Final answer:
Untreated joists should be at least 18 inches above the ground to prevent issues like rot and insect damage, ensure proper ventilation and comply with most local building codes.
Step-by-step explanation:
Untreated joists should be positioned at least 18 inches above the ground to prevent moisture-related issues such as rot and insect damage. This is especially important for outdoor structures like decks and porches. Having a substantial distance between the untreated wood and the ground allows for proper ventilation and reduces the potential for moisture accumulation, which is a prerequisite for wood deterioration. In some local building codes, the required clearance may vary, but it is generally recognized that at least 18 inches is a safe standard to prevent ground contact that can lead to premature decay of the wood.