Can shingles be installed on a wet roof?

Shingles should never be installed on a wet roof deck or during rainy conditions, and doing so violates manufacturer installation instructions and building codes. Professional roofing contractors will stop work if the deck is wet. There are several important reasons. First, wet plywood or OSB decking can trap moisture underneath the underlayment and shingles, leading to wood rot, mold growth, and reduced nail holding strength as the wood swells and later shrinks. Second, most underlayments, especially synthetic ones, become extremely slippery when wet, creating a serious fall hazard for roofers. Third, asphalt shingles have a thermally activated sealant strip that needs sun and heat to bond properly. Installing them wet prevents proper sealing, making them vulnerable to wind blow-off. Fourth, roofing cement and sealants will not adhere properly to wet surfaces. Fifth, wet conditions can cause the shingle backing to trap moisture, leading to blistering later. The roof deck must be dry, and the underlayment must be installed on a dry deck. If it starts raining during a tear-off, a professional crew should immediately cover the exposed deck with a tarp and stop installation until the deck dries completely. Installing over damp morning dew is also not recommended. Always wait for a dry day with no rain forecast for at least 24 hours.