Will my horizontal fence sag with 10 foot boards?
If you are using 1X4X10 horizontally, you may notice a bit of sag over time. The wider the board, the less likely it will sag. 1X8 would be a good choice, but if you think you are going to keep a consistent 1/4" gap across all boards, that will not happen. Not all the boards will be straight to begin with, and when moisture gets introduced into the mix of things, there is no way to predict what will happen with a gap that may be good at the time of install.