The best surface for your fire pit would be on a non-combustible material such as a concrete or gravel surface.
Decking - We generally do not recommend this, as heat can transfer through base and affect the condition of your decking. To prevent this you would have to place some concrete pavers underneath your fire pit, exceeding the overall width of your fire pit. This does not fully protect the decking from any stray embers.
Grass - Heat can transfer through the base and affect the colour of the grass. To prevent this you would have to place some concrete pavers underneath your fire pit.
Undercover - We do not recommend this, as any smoke emissions may discolour the roofing.