Bad weather always seems to happen at inopportune times. This includes rain showers during the middle of a roofing project. This is one of the times where you do need to take some time off, or find a different project to work on, and wait for sunshine. Installing shingles over wet felt or roof sheathing can lead to later problems and may even pose a safety issue.
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Keep it Dry
Keep the roofing materials dry during transportation and storage. This includes the shingles and roofing felt. If the materials inadvertently are exposed to rain, allow everything to dry before attempting to install them. Make sure the roof deck is dry of any dew before beginning work in the morning. This often only takes a short period of sunshine.
Mold and Mildew
Roofing felt applied over a wet roof deck or shingles applied over wet roofing felt traps a layer of moisture within the roof. This moisture can lead to mold and mildew forming underneath the shingles. This shortens the effective life of the shingles and can adversely affect the roof sheathing. Roof replacement is expensive. Install the materials correctly and reduce the frequency you repeat the project.
Slipping and Sliding
A wet roof deck is also a slippery deck. For safety reasons, it is best only to work on the roof when all surfaces are dry. Wear soft-soled shoes and follow all common safety practices when working on the roof. To gain access to the roof, use proper ladders or scaffolding, which should also be dry to prevent slips.
Roofing tar is often used to seal shingles around vents, chimneys and other structures that extend through the roof surface. Roofing tar adheres best when applied to dry surfaces. This helps provides a waterproof seal and prevents leaks in these areas.