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Lumber end cuts shows how boards were sawn and bark vs pith side (C) Daniel Friedman Bark Side Up or Down Answer
When Should Wood Boards Be Placed Bark Side Up or Bark Side Down?

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.

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A Sensible Answer to the Bark Side Up or Down Debate for Deck Board Placement

End grain view of flat sawn board (C) Daniel Friedman

Confusion about shrinkage, cupping, and twisting in wood in response to moisture changes has plagued both researchers and carpenters since at least the 1940s and probably for centuries.

In large part, this is because there are a number of factors that affect wood behavior as it becomes more wet or more dry including, even, wood boards milled from the same individual tree but cut from different positions in the tree. Wood cupping or twisting is also affected by the wood species and, in our OPINION largely by the actual in-situ conditions to which a wood board is exposed.

For example, wooden deck boards on a deck built right on the ground in a northern climate such as Minnesota are exposed to near constant wet conditions on their under-side and more dry conditions on their upper-side, while a wood deck in a hot dry climate like Arizona and built four feet above ground doesn't see much change in moisture once the boards are nailed or screwed in place.

Cupped wooden walkway boards on ground (C) Daniel Friedman Paul Galow at InspectApedia.com

Above: nearly all of the boards on Paul Galow's Poughkeepsie, New York on-ground wooden walkway are cupped. The upper surface of these boards is somewhat protected by a wide roof overhang while their underside is just a few inches above the soil.

Algae growth on shaded deck built on the ground (C) Daniel Friedman & Paul Galow InspectApedia.comThese on-ground decks built by our friend Paul Galow (Poughkeepsie NY) are mostly shaded.

That green color you see is algae that, when wet, is the slipperiest substance known.

Barkas explanation of wood shrinkage and thus cuppingThis article series 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.

In 1941, Barkas described wood shrinkage and related cupping.

We think that wood shrinkage affects wood, of course, as its moisture content falls from the high moisture content of green wood, but we also think that these same effects explain wood expansion in response to rainfall or in response to being placed on or close to wet soils.

So Barker's "wood shrinkage" is probably reversed and becomes wood "expansion" if relatively dry boards are nailed or screwed onto a deck and then exposed to wet conditions.

"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 usually attempted.

These differences in shrinkage may be accounted for by the presence of ray cells in the wood ...
" - Barkas (1941) [11]

Bottom Line: Flatsawn Boards Go Bark Down Unless ...

Deck board rot on a ground level deck, Green Cabin, Two Harbors MN  (C) Daniel Friedman at InspectApedia.com

Above we show details of rotting treated wood deck boards on a ground-level wood deck as conditions existed in October 2022. This entry porch on a small cabin in Two Harbors Minnesota was constructed in 2006. Yellow arrows mark the worst areas of rot developing in these deck boards.

Below we show the end of one of the most-rotted boards on this deck.

Rotted on-ground deck board, bark side "down" (C) Daniel Friedman at InspectApedia.com

The on-ground location of this deck is likely to be an important contributor to deck board rot. However let's take a close look at two adjacent boards, one badly rotted, the other in much better condition.

We empasize that as every wood board is unique, the reasons that the right-hand deck board shown below is more-rotted may include more than having been placed bark-side "up" - for example the board may have been less-penetrated by wood preservative, or it may have had a more-soft core at the time it was milled.

Rotted on-ground deck board, bark side "down" (C) Daniel Friedman at InspectApedia.com

Nevertheless, just above we can see a clear example in which a bark-side down board survived better than her sister who was placed bark-side up.

Below we show another board in this same deck, one with notable cupping that holds water, inviting greater algae growth and possibly rot.

Cupped deck board, Two Harbors Green Cabin (C) Daniel Friedman at InspectApedia.com

Why do some 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 and its center to "cup" down.

Thus the experts argue for bark side down, so that the "arch" of the board will be facing up.

while

regardless of whether a board is placed bark-side up or down, the wetter side of the board tends to expand more than its more dry surface, causing the wetter-side of the board to "arch".

So how should you place each wooden deck board or wooden stair tread?

Look at each board.

Don't install a deck board with a significantly concave surface facing up.

In the case of the stair tread shown just above, it has a concave upper surface. If the board is already very cupped "the wrong way" and you install it with the concave surface up, we don't think that frequent wetting is going to be enough to flatten the board or make it arch upwards.

Watch out: Worse, because the air flow and sunlight exposure are much less on the underside of a deck, especially for a deck built close to the ground; in a wet climate the underside of the board on a low deck, say just 1 meter or less above ground, is likely to stay wetter longer than its upper surface.

That means the board's upper surface tends become concave, holding water enough to grow algae and become slippery.

If possible, I'd have left this tread in the photo above in the lumber yard and picked a different one.

Synonyms for "bark side up" board position are "convex" or "arch side up" or "rainbow".

Which way should the bark face when stacking firewood?

Stacking firewood has different considerations from placing wooden deck or stair boards bark-side up or down.

When stacking firewood outdoors, exposed to the weather, we stack firewood with bark-side "UP" so as to better shed-water. If you store firewood exposed to the weather and bark down, the wood tends to hold water longer and may dry less, burn more-poorly, and rot faster.

But when stacking firewood under-shelter (in the wood-shed) you can stack bark up or down according to your preference.

Deck Board Bark Side Up vs Down Research

Deck with some boards bark up and others bark down (C) Daniel Friedman at InspectApedia.com

Photo: for some wood construction like these factory-milled tongue-and-groove 5/4" deck boards used on a stage we build in northern Minnesota, the builder has no choice in which side of the board must be place "up" - the factory-cut T&G allowed only one side of the boards to face upwards.

But the factory paid no attention to which side of these boards was "up" when they were milled. The result is shown above. Some boards have bark-up and others, bark-side down.

This stage is now about twenty years old and has withstood the Minnesota weather very well, but in our OPINION some of the bark-up boards that cupped upwards show a bit worse wear.

Hoadley's Understanding Wood, A Craftsman's Guide to Wood Technology, using the book cover and annotatios by InspectApedia (in yellow) illustrate some of the basic features of wood and wooden boards or timbers. Cited & discussed at InspectApedia.com

Illustration, adapted from the cover of Hoadley's book cited just below.


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