Shrinking Top Railing Board Question

Rookie question here..... My top railing board shrunk at the corner.

I recently attempted to install new pressure treated deck boards for my top railing. After a couple days of the boards drying, my 45 degree cuts didn't look so nice. They were never perfect but they certainly looked better than this. What did I do wrong?

Side note: I installed the boards the same day I purchased them. The wood was pretty wet when I did the install. Was this my mistake? Any help is appreciated!

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Editor's Comments:

This is such a great question and fundamental to building with wood - especially outdoor projects like decks.

Dimensional lumber shrinks at different rates along its length, width and thickness. Think of this three dimensions as an x, y and z axis.

What Happened To Your Board

Almost all lumber is flat sawn because that is the way mills get the most lumber from a log. They get less waste. Unfortunately, a flat sawn board with its wavy open grain appearance is physically configured to experience most of its shrinkage along the width of the board. This movement is juxtaposed with the minimal shrinkage of the length of the board.

A Perfect Storm Of Amplifying Forces

A board shrinks up to 4-6% across grain lines, which happens to be thickness for a flat sawn board. It shrinks up to 8% tangentially or parallel between grain lines which is the width of a flat sawn board. It only shrinks about 2% along its length.

So with a flat sawn board you have a perfect storm of shrinkage converging in a way which visually amplifies the movement. This is most evident when you cut a 45 degree angle on the end of the board just as you have.

The length of the board has not significantly changed. The outside length compared the length of the inside edge are much the same as they were a few days ago.

But what has changed is the width of your board as likely shrunk from 5-1/2" to 5-1/4". Doesn't sound like much but it dramatically changes the angle from 45 degrees to maybe 42 or 43 degrees. And this leaves a big gap.

To learn more about how a log is cut and how it shrinks I explain this in detail in another article.

Different Rates of Shrinkage For Different Species

Different types of wood shrink more than others. Pine, but particularly southern yellow pine which grows very fast with growth rings as much at 1/4" retains so much water and this water is released very quickly during the weathering process. As a result you will see yellow pine twisting like a licorice stick in worst cases.

Other species like cedar, redwood and other tighter grained wood tend to move less as the water evaporates.

Tricks of The Trade

Even if you use pine, there are some ways to get those forces in the perfect storm I was talking about to cancel each other out - or at least diminish each other a bit.

Quarter sawn lumber in any species will be more stable and shrink less along the width of the board because of the grain direction, vertical instead of flat.

Your Situation

You have one board that appears to be more like a quarter sawn board and therefore probably did not shrink width wise as much as the other board which is clearly flat sawn.

I am willing to bet that the width of the flat sawn board is less than the quarter sawn board. If both of the boards had been flat sawn the gap in the boards would have been even more.

So this is a good lesson for anyone planning to build their own deck or railing using common lumber and miter cuts. You can get a better sense of this in this more detailed article.