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?
     


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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
  • For flatsawn boards if we want to follow the advice of wood experts cited at REFERENCES for this article we'd put the bark side down when setting deck boards or wood stair treads.

    The technical basis and scientific support for bark side down are detailed at BARK SIDE DOWN ARGUMENT.

    If we prefer to follow the opinion of many deck builders whose opinion arises from years of field observation, we'd put the bark side up unless the board is already cupped - that is, we like to place deck boards such that the arch shown by the board's growth rings (visible at the board end cuts) tends to be "up".

    In sum it seems that carpenters like bark side up and most people of either camp will agree that moisture differences in practice tend to dominate cupping effects.

    Some reasons that builders like bark side up, a less technically-supported position and one that has been my [DF] own for a long time, are at BARK SIDE UP ARGUMENT.

    Why do 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 (thus arguing for bark side down) while frequent soaking of the upwards facing surface of a deck board wants to make the upper surface of the board expand more than its bottom surface, causing the board edges to curl downwards toward the tree heart. The ultimate effect may be flatter deck boards or less cupping in actual use.

    Look at each board. Don't install a deck board with a visibly concave surface facing up. In the case of the stair tread shown just above, it has a concave upper surface. If possible, I'd have left this tread in the lumber yard and picked a different one. Apparently the manufacturer who pre-grooved tread walking surface patterns on the intended "upper surface" of the tread was not paying attention to the end grain pattern of the boards used.
  • For quartersawn boards or boards whose end grain shows that the quartersawn board pattern dominates the board, look at the board by eye and place its most arched side with the arch up.
  • The wetter side of boards will tend to arch upwards as that side of the board expands more than its more dry side.

    A result is that boards used on wood decks whose upper surface is regularly more wet than under surface may tend to develop concave cupping in the deck surface. Our field experience is that this effect is less in boards placed as we described for flatsawn just above.

    Barkas explanation of wood shrinkage and thus cuppingAssuring that the deck surface drains adequately (don't but boards too tightly) reduces this cupping effect as well as other slip trip fall hazards such as slippery algae growth (SLIPPERY STAIRS, WALKS)

    [Click to enlarge any image]

The rest of this article 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.

R.W. Barkas, back in 1941, addressed the question of the directions of wood grain shrinkage, stating:

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

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