but quite a few deck builders had a different field experience.
Those disagreed with the "bark side down" view so I asked Mr. Bliss for more help with this question. In the text below we both, along with the US FPL came to the conclusion that the opposite approach : the bark side of deck and wood stair or ramp boards should generally face up, is correct - a change with which Mr. Bliss concurs - good research. However plenty of wood experts argue about the direction in which flatsawn boards will tend to cup - views that we report and whose sources we cite in the article below.
If you click to enlarge the photo of an old, rotting deck structure at left you might notice the following very interesting details:
To understand why the orange-arrow-marked boards arched backwards, with the bark-side cupped (slightly) consider how these 2x boards were used. To form a girder the two boards were placed facing one another to form a sandwich. There was no top flashing to keep water (rain or snow) from entering the space between these boards.
As a result, the space between the boards catches and holds water while the opposite side of the boards is exposed to more airflow. Therefore arching on the wetter side and cupping on the more dry side of these boards is quite consistent with our explanation of Why Boards Cup in the First Place.
Cupping with bark side convex occurs in some misbehaving boards
Watch out still further: while generally placed outdoors such as on a deck, the bark side of treated wood and much other wood arches "up" as the wood dries, as our photo at left illustrates, nobody has told any of the the boards or framing lumber that's what they are supposed to do. The board shown in end-grain in our photo at left has its cupped concave surface towards the bark side (see Wood Experts Who Argue for Bark Side Downbelow in this article) .
For any wood cupping rule you will on occasion find a board that behaves opposite to the usual rule.
The pre-cut pressure-treated wood stair step or tread in our photo (above left) is cupping "wrong way" arching towards the tree center rather than towards the bark side.
Therefore, if during construction of a deck, stair, or other wooden walking surface you see that a board is already significantly cupped in one direction, we recommend placing that board with the arch of the cup or the convex side facing up for better drainage.
Field Experience and Carpentry School Argued for "Bark Side Up" - place the arced wood-grain facing convex surface upwards
Question: isn't it better to place boards with the arc formed by tree growth rings facing "up" - that is, with the convex side facing up when building stairs or a wood deck deck walking surface ?
In carpentry school I [DF] was taught to always place 5/4 & 2x lumber wood stair treads with the wood grain arc visible at the end cut of the tread such that the arc faced "up" - that is bark side up. An intuitive basis for this view is the simple observation that tree trunks are more or less round and that their bark is on the outside, the convex surface of the tree. It seemed natural that this same "round" or convex tendency would be innate in flatsawn boards cut from trees.
Bernie Campbalik, our instructor, opined that wood (at least exposed to outdoor conditions) tends to cup towards the center of the tree - backwards from the explanation previously published in some sources.
And when a piece of framing lumber was twisted (top board in our photo shown at left) we'd use clamps to force the board in place and we'd fasten it with screw-type connectors. Or if a devil-board was too wild, we'd cut out shorter, usable sections, or in the worst case, it was used for firewood. At the lumber yard Henry Page used to yell at me if he caught me sorting through the framing lumber for my deck, porch or stairs. "Ya ain' t building furniture" he'd shout. As a result, plenty of furtive sorting went on when the boss was not looking.
Original sketch (left, edited with "corrected" text) and flipped sketch (right) adapted from the original sketch provided by S. Bliss, Best Practices in Residential Construction. Edited by InspectAPedia)
Indeed I [have built quite a few outdoor steps and decks and made a point of following Bernie's advice. And when inspecting existing wood decks and steps outdoors (I've of course inspected many more than I could ever have built) in my OPINION I pay close attention to boards that are cupped.
For those deck or stair tread boards whose end grain I could see, 9 times out of 10 my recollection is that the deck or step boards that were cupped (that is concave with a cup that tends to collect water with the cup up) had, by their end grain information, been placed bark side down - the "wrong way" in my book.
Cupped ramp, deck or exterior stair boards (or landing and platform boards) hold water and form algae or ice more quickly and for a longer interval than boards that drain properly. They sometimes rot faster too.
Our photo (left) 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.
When building a wood deck, ramp, or exterior wood stair treads, take a 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 these lines arch "upwards" (bark side up) when the board is placed, most boards will also be curved upwards (convex) and will drain better.
On occasion when inspecting exterior wood structure surfaces I found what I considered an exception to the "bark up" rule: a board that cupped in the "wrong way" - wood boards, having been made by mother nature, don't always follow the rules, but usually they do.
Steve: given these views, do you know of more research or do you have some experience or photos that shed light on my dim confusion? - Daniel Friedman
My [S. Bliss] original bark-side-down advice was based on conventional wisdom, which, as you know, is often wrong. You obviously have far more empirical data to go on, so I’d go with your advice. I remember we pitched this question to one of our wood gurus at JLC [Journal of Light Construction] who came to the conclusion after years of study and debate that you should ignore the bark issue altogether with exterior decking and “pick the best face and install your decking best-face up. Securely fasten the deck boards and apply an annual coating of water repellent.”
The wood gurus at FPL seem to support the bark-side-up approach,  more to prevent shelling and the exposure of untreated sapwood than to prevent cupping.
Two More reasons to place wood boards bark-side up: avoid shelling problems & improve preservative penetration
The same US FPL document continues with explanation of other reasons for placing wood with bark side up and pith side (tree center side) down:
[The moisture level of the bottom of the deck or step boards is less changeable than the upper surface because 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 and possibly more wind exposure as well. For example in the ramp and entry platform shown at left, there is much less air circulation and no sun exposure on the under-side of this wood ramp. - DF.]
So I [SB] stand corrected and will install future decking bark side up – or more likely avoid the issue altogether and use bark-less composite materials.
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.
Watch out: Don't follow this bark side up rule blindly. Take a good look at your actual individual boards or stair treads when you are building an outdoor structure. If we are installing a deck board that is already rather cupped, we prefer to install it with the convex or "outward curve" side facing up for best drainage. The FPL wood surface chemists agree, commenting in the same document:
How interesting, thanks. I will solicit more data from readers too. Contact Us
Our photo (left) illustrates a deck constructed more or less on-grade, in a shaded area, with minimal ventilation beneath. We expect significant moisture differences between the hidden underside and the exposed top side of these deck boards. Photo courtesy Paul Galow.
In my OPINION [DF], problems with relying on wood treatment to reduce wood cupping on steps and decks or ramps include:
Next: BARK SIDE DOWN ARGUMENT
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