The function of the gap is to permit thermal expansion of the pipe. Without the gap, the heat of welding can cause the pipe to expand at a faster rate than the fitting. If the pipe bottoms against the socket fitting, the thermal growth of the pipe strains the weld, possibly producing cracks in the root.
Failures have also been observed in socket welds subjected to thermal and mechanical cycling during service. The mechanism is similar to
failure during welding. Rapid temperature changes cause the pipe to expand against the bottom of the fitting, straining and cracking the weld. Construction and Military codes such as ANSI, ASME, NAVSEA-250-1500-1, MIL-STD-278 and MIL STD-22 permit socket welds to be used, provided a minimum 1/16" (1.6 mm) is established between the pipe end and socket prior to welding.