What should I know before building a horizontal slat fence or privacy screen?
Short answer
Horizontal slat fences and screens look clean and modern, but they are less forgiving than a picket fence. The boards span post to post with nothing holding the middle, so posts need to be closer together, the wood needs to be straight and dry, and every gap has to stay even as the boards move. Choose a stable species, keep spans short, use a hidden vertical cleat mid-span, and plan the gap for the privacy you actually want.
Why horizontal is harder than it looks
A traditional fence hangs short vertical pickets on horizontal rails, so each board is only a few feet long and the rails do the structural work. Flip the boards sideways and every slat becomes its own little beam, spanning from post to post with nothing but its own stiffness to keep it straight. A six or eight foot run of thin cedar will sag in the middle, cup in the sun, and open its gaps unevenly.
The clean lines that make the style attractive are also what make flaws visible. One slat that twists a quarter inch reads from across the yard.
Posts, spans and the hidden cleat
We set posts closer than we would for a picket fence, typically in the range of four to six feet depending on slat thickness, and we add a vertical cleat behind the slats at mid-span so each board is supported at least every three or four feet. The cleat is hidden on the back side or dressed as a design element on the front. Either way, it is what keeps the gaps even five years from now.
Posts for a horizontal fence also have to be dead straight and plumb, because the slat lines will trace any wander. A steel post sleeved in wood is a sensible choice here: it stays straight, does not rot, and gives a solid screw base.
Picking the wood
Stability matters more than anything. Vertical-grain cedar and redwood, clear or near-clear, move far less than flat-sawn boards with wide bands of soft grain. Thermally modified wood and some hardwoods are very stable too but need pre-drilling and stainless fasteners. Kiln-dried stock is worth insisting on; green lumber will shrink and open every gap after it is installed.
Thickness helps. A full one-inch or thicker slat resists cupping and sag far better than nominal one-by fencing boards, and the shadow line looks richer.
- Vertical grain over flat grain.
- Kiln-dried, not green.
- Thicker slats for longer spans.
- Stainless screws, pre-drilled near the ends.
Gap, privacy and airflow
The gap between slats sets the character of the fence. A quarter-inch gap reads as nearly solid and gives full privacy at normal viewing angles. A half inch to an inch lets light and breeze through and softens the wall effect, but a neighbor standing close can see through. For a screen around a hot tub or an outdoor shower we often run tight gaps at eye height and open them up above and below.
Gaps also matter for wind. A solid screen near the coast takes a real load; even a small, consistent gap sheds a surprising amount of it. We talk about that more in our answer on wind and coastal fences.
Finishing and keeping it looking sharp
Horizontal slats hold water on their top edges, so a penetrating oil finish applied before installation, on all four sides and the cut ends, makes a big difference. Plan on a maintenance coat every couple of years on sun-facing runs. If you prefer the silver look, a clear UV sealer slows the checking while the wood grays evenly.
Leave the slats a touch short of the ground. Soil contact and splashback are what turn the bottom board black in a season, no matter how good the wood is.
Related questions
Can I convert my existing picket fence to horizontal slats?
Usually not without new posts. Picket fence posts are typically spaced too far apart for horizontal boards, and older posts are rarely straight enough. Reusing the post holes with new, closer posts is often the practical route.
Do horizontal fences count the same as vertical fences for height rules?
Yes. Height limits are about the fence, not the board direction. Check your city's zoning rules and any HOA guidelines before you design, especially for a screen taller than the fence it joins.
Will a horizontal slat fence sag over time?
A well-built one will not. Sag comes from spans that are too long, thin slats, or green wood. Short spans, a mid-span cleat and dry, thick, vertical-grain stock keep the lines straight for the life of the fence.