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

Written 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 powerful natural external forces that causes the most movement to happen. These forces, UV rays and moisture, wreak the most havoc on hardwood deck boards. This is especially true with exterior woods, where there are many weather factors in play.

How and why decking boards cup

The two most powerful conflicting external weather forces; solar rays and moisture, are at war with your decking. The heat on the top of the deck draws up the moisture from below the deck.

And then there is the poor deck board, caught in the middle of this epic struggle, literally pinned down in place. The top of the board is shrinking, and the bottom of the board is expanding. Help!!

If the deck boards could talk, they would be screaming in pain. When the external forces become too powerful for the wood, the wood will move, typically cupping upwards at the outside edges.

Key Factors Causing Deck Boards to Cup

Too much sun on the top, combined with too much moisture below.
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 internal stresses 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.jpg Unventilated, poorly drained deck shows wood deck boards cupping

We created a convenient PDF for you of this information:

Download PDF: Why Do Boards Cup and How To Prevent It?

How Can You Minimize the Amount of Cupping in Deck Boards?

Creating a balanced deck design and structure starts in the planning and design process. Short of putting a roof over your deck, there's not a whole lot of things you can do to minimize the sunlight. So, controlling and minimizing the amount of moisture below your deck is your best option.

Here is a list of some 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 below your deck. pitch the ground away from your home or structure. Get the water out of there! Standing (or sitting) water creates an unacceptably moist below-deck environment.
Make sure there is adequate drainage to allow rain and bulk water to escape from below the deck. Some landscape architects and master deck builders use landscape fabric below the deck to keep the moisture in the ground.
Some experts will then place a layer of gravel on top of the fabric to handle rain water from above. And since the ground is already pitched it flows out from below the deck at a controlled rate.
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.

Or, if you're a big risk taker, you can 'roll the dice' and take your chances, and see what happens. Like this failed installation:

Here's what can happen without proper prior planning

Oh my! When the cupped deck boards in this 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.

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

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 much 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.