Why Are My Deck Boards Cupping? 5 Common Causes and How to Fix Them

Your deck once looked flawless, but now the boards are curling at the edges or forming a weird concave shape. If you’re frustrated and wondering why your deck boards are cupping, know it is not just a cosmetic issue. Cupping can signal deeper issues that, if ignored, may lead to severe deck damage. Here is what causes cupping and how to stop it in its tracks.

1. Uneven Exposure to Moisture

One of the most common reasons for deck board cupping is moisture imbalance. When one side of the board absorbs more water than the other, it dries unevenly, causing the curl upward.

The Solution

• Avoid blocking airflow with skirting or storage under elevated decks.

• Seal all sides of the deck boards, including the underside, to limit moisture absorption.

• Slope the ground beneath a floating deck to encourage water drainage and avoid pooling.

2. Improper Board Installation

Deck boards installed too tightly, upside-down, or without proper spacing can lead to cupping as the wood naturally expands and contracts.

The Solution

• Check the crown to ensure boards have the growth rings curving down, not up.

• Allow proper spacing between boards of ⅛ inch to ¼ inch for expansion.

• Reinstall boards if they are upside-down or tightly jammed together.

• Use screws instead of nails to secure boards more firmly and reduce movement.

3. Inadequate Sealing or Staining

If your boards are only stained or sealed on the top surface, moisture can still creep in from the bottom or sides, leading to imbalance and cupping.

The Solution

• Seal or stain all sides of each board before installation.

• Reapply sealant regularly every 1 to 3 years, depending on climate and product.

• Use a high-quality water-repellent sealant designed for outdoor decks.

• Don't skip sealing the ends of boards; they are often the most vulnerable to moisture.

• Consider hiring a professional to seal your deck.

4. Poor Drainage Around the Deck

Water that collects around or under a floating deck contributes to constant moisture exposure, causing boards to cup and warp.

The Solution

• Grade the soil to direct rainwater away from the foundation.

• Install a gravel bed or landscape fabric beneath the deck to improve drainage.

• Clean out gutters and downspouts nearby to reduce water runoff pooling near the deck.

• Add a French drain or trench to divert water away from the structure.

5. Using Wet or Green Lumber

If your deck boards were installed while wet or not properly dried, they can warp or cup significantly as they dry out and shrink unevenly.

The Solution

• Buy kiln-dried or seasoned lumber when building or replacing boards.

• Let boards acclimate on-site for a few days before installation to adjust to local humidity.

• Replace severely cupped boards if drying alone doesn’t resolve the issue.

• Check the moisture content using a meter; wood should ideally be below 19% when installed.

Signs of Patio Damage Problems

Cupping is one red flag, but there are often other clues that your deck struggles with moisture or structural issues. Here’s what to watch for:

• Boards that curl up at the edges or feel uneven underfoot

• Splintering or cracking, especially near the board edges

• Boards pulling away from fasteners or rising above neighboring boards

• Mushy, soft wood or signs of rot underneath

• Persistent moisture or mold under the deck

• Stain or sealant that wears unevenly or peels prematurely

When to Call a Pro

If large areas of your deck are cupping, it's time to hire a pro. A contractor can inspect for rot, structural damage, or foundation issues and help you decide whether your deck is worth repairing. Homeowners can expect the average cost for deck repairs to be around $2,160, making it a more affordable option than a complete replacement if the damage isn't too far gone.

If repairs aren't enough, building a deck costs about $8,250 on average, depending on the size, materials, and labor cost in your neck of the woods. Wondering who can build a deck? Reach out to a local deck-building professional. These experts can walk you through design options, code requirements, and long-term maintenance plans.