Wood Grain Orientation to Prevent Cupping

[Todd Crawford]

I'm putting up some privacy fence in the next couple of weeks. What would be the correct orientation of the wood grain to prevent cupping of the vertical boards. It will be constructed as a "board on board" fence. I believe I remember it being that you turn the inside of the curve of the grain towards the support member. Is this correct?

[Todd Crawford]

Also any experience concerning nailing through both boards at the same time.

[David Freed]

First question - Yes.

What species of lumber are you using?

[Todd Crawford]

Pressure treated Southern Yellow Pine - Thanks

[David Keller NC]

"I believe I remember it being that you turn the inside of the curve of the grain towards the support member. Is this correct?"

Nope - that's completely opposite. The reason is that wood shrinks/expands most along the growth rings (i.e., tangential expansion/contraction). Because those growth rings are longer on the outside of the tree, and the shrinkage/contraction is a percentage of the length of the ring, the board will cup towards the outside of the tree and away from the heart as it dries.

If you want to restrain this tendency, you will need to have the heart of the tree face away from the support, though keep in mind that you can't restrain wood movement over the long haul - boards mounted this way may crack down the center as the edges put pressure on the center, and if you mount them the other way, they will cup.

[Chris Padilla]

Mount the boards with the best face that YOU will see and don't worry about growth rings and any potential cupping that may happen. It is a fence, after all, and if properly dried, should stay relatively flat. Good luck nailing through SYP...it could be challenging. :)

[David Keller NC]

Tood - One other thought. Since this is a fence, I'm assuming you're working with treated SYP. This product is supplied very wet. That's fine if you're whacking together just about any outdoor project, but if you're building for appearance, you might wish to stack and sticker the wood several weeks in advance.

It will shrink quite a bit without allowances for drying it out, and that may result in gaps between the boards of your fence that you find unacceptable.

As to nailing SYP, you can certainly do it with a good 20 oz framing hammer, but if your fence is of any size, you might consider renting a framing nailer and a compressor. Also recognize that you must use special nails when fastening treated lumber. The new MCQ stuff is kinder to fasteners than the ACQ stuff that was used as an interim solution to the older arsenic copper chromium treated wood, but it's still corrosive. For that reason, one needs to use coated nails (not galvanized - the chemicals in the wood will eat up a zinc coating in a few weeks).

[Leigh Betsch]

More important is how you nail the boards. There is a specific way to nail the boards onto the rails. IIRC you put one nail through the center of the board, never nail the edges or the boards will split when they shrink. Somebody else probably knows more.

[Todd Crawford]

The board have been drying for about 3 weeks now and it will probably be about 2 more weeks before I begin nailing them. Thanks for the advice. I just purchased a Paslode framing nailer, and have the correct nails (ring shank for treated lumber) I'll post picks of it when I finish. I finished setting the post today, but have to leave for some training in Denver and will not get much time to work on it until Memorial Day weekend.

[David Freed]

I believe I remember it being that you turn the inside of the curve of the grain towards the support member. Is this correct?


Nope - that's completely opposite

I agree. I answered wrong at first because I was thinking of the inside of the curve of an already cupped board, not the curve of the grain. I need to learn to read better.

I also agree with Leigh; one nail through the center. You can add more in a couple months. Even though you are letting it air dry now, it may still shrink more after it is installed. I built a wooden bed for my pickup with 6/4 poplar kiln dried to 6%. After a couple months, the boards totaling 8' in width shrank 2 more inches.