New building access ramp (C) Daniel Friedman Bark Side Up Argument
Why Should Wood Boards Be Placed Bark Side Up?
     


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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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Origins of the Bark-Side Up view of wood deck or stair board placement

Empirical Evidence for the Bark-Side Up View of wood deck or stair board placement

Deck board rot (C) Daniel Friedman

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.

[Click to enlarge any image]

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:

  • Boards marked with green arrows: most of the deck boards were placed bark side up, arched convex side up (slightly) and show less rot than the red arrow-marked boards.
  • Boards marked with red arrows: the two most-rotted deck boards (red arrows, photo center) were installed with deck boards cupped upwards or "concave";
  • Boards marked with orange arrows: You can see the end grain in the two 2x12's forming the deck girder (orange arrows photo lower center). These two boards arched backwards from our anticipated curve pattern. You'll see by the wood end grain pattern that the girder right-hand board has it's "bark side" facing right, and the left hand board has its "bark side" facing left. Both of these boards curved towards the "bark side" - an example supporting the "bark side down" wisdom when building an exterior deck or stair.

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.

Wood step cupping wrong way (C) 2013 Daniel Friedman 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

Twisted framing lumber (C) 2013 Daniel FriedmanQuestion: 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.

Figure 1-31: Cupping of flat sawn lumber (C) Wiley and Sons, S Bliss Figure 1-31: Cupping of flat sawn lumber (C) Wiley and Sons, S Bliss

Bark side up on an exterior deck floor may balance tangential. wood shrinkage against the effects of high moisture on the rained-on side of boards, pitting one force against the other.

For a deck whose underside is likely to remain more wet than its upper surface you might prefer bark side down. .

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.

Lumber end cuts shows how boards were sawn and bark vs pith side (C) Daniel FriedmanFor 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

US FPL Support for Bark Side Up, Pith Side Down

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, [38] more to prevent shelling and the exposure of untreated sapwood than to prevent cupping.

Wood edge or straight grain vs flat grain - U.S. FPL

Cupping results from the shrinking of the top and swelling of the bottom of the wood, which are caused by exposure to moisture and drying by the sun. These changes do not occur evenly throughout the wood.

The amount of initial cupping depends on the difference between the moisture content of the wood at the time it is cut compared to the moisture content in service. As the boards weather and continue to experience wetting and drying cycles, cupping continues to increase. The boards take on increased “set.”

Sketch at left illustrating the different log locations and wood grain properties of "edge grain" or "straight grain" wood (a) and "flat grain wood" (b) has been adapted from one provided by the U.S. FPL [38]

Even during periods of rain, when the moisture content rises in the surfaces, boards remain cupped because the surfaces undergo a permanent set. During periods of increased moisture content, the surface cells are slightly crushed; during drying periods, small cracks (checking) appear in the wood because the surface cells remain crushed.

Vertical-grained lumber tends to have fewer problems with cupping than does flat-grained lumber, but both kinds of wood will cup when the top or outside of the wood is lower in moisture content than the opposite side. In the case of decking, the top of the board dries much faster than the bottom; in fact, water may be driven down by the drying process.

Water-repellent treatment also helps to keep the wood dry. In tests at the Forest Products Laboratory, the bottom of deck boards was about 2% wetter than the top. The better the water repellent, the greater the difference [in moisture absorption between the two sides of the board - Ed.]

Contrary to popular notion, there is only a small advantage to placing boards bark-side up with regard to cupping. - copy available at [38]

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:

1. However, it is advisable to place the bark-side up for other reasons. The pith side is more prone to a severe type of raised grain, called shelling, particularly in species that have dense latewood growth rings, such as southern pine.

See Shelling Damage to Walking Surfaces on Wood Decks & Stairs below.

2. The second advantage to placing deck boards bark-side up relates only to wood that has been pressure-treated with preservatives and not incised.

Pressure treatment does not penetrate the heartwood of unincised wood very well. In a flat-grained board, the bark side may consist of 100% sapwood and therefore be fully penetrated with treatment, whereas the pith side may contain a portion of poorly penetrated heartwood.

When the deck is exposed to the weather, checking of the heartwood surfaces can expose the untreated heartwood to attack by decay fungi.

The bottom of the deck is not prone to checking because its moisture content is less changeable than that of the surface.

New building access ramp (C) Daniel Friedman[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

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.

Exceptions to the Bark-Side-Up Rule for Placing Wood Boards on Steps & Decks/Ramps

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:

These factors, although important, can be overridden by a third factor—the quality of the pith side compared to that of the bark side. If the pith side is clearly better, place this side up.

Low wood deck (C) D Friedman Paul GalowFollow-Up: more support for bark side up.

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:

  • Treating just the up side leads to differential water absorption - often only the "up" side of wood is treated after construction
  • Cupping may occur on unevenly-treated lumber as we see in both indoor and outdoor cupped flooring and boards - the more dry side of boards typically can be identified by center cupping in the board and board edges curling up.
  • Realistic expectations: nobody is going to apply waterproofing treatment coating to decks, stairs, or ramps every year
  • Good design IMHO is for what people actually do, not for what they should do.

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