What Makes Hardwood Decking Cup and How Can I Prevent It?

Posted by Chris Nolan on Wed, Sep 11, 2019 @ 06:09 AM

When there is an imbalance in a wood board, it may tend to cup. How does this happen? Occasionally (mostly with softwoods) the orientation of the grain in the board facilitates the board to cup. But it is external forces that causes the movement to happen. These forces (especially moisture) wreak the most havoc on hardwood deck boards. This is especially true with exterior woods, where there are many weather factors in play.

Garapa hardwood deck in Connecticut.jpgAdequate ventilation, proper drainage and proper fastening helps decks perform best  

Key Factors Causing Deck Boards to Cup

  • Too little clearance between the ground side moisture and the bottom of the deck boards 

  • Inadequate ventilation below the deck

  • Improper drainage below the deck

  • High exposure to sunlight above the deck

  • Minimal gap space between the deck boards

  • Insufficient fastening of the deck boards

Any of the individual factors listed above can create conditions for wood movement and deck boards to cup. When several of these conditions are present together, you can expect problems.  The combination of 'too moist' conditions on the bottom of the deck board and too dry on the top, creates stress in the wood. The wood will cup upwards naturally to relieve the internal stress.

Hardwood decking cupping- moisture below and sunlight above creates imbalance causing wood to move.jpgUnventilated, poorly drained deck shows wood deck boards cupping

How Do I Minimize the Amount of Cupping in Deck Boards?

Creating a balanced deck structure starts in the planning and design process. Here is a list of best demonstrated practices for long-lasting deck performance

  • Build decks as high above grade as possible. 30” is considered the minimum for best performance. Anything less than 30” is considered a ‘low clearance’ deck.

  • Make sure there is proper grading and adequate drainage to allow rain and bulk water to escape from below the deck. Standing (or sitting) water creates an unacceptably moist below-deck environment.

  • Plan adequate below-deck ventilation. Ventilation on three sides is ideal. Proper air flow helps eliminate unwanted moisture. Two sides with cross-ventilation is next best.

  • Always properly acclimate your deck boards to site conditions before installation.

  • Pre-finishing your deck boards with an appropriate sealer on all four sides, before installation, helps protect your investment.

 Cupped deck board was removed revealing wet board and mold growth below due to lack of ventilation and drainage problems.jpg

When the cupped deck boards in the previous picture were removed for inspection, the top of the boards were bone dry. The bottom of the deck boards are literally soaking wet. In addition to the expected cupping in these conditions, you can see black mold growing where the deck boards were sitting on the joists. This is a bad environment for the decking and the joists.

Additional Requirements for Best Performance of Low Clearance Decks

Low clearance decks are the worst-case scenario for deck boards. Limited air flow and imbalanced moisture conditions are extremely tough on wood decking. Special steps and design considerations are needed for best performance.

  • Use thicker deck boards. 21 mm or 5/4 thick nominal decking is heftier and less likely to cup as much as a thinner (1 x nominal thickness) deck board

  • Narrower boards work best. 4” nominal (3-1/2” actual) wide deck boards perform better than wider (6” nominal) deck boards.

  • 4” nominal deck boards expand and contract less than wider boards

  • Because there are more gaps between the deck boards, 4” nominal deck boards provide about 44% more top of deck ventilation than 6” nominal decking.

  • Face screws provide more secure fastening than hidden fasteners. This is especially important for low clearance decking.

We hope you enjoyed this article. Please visit for additional information about hardwood decking options, deck photo galleries, deck installation guidelines and lots more.

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Topics: Hardwood Decking