The best surface for your fire pit would be on a non-combustible material such as a concrete or gravel surface.
Decking - We do not recommend this, as heat can transfer through base and affect the condition of your decking or be a fire hazard. Also hot ash and coals could spill over the side of the fire pit.
To prevent this you would have to place some heat resistant insulating material underneath your fire pit, exceeding the overall width of your fire pit by a sufficient amount to protect against ash and coals falling out.
Grass - Heat can transfer through the base and affect grass. To prevent this you would have to place some concrete pavers or other heat resistant material underneath your fire pit.
Undercover - We do not recommend this.