Generally, a 1/4-inch gap between deck boards is suitable. In addition to being pleasing to the eye, this gap will allow proper drainage of the deck without being so large that the ground under the deck shows through. A 1/4-inch gap is also ideal because as the deck boards expand and contract over time, this size of gap will allow them to do so without infringing on each other.
Setting the Space
When fastening the boards to the surface of your deck, using a tape measure or ruler to ensure you have a 1/4-inch gap is too time consuming. Instead, make several spacers out of wood that each measure 1/4-inch wide. After laying one board, have assistants hold the spacers along the edge of the board while you push the next board firmly against the spacers.
It's acceptable to use a 1/2-inch gap on decks, too. The visual difference between a 1/4-inch gap and a 1/2-inch gap is minimal, but certain situations call for a 1/2-inch gap. If you have a significant number of trees around the deck, their leaves can fall on the deck and block the gap enough that water will not drain. In this case, a 1/2-inch gap will promote drainage to avoid having standing water on the deck.
Being wet results in cut pieces of wood expanding. As such, avoid building the deck when the wood is wet. Instead, begin the project on a dry day with dry wood. Doing so provides you with a truer idea of the actual size of each board, which makes setting the space easier. If you worked with wet wood, for example, it would contract upon drying and you might not have the gap you desire.