When most people decide to purchase decking material, the first thing they consider is either the appearance of the wood or its cost. An unfortunately overlooked aspect of decking material is the wood’s structural stability. If you choose an unstable decking material, you run the risk of the deck boards warping or checking. Warping is the tendency of wood deck boards to curl inwards along their lengthwise edges. End checking refers to the ends of deck boards cracking and splitting as they absorb and expel moisture. Both warping and checking are big problems for a deck owner, and can cost a lot of time and money to fix.
Luckily, the stability of a wood species is relatively easy to measure. There are certain standardized measurements that are used to determine the mechanical properties of each wood species, and those properties tell us the stability and durability of the wood in question.
Measures of Stability
The two main properties used to measure stability are tangential and radial shrinkage. Both radial and tangential shrinkage refer to how much a wood species tends to shrink during the drying process. Radial shrinkage occurs in the direction from the center of the tree to the bark, while tangential shrinkage occurs tangentially, or parallel, to the tree’s growth rings. Both radial and tangential shrinkage are measured as a percentage. The lower the percentage, the less the wood will shrink during the drying process and the more dimensionally stable the board will be when installed.
While both radial and tangential shrinkage percentages are important on their own, the best way to measure a wood’s stability is to consider these properties together. The smaller the differential between the two measurements, the more stable the wood. For instance, if a species of wood shrinks twice as much along its width than it does through its thickness, the boards of that wood would warp and bend much more than the boards of a wood with much lower radial/tangential differential.
Stable Wood Species
Now that we’re familiar with how stability is determined, let’s look at the most stable types of domestic and exotic decking woods.
Domestic woods are generally softwood, which means they are less dense, less hard, and can’t stand as much weight. However, this doesn’t mean there aren’t great softwood decking options available. Two of the most stable softwood decking options are Western Red Cedar and California Redwood.
Western Red Cedar
Although cedar is a softwood, it is still very structurally stable. Both the tangential and radial shrinkage are quite low, and the differential between the two is very small as well:
Tangential Shrinkage: 5.0%
Radial Shrinkage: 2.4%
Cedar’s other mechanical properties – strength, stiffness and density – are also relatively high, making this a durable species good for most decking projects.
California Redwood is another very stable decking material. The tangential and radial shrinkage values are even lower than those of Cedar:
Tangential Shrinkage: 4.9%
Radial Shrinkage: 2.2%
Even though these percentages are lower than those of Cedar, the differential is 0.1% larger, meaning it is slightly less stable than the other species – but not by much.
Exotic hardwoods are denser, heavier and tend to have higher tangential and radial shrinkage percentages. Does this make them less stable? Not always. The differentials for some exotics are lower than their domestic counterparts, making them more structurally sound.
The tangential and radial shrinkage percentages of Ipe are much higher than both Cedar and Redwood; however, the differential is much smaller, making it the more stable wood.
Tangential Shrinkage: 8.0
Radial Shrinkage: 6.6%
A 1.4% differential is great, and combined with Ipe’s naturally high density, strength and stiffness, makes it naturally suited for decking purposes.
Merbau is another hardwood with excellent structural stability. The tangential and radial shrinkage percentages are:
Tangential Shrinkage: 4.6%
Radial Shrinkage: 2.7%
These percentages are quite a bit lower than those of Ipe, and even though Ipe’s differential is only 1.4%, a 1.9% differential is still very good. The low percentages combined with the relatively low differential makes this the most stable exotic decking choice. However, Merbau comes with an unrelated and unfortunate downside – read on to find out what it is.
For domestic woods, the most structurally stable option is Cedar, though Redwood is a close second. For exotics, Merbau is the more stable option, with Ipe decking being a fine second choice.
Between Merbau and Cedar, the obvious choice is Merbau. The shrinkage percentages are about the same as Cedar, but the differential is much smaller. Also, having a higher density makes the wood more naturally resistant to rot and decay.
Besides structural stability, there are a number of things to consider when choosing the right decking material. Here are some important facts on the previously mentioned wood species
- The shrinkage values for Cedar and Redwood apply only to materials made of pure heartwood. Many manufacturers will include sapwood in their boards, which greatly reduces the stability and the strength of the wood. When choosing a softwood, check to make sure there is little to no sapwood content.
- While Merbau is an extremely stable hardwood, it does have one unpleasant quality – it bleeds. When wet, tannins in Merbau boards will bleed out an intense red color from the wood and can stain any light-colored surface. Merbau tannin bleed can ruin driveways, cars, and other living spaces. Merbau decks should be treated with a high-quality wood sealer to minimize tannin bleed. Just to be safe, don’t use Merbau over a living space, driveway, concrete slab, or car port.
- While structural stability is certainly important, it shouldn’t be the only factor you consider when choosing a decking material. Merbau may have the lowest tangential and radial shrinkage percentages, but Ipe has an incredibly high density. This density – which naturally protects the wood against boring insects, mold and weather wear – causes many consumers to choose Ipe decking over Merbau. Ipe also has no tannin bleed, making it safe to use over carports and other living spaces.