As often as every deck is built the question arises should you place the bark side up when you install the decking or should you place the board so the bark side is down.? The reason for this is to reduce cupping or warpage of the board. In shingling the same is true there is an inside and outside of many shingles.
Here we will take a look at how to identify the growth pattern of a board and try to understand what advantage you can see by installing it one way or another.
When a tree grows it forms rings in the trunk. The smaller radius rings are on the inside of the tree and as you look out towards the bark of the cross section you will see tighter rings with a larger radius. Growth pattern rings occur every winter when the tree goes dormant. In the spring new soft growth happens in the center of the trunk to allow water from the roots to be distributed to the branches and leafs.
Because older wood is forced to the outside of the trunk as the tree grows the outside wood becomes denser and stronger then the inside wood. You might have seen a tree in the woods that has fallen and the center of the trunk gets rotted out first.
Well in this same idea builders want to place their boards on the deck floor so the bark side or more dense wood is up and will weather better. The denser wood should shed and repel water better and standup to more foot traffic.
This seems to be a good idea but the advantage is very small. Tests have shown that a bare board will have about 2% higher water content on the bottom of the board then it will on the top. Even coating the top of the board with stain sealer will only have a slightly better result.
So, this is a difficult question to answer because boards are not cut in a way that can always allow you to place the bark side up but in studies by the Forest Service it seems that you have a very small advantage by placing the bark side up in decking or out when applying wood shingles or siding.
The real factor of cupping cracking checking is the difference in moisture from one side to the other and no matter which side is facing the weather you are always going to have different moisture levels. Additionally when small trees are harvested you may have a larger difference across the board width. If you have bark side on the left and pith on the right that will increase the chances of cracking and splitting.
So, in best practice try to take a second and look at the growth ring pattern on the end of the board but don’t let that overrule using the best looking and lower defect side as the up side.
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