How much spacing you use to install your decking will depend on what kind of decking you are using and how much it will shrink. After wood decking is installed and has had an opportunity to stabilize with the atmosphere moisture conditions you want to maintain between 1/8" and ¼" gap between boards to promote drainage. Leaving a gap will also allow debris like dirt and leaves to fall in between the boards. The more wet the decking is the more it will shrink. If the decking material has been kiln dried you should gap them at least 1/8" because the boards will not shrink very much. Hidden fasteners will usually require a larger amount of spacing between boards. Check with your lumber supplier to see what they recommend for your choice of decking factoring in the regional climate. The thermal characteristics of wood will not allow it to expand with heat like plastic. If you are using a composite decking material you should make sure to follow the manufacturer’s installation instructions for decking spacing to properly account for thermal expansion.