I'm building a fence with horizontal slats made from cedar with pine posts. The posts have dados cut into them and I'm planning to slide the slats down the dado and affix them to the post using a pocket screw jig with decking screws. The slats will be 10 feet long with a quarter inch spacing between each. What I want to know is if the slats are too long such that they will sag in the middle and what I should do to prevent this short of just making shorter fence panels. Thanks in advance.

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    I'm confused about sliding them -down- into the dados, do you mean horizontally? Also, the answer depends on the size of the slats, please provide the dimensions – Ack Apr 2 at 0:21
  • You really won't need to pocket screw the slats in place. If the dadoes are 3/4" to 1" deep, you could pin the slats in place with stainless steel finish screws, or at least a finish screw that is compatible with cedar, so the tannins in the cedar don't start making gray streaks at the fasteners. The pockets would be quite detectable, compared to using trim head screws. – Jack Apr 2 at 4:28

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.

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