Why your fence panel is cupping.

A cupped picket is a picket that has gone from flat to shallow-dish-shaped, with the edges curling away from the back rail. It’s the single most common complaint a fence manufacturer hears, and nine times out of ten it’s not a defect in the wood. It’s a moisture problem the install created.

Wood moves as it takes on and releases water. Every board that leaves our mill is at roughly 12–15% moisture content — the Ontario equilibrium for outdoor exposure. Once the panel is installed, one face dries faster than the other; the faster-drying face contracts, and the picket cups toward it. That’s the whole mechanism.

The first cause, which accounts for the bulk of the problem, is growth-ring orientation on the picket. Every picket has a growth-ring side (the face that curves toward the centre of the tree) and a bark side (the face that curves away). The growth-ring side is less stable; it will cup toward the dry face. Our mill orients every cedar picket bark-side-out on the panel face, so if a picket does cup, it cups inward toward the rail, which the rail resists. But if an installer replaces a picket on-site and flips it — or if a homeowner repairs a panel with loose stock from the yard — the picket cups outward, and the edges lift visibly. If you see one picket in a panel that’s cupped and the others aren’t, look at it closely; it’s almost always a flipped board.

The second cause is fastener pattern. A picket wants to move — expanding in humidity, contracting in drought. If it’s nailed at two points on a single horizontal rail, one nail becomes the pivot and the other becomes the anchor; the board rotates around them and the edges lift. Our panels use three fasteners per picket per rail — two on the outer edges, one in the centre — and we specify a ring-shank rather than a smooth-shank because ring-shank holds the pulled-back board from re-rotating. An installer who replaces pickets with smooth-shank nails or uses two fasteners instead of three will see the problem come back.

The third cause is what the wood sits against. If the back of the picket sits directly against a solid rail with no air gap, the back face can’t dry — the rail traps moisture there — and the front face dries rapidly in the sun. Big moisture differential, big cup. This is why shadowbox panels cup less than back-rail panels: air moves freely across both faces of every picket. The best antidote on a solid-face panel is a 1/16-inch shim or washer between the picket and the rail, which is labour-intensive to do on-site but is how the pre-milled stringer on our Privacy panel is designed.

A fourth cause people blame but rarely verify: bad lumber. We cull heavily at the mill, but no manufacturer catches every short-grain board. If a panel cups uniformly across all pickets at year one, it’s worth photographing and sending to us. We replace defect-cupped panels panel-for-panel under warranty (details on /warranty).

The remedies for an already-cupped picket are limited. You can sometimes flatten a mildly cupped picket by soaking the concave side with water and letting it re-equilibrate for a week, then re-fastening with a third ring-shank nail. Badly cupped pickets are replacements, not repairs. Cupping almost never gets worse on its own if the cause is fixed (flip the flipped picket, add the third nail, open the back to airflow); it just stays where it is.

The best prevention is the install, not the lumber. Set posts at the right depth, leave a half-inch gap between the bottom of the panel and grade for airflow, and don’t over-fasten. Our panels ship ready for this; we’d rather a yard sell the install-guide with the panel than an extra panel without it.