The question inevitably comes up when laying wooden decking, “does it matter which side faces up”? Well, it actually does. The direction of the wood grain plays a significant role in terms of how the board may change shape in the future. Repeated cycles of rain, sun, hot and cold, will test the durability and stability of any material – wood is no exception.
Wood will attempt to straighten itself out with respect to its grain. This means that depending on the direction of the grain, the wood will either form a “cup” shape or a “crown” shape as the level of moisture content fluctuates. As liquid enters or leaves the wood, it will either expand or contract respectively. A direct consequence of this is that almost certainly at some point the wood will change shape in an undesirable form.
Shape-change in wood comes in several different forms. Some of the more common ones are bow, crook, kink, cup, and twist. However, the only one that’s relevant to this particular article is the “cup”. Cupping creates a “cup” shape across the cross section of a given 2 by 4 (or any board), and is a fairly common occurrence with wooden decking.
Technically speaking, there is both cupping as well as crowning. You can think of cupping as a smile and crowning as a frown. There is some debate concerning which one is the lesser of the two evils, but I think you should decide for yourself after all the major angles have been covered. There are both obvious pros and cons as well as ones that are more hidden.
If you have the budget to be picky as to the wood you use for decking, it’s definitely better to select what is called “quarter-sawn” (see image – edge grain) boards. Quarter-sawn boards are those whose grain run perpendicular to the longer of the two edges of a board’s cross section. These boards are generally more expensive and far less common than their counterparts – flat-sawn.
Quarter-sawn boards are considerably more stable than flat-sawn due to the direction of its grain. Swelling and shrinking of wood takes place mainly in the direction parallel to the growth rings, which is why you can easily see how quarter-sawn boards will be more stable than flat-sawn. However, procuring a full set of quarter-sawn decking boards may not be feasible.
Cupping, which is the “smile-shaped” wood warp, results from what’s called the “bark-side up” installation method. This method is arguably the one of choice for many professionals, due to possible splintering if done “bark-side down” – also known as shelling (see further below). But there are also many experts who prefer the “bark-side down”, so I think it’s more a matter of priority and personal taste after considering the various pros and cons.
Having touched on the less-than-favorable results of shelling (due to crowning), cupping has several downsides to it as well:
- People can trip on the protruding geometry.
- Water accumulates in the “cup” and is cause for premature rot.
- Though subject to personal opinion, it generally doesn’t look very good.
- Depending on the severity of the cup, the fasteners can be pulled partway through the board causing them to loosen their hold. I’m sure you can see how this can cause problems down the road via loose and “rocking” boards. This “see-saw” motion will, if severe enough, slowly loosen the board to the point of complete failure.
- Cupping results from bark-side up, which means the sapwood – not the heartwood – is the face that is exposed to the elements. There are both pros and cons to this arrangement. Sapwood, although generally less resistant to decay and insects, can absorb anti-decay treatment better than heartwood.
Perhaps the most important consideration in the bark-side up or down debate – or why some promote bark-side up – is that bark-side down (crowning) often results in what’s called shelling. Let me explain. There are two main components in a tree’s growth rings – earlywood and latewood. Earlywood is formed in the early parts of the growing season, and is the part nearer to the heartwood (tree center).
Latewood is the outer part and is formed in the later parts of the growing season. Shelling occurs when these two parts – earlywood and latewood – split apart due to repeated cycles of wet and dry. This splintering of the top surface results in damage from where further rot and damage can take place – not to mention the painful consequences of walking bare-foot on such surfaces.
This is the “frown-shaped” type of wood warping, and can be thought of as the opposite of cupping. Experts will often advise against this form of deck installation due to the possibility of shelling. However, there are certain types of wood that are more prone to shelling and there are those that aren’t. Some say that shelling is limited to certain select softwoods such as Southern yellow pine and Douglas fir.
Having discussed the above-mentioned con of shelling, let’s talk about the pros of the crowning form of warping:
- It generally looks better when compared to cupping.
- There’s no trip hazard.
- Water drains off easily and therefore dries quickly.
- Although the center of the board will mound up and therefore put some tension on the fasteners, there won’t be any “see-saw” motion. In either case – cupping or crowning – the fasteners will most likely be pulled through the wood to a certain degree. The difference is that with crowning, there won’t be any additional stresses due to the rocking “see-saw” motion.
- Due to the board being bark-side down, the heartwood, as opposed to the sapwood, will be exposed to the elements. As you may know, heartwood is generally more decay and insect resistant – though as mentioned above, it doesn’t absorb anti-rot chemicals as readily as sapwood. This is the better choice if you don’t plan on treating the boards.
After all is said and done, there are proponents for both sides of this debate and one can only decide for themselves after taking into consideration the various points and angles. It seems there are experts on both sides of the fence as well, and there doesn’t seem to be a clear or definite better option. However, taking all things into consideration, it is my personal opinion that the bark-side down is the better option. The following page has additional information that supports this opinion – Wood Board Cupping.
Other Tips when Fastening the Decking to Joists:
- When fastening 2 by 4s to the joists, always use two fasteners per joist. If you’re tempted to minimize the number of fasteners, think again. The width of a 2 by 4 is much too wide to try to scrape by with only one fastener. Even with the bark-side down installation method (growth rings in frown), higher levels of humidity below the deck when compared to above, will cause the lower surface of the board to expand parallel to the grain. This lopsided humidity and expansion can cause the board to cup regardless of direction of grain. Two fasteners – as opposed to only one – per joist will pin down the 2 by 4 closer to the edges, greatly minimizing the amount of cupping.
- Use fasteners with wide heads for better hold. This will minimize the degree of pull-through when the wood tries to twist and turn.
- Don’t sink the head past the surface of the board. Doing so will allow water and debris to pool causing premature decay. The trick here is to ensure the 2 by 4 is sitting tight on the joist so you don’t have to add additional torque to pull the two against each other. It’s often tempting to just go ahead and wring the fastener a few millimeters past the surface in order to tighten the board against the joist, but this will cause problems down the line. With harder wood you may need to tap out a hole prior to fastening as not doing so can cause the 2 by 4 to “float” even after the screw head is flush with the surface.