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New building access ramp (C) Daniel Friedman Wood Board Cupping
Why do Boards Cup; Should Wood Boards Be Placed Bark Side Up or Bark Side Down when Building Exterior Decks or Stairs?

Bark side up or down on exterior wood decks & stairs:

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 DOWN" - that is pith side (tree center side) of the board facing up.

We include a warning about "shelling" damage on decks and steps - a phenomenon that occurs more on the pith side of boards.

Page top photo: in this wood-framed wall the top plate has been nailed "bark side up" - which matters much less in wood frame construction that will be covered by final building roofing and exterior walls. In framing walls, our opinion was that all else being equal, we'd nail the top plate or sole plate of a wall with its arch "up" or "convex" simply because it was easier to nail the edges of the board down to the studs below.

There has been plenty of argument about bark up vs bark down. Field experience, FPL experts, and further discussion here all agree that because a bark-side-down deck board tends to arch upwards or actually to flatten as the wood dries, "bark side down" is the usually the best practice when building wood decks, steps, ramps, platforms.

Really? Well not always. We make an exception when flooring a deck or porch, especially outdoors, if a board is already so extremely cupped that it's not likely to flatten out if nailed with its cup facing up. We flip those boards over so that their high arch is "up" regardless of where the bark side is found.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Cause of Outdoor Deck Board & Stair Tread or Wood Trim Cupping & Shelling

Why Boards Cup in the First Place: moisture differences dominate wood board cupping

End grain view of flat sawn board (C) Daniel Friedman Which side should go "Up" when placing wood deck boards, stair treads?

The two fundamental factors affecting 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. Although we see grooves cut by the manufacturer to serve as a walking surface (facing up in the photo) the board has cupped its outer edges upwards towards the bark side of the tree.

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 like the one above will cup more than quarter sawn boards, and board cupping also varies by wood species, where the board was cut from the tree, and also by variations in the installed location and position of the board with respect to weather, rain, ground moisture, wind drying, sunlight and other factors.

Board guardrail top cupped towards the bark side (C) Daniel Friedman & Paul Galow, InspectApedia.comPhoto: Paul Galow's deck guardrailing top board, cut from near the heart of the tree, is almost quarter-sawn, and has still cupped significantly towards the bark side - the under-side of the board in this case.

Depending on from where in the tree a board is cut and whether it is flat sawn or quarter sawn, the board's future weather and water exposure, it may absorb more water in its wider, center tree-growth rings, expanding more in the center, tending to "cup" (towards the bark side) and so if the board's bark side had been "arched" upwards, it might flatten out.

Article series contents

If you're unsure about the how the wood grain will show up in different lumber saw-mill cutting methods, see the definitions of "flatsawn" or "flat-sawn" or "plain sawn" boards or lumber given

at LUMBER SAWING METHOD DEFINITIONS

Definition of Concave vs Convex Board Shapes

If you're not sure about cupping direction or the definitions of concave and convex, this U is concave (think "arch down" or think "fruit bowl") and this n is convex (think "arch up" or think "rainbow") - courtesy of James MacQueen Contractors cited below.

A concave board has its outer edges curled upwards while a convex-cupped board has its outer edges curled downwards.

Flat-sawn boards cup more than quarter-sawn boards.

Wood Cupping & Uneven Moisture Content

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

Above: this ground-level deck board on an entry to "the Green Cabin" in Two Harbors Minnesota, is one of several that was nailed bark-side up and has cupped further so that in 2022, sixteen years after it was first nailed in place, the board is cupped enough to hold water after a rain, encouraging more algae growth and ultimately, rot.

Other boards of this deck that rotted before their sisters are illustrated at ANSWER to BARK SIDE UP or DOWN

We think that an on-ground or close-to-ground deck board stays wet longer on its under-side so may tend to cup up regardless of bark position. Below we explain why.

Two wood board cupping truths that individual boards may not always obey:

  1. The wet side of of a wooden board tends to expand more than its dry side.

    The "expanding" side, by getting wider, will tend to curve or cup such that the center of that side arches or becomes convex.

    The moisture level may, in some boards, overcome the effect of the natural cupping towards the bark-side - point 2 below.
  2. Flatsawn boards tend to cup (form a concave or bowl-like surface) towards the bark side.

    The bark side tends to cup "towards" the outside of the tree or in the "bark" side direction.

    So if a board is installed "bark side down", the bark side of the board tends to form a "cup" with an "arch" facing "up".

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 are 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. [38]

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. That is, a low deck's under-side may remain more-wet longer than its upper surface.

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.[7]

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

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

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 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 of winter wood (red arrows) occur in the center of the board and arch upwards towards the bark side of the tree.

The thicker rings of growth wood, aka "summer wood" are marked by blue arrows in our photo below.

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

The darker, thinner lines in our illustration above 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 (as wood dries) 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

Hazards Caused by Cupped Wooden Deck Boards or Stair Treads

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.

Watch out: Wetter boards also collect more algae and are a serious 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.

Shelling Damage to Walking Surfaces on Wood Decks & Stairs

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.

See STAIR TREAD NOSE DIMENSIONS and

see SLIPPERY STAIRS, WALKS, ROOFS for details.

Bark Side Up or Down for Stacking Firewood?

Stacke firewood bark side up or down (C) Daniel Friedman at InspectApedia.com

Above: in Nancy and Edna's New York woodshed, the firewood is stacked with bark side up and down and mixed (left side of photo) but as the wood is reasonably-protected from the weather its position is of less concern.

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 [DJF], and split and stacked a lot of wood, it's easy to understand the argument. [39]

Stacke firewood bark side up or down (C) Daniel Friedman at InspectApedia.com

These firewood logs are stacked roughly bark-side up and would, exposed to some wind-blown rain entering this open firewood shed, be a bit better at shedding water without soaking the wood. Stacked in the open this would be perhaps more important.

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.

Stacke firewood bark side up or down (C) Daniel Friedman at InspectApedia.com

Above: smaller-diameter round log firewood that has not been split can be stacked without worrying about "bark side up" - but of course will also dry out more-slowly than if it had been split.

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. Stack 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, ROOFS we discuss the SCOF (static coefficient of friction) for wet algae-covered surfaces.

IPE Wood Deck Board Cupping

Reader Question:

Cupped ITE deck boards (C) InspectApedia ET

Three months after constructing a new Ipe deck some of the boards became cup shaped (see enclosed photos).

Soaking two pieces of the leftovers resulted in one of the boards sink and the other float.

Can this 'experiment' prove that some of the boards are not Ipe?
Awaiting your expert advice. E.T. by private email 2016/01/18

Cupped ITE deck boards (C) InspectApedia ET

Reply:

My opinion is that IPE, described by IPE Depot as " Ipe Decking. Ipe is an exotic hardwood that is naturally resistant to rot and decay, is 8 times harder than California Redwood, and is guaranteed for 20 years without preservatives!" - http://www.ipedepot.com/compare.htm

Should not usually float. A cubic foot of water weighs about 62.3 pounds. According to IPEDepot, a cubic foot of IPE weighs 69 pounds - it would sink in water.

Cupped ITE deck boards (C) InspectApedia ET

However as this is a "natural" product, I would not assume that all IPE boards are absolutely uniform in density or weight. So some boards or board cutoffs may be a bit under-weight.

Gibson (2000) points out that IPE is actually a broad term for a number of similar wood species. And White (2007) points out a range of densities among woods classified as "IPE". So your particular IPE deck boards may be less dense.

Furthermore, any natural wood deck board is exposed to cupping if the moisture differences across the board are significant.

Cupping will usually show the concave side of the cup facing the more dry side of the board - which is usually "up" on a deck.

Boards can misbehave and cup "backwards" for other reasons as we discuss ad-nauseum in our articles on deck board cupping beginning at the top of this page.

I'd look into the cause of cupping rather than thinking that if you'd had "better or more real" IPE no cupping would have occurred.

Watch out: there may be environmental and protected wood species issues with exotic woods used in decks or other construction. See Gibson (2000) who discusses chain of custody certification for IPE wood sold in the U.S.

Reader follow-up: Use fluorescence testing to identify IPE wood boards?

Thank you for your detailed answer with the valuable information.

It appears that the 'soaking test' is not absolutely conclusive to determine the exact identity of the boards. Will the 'fluorescence test' (as described in links given below) - http://www.wood-database.com/wood-articles/fluorescence-a-secret-weapon-in-wood-identification/, http://www.wood-database.com/lumber-identification/hardwoods/ipe/, http://www.wood-database.com/lumber-identification/hardwoods/cumaru/) be conclusive? - E.T. 2016/02/16

Reply: apparently not

I have not found IPE listed among woods that are easily identified with fluorescence testing. (Dyer 1988, Sum 1991, Pandey 1998, et als cited below).

Have you gone to the supplier of your wood to ask for documentation of where the wood was purchased and where it was harvested?

Wood Properties & Wood Identification

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

In this illustration, adapted from the cover of Hoadley's book cited just below, our yellow annotations point out some of the features of wood using Hoadley's cross-section of a tree.

[Click to enlarge any image]

What Wood is That - Edlin - at InspectApedia.com

Wood identification samples  for 41 common species (C) InspectApedia.com & Herbert Leeson Edlin

Mr. Edlin's wood identification guidebook includes actual samples of wood, as we illustrate just above, adding our own addition of Hickory flooring. - Ed.

Understanding Wood, R. Bruce Hoadle, Taunton Press - cited & discussed at InspectApedia.com

Timber and Trees Laslett's 1894 book available as a PDF download cited & discussed at InspectApedia.com

Wood Board Cupping & IPE Deck Board Properties

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2023-06-07 by InspectApedia Publisher - IPE decking cupping or warping & checking

@Wayne Perrier,

Thank you for a helpful question about IPE decking cupping or warping & checking.

I'm sorry, but in my OPINION the process you described is almost guaranteed to cause deck board cupping.

You don't say how high above the ground is the under-side of your deck nor do you give country, city, climate, surface runoff conditions or other features that would affect the level of moisture under and around the deck, and I agree that often, once installed and initial drying has occurred, the area under a deck *might* be more-moist than above it - contributing to cupping.,

But in this case, coating one side of a board and then setting it out in the weather pretty much guarantees that there will be significant moisture differences across the thicknesses of the board. Just picture what happens at the first rain! We soak the board but its bottom is sealed - it's nearly a "cup" for holding moisture.

The fact that you said you expected the one-side-coated boards to "dry out, then come bacck and sand the tops and stain it" tell me that the boards were not dry at time of installation and that you coated one side - guaranteeing un-even drying.

If that's the primary moisture source (haven't had much rain?) then the boards would be expected to cup or arch "down" as the upper side dried more than the under-side.

But there's an opposite possibility: help me out - and embarrass me if you like - by telling me: are the boards cupped "arch down" or arched (convex or "arch up") ?

If there has been rain, then I would expect the boards to be absorbing more moisture on their un-coated side, causing more expansion of that side, that ought to make them arched, but, then I'm not sure I've got the whole picture here.

Now what are we going to do about it?

Given the cost and trouble involved I suspect you're going to end up trying to protect the deck from rain, let it dry out, and in the driest of your season (where are you ?) if necessary sand or even plane the exposed board surfaces flat, then for heaven's sake, seal all of the board surfaces uniformly.

An alternative would be to dis-assemble all of the deck boards, dry them out in your garage - to below 18% moisture- , then run them through a planer to flatten them out and assure that they are all of both uniform thickness AND that the clip-gaps along their sides are at a uniform distance in the board so that on reinstallation they'll be uniform and flat, stain/coat ALL sides of the boards, and re-install them.

Post some photos (one per comment) to show us what's going on.

For other readers, IPE decking is a Brazilian hardwood, also referred to as Brazilian Walnut. Although this is a hard wood, it's generally expected to air dry rapidly and with only minor checking, warping, cupping, or arching IF your boards are not particularly thick.

Thicker IPE wood such as 4x4 IPE posts or wider IPE boards like a 2x12 IPE are much more prone to cupping and have been reported cupping badly-enough to actually pull deck screws right out of the deck joists below.

Sealing ALL sides of your cut-to-fit IPE deck boards at the same time and staying away from wide boards like 2x12s can minimize the cupping problem.

But

Watch out: if you coat or seal the surface of just one side of ANY board it's going to be more-likely to cup or arch.

IPE Lumber Resources

CHARACTER GRADE IPE DECKING, LUMBER AND TIMBERS [PDF] from Tropical Forest Products, 1-905-672-8000 or 855-344-4500 info@weraetropical.com


Also see the US FPL WOOD HANDBOOK [PDF] that begins discussing IPE lumber on page 2-25.

Below: this photo is an Example for other readers - image of IPE Brazilian Walnut is from novausawood.com/

IPE Brazilian Walnut ovausawood

On 2023-06-07 by Wayne Perrier

hi, I have a deck being built that is 5/4x6 IPE decking and it is slotted on the sides so that a cleat system is used to hold the boards down instead of driving screws through the face of the wood. The deck builder told me that it was the "nicest" IPE he has ever received.

We have alot of long boards, some 20 feet. the bottom and sides were stained with an oil or solvent-based transparent stain. the tops were left unstained and the idea is to let the boards dry out, come back and sand the tops and then stain it.

We are starting to see cupping -- it's only been installed about a week. My deck guy says that if we had stained all sides "before letting the IPE dry out" then we would get alot of splitting.

I'm getting worried that the cupping will not go away on its own. Can you advise? It's about a 600 sq ft deck and we've put alot of money into it. Thank you in advance. wayne@perrierfamily.com

On 2022-09-17 by InspectApedia-911 (mod) - do I lay the board with bark side up or down for decks exposed to winter?

@Tom Ambalam,

Thanks, we're reviewing the text and will clarify it.

The topic is confusing and the right answer is found in detail at ANSWER to BARK SIDE UP or

There we state

For flatsawn boards, if we want to follow the advice of wood experts cited at REFERENCES (https://inspectapedia.com/decks/Wood_Board_Cupping_Answer.php#reviewers) 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.

BUT THERE ARE EXCEPTIONS to THE RULE:

Our object is for the arch or convex side of the board to face "up" so as to drain better and last longer, but we have seen in practice that sometimes a board doesn't cup the way we expect, so we inspect each board at the time of construction and if it's visibly cupped we will put the "arch" "up" regardless of the tree rings visible on the end of the board.

Thank you for the question; I'm working on the text.

If you return in an hour or more and clear or refresh your browser text you'll see the updated page and we will appreciate any further questions or critique.

DF & AC

On 2022-09-17 by Tom Ambalam

I read this article a couple of times but I am confused about the bottom line - on whether to lay the board with bark side up or down for decks exposed to winter.
The article starts with a premise that for decks , bark side should be on top. But the text below is so different.


...

Continue reading at ANSWER to BARK SIDE UP or DOWN or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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