A. Michael Casey, a licensed plumbing contractor in California and Connecticut, responds: Prolonged exposure to the sun may cause white PVC to discolor slightly - turning it yellow or tan - but there's little potential for actual damage. Tests have shown that prolonged exposure may in some cases slightly reduce the impact resistance of pipes and fittings, but not enough to cause system failure.
Still, nearly all plumbing codes - including North Carolina's - require DWV piping to be installed according to either the manufacturer's instructions or a recognized installation standard. PVC pipe manufacturers suggest applying a coating to protect against UV when long-term exposure is expected. Specifically, they recommend cleaning the pipes...
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