For lavatories not affected by side walls, the minimum air gap with an effective opening not greater than 13 mm diameter is _____.

The minimum air gap requirement for lavatories not affected by side walls is established to prevent backflow and ensure proper drainage while maintaining sanitary conditions. An air gap functions as a physical separation between the water supply and any potential contaminants, effectively preventing the reverse siphoning or backflow of wastewater.

In scenarios where the effective opening is not greater than 13 mm in diameter, the specification calls for a minimum air gap of 25 mm (which is approximately 1 inch). This dimension is essential to guarantee that the gap is sufficient to safeguard against any backflow issues that might compromise the water quality.

When considering the other choices, ½ inch is slightly less than the minimum requirement, while 38 mm and 2 inches exceed the necessary air gap. While a larger gap can provide even greater protection, building standards outline specific minimums to maintain efficiency and uniformity in installations. Hence, the correct answer of 25 mm aligns with the standard regulations governing sanitary installations.