What deck thickness should I get?

Hybrideck wood plastic decking in residential is 19mm thick .

Entrance step in Hybrideck wood plastic composite decking boards

This is the ideal thickness if you are replacing existing timber decking as most timber decking manufactured  specifically for the purpose of  deck builds is this thickness. The thickness of the board will determine the height restrictions that you have to take into consideration .  Additionally if you have an existing deck with the boards that need replacing you may have to be mindful that the swing opening to your door, the step up to your slider, the height of your door entries or even your windows are not affected if you end up using a thicker material.

Here plastic wood steps have been made to the correct height to meet legal requirements

This can also be the same if you have exit steps leading off the deck, as a thicker material could mean the step may become illegal if outside of the allowable ‘riser’ height.  When considering wood composite decking as a suitable  replacement material, while its no maintenance and good looks are important, the thickness and ultimately the height of the installed material could be the most important factor.