Deck boards warping or cupping — San Jose, California

Deck boards that bow upward (cupping), twist, or warp are typically caused by moisture imbalance — the bottom of the board absorbs ground moisture while the top dries in the sun. Pressure-treated lumber is especially prone to warping in the first 1–2 years as it dries from its initial chemical treatment. Other causes include improper joist spacing (joists should be 16 inches on center, or 12 inches for composite), boards installed without adequate gaps for expansion (1/8–1/4 inch between boards), or fasteners that don't allow the board to move naturally. Mild warping can sometimes be corrected by flipping boards, adding mid-span blocking, or using deck board straighteners. Severe warping requires board replacement — $5–$15 per linear foot for pressure-treated, $10–$30 for composite.