Unfortunately the organic elements of tar paper require a perfectly dry day to lay flat.
Rain on partially shingle roof.
When tar paper gets wet it buckles and ripples.
While the completed shingles may not be in place during an unpredicted rainstorm there should be a protective layer on your roof that keeps rain from getting in.
In the midst of your roof installation joe hall roofing can weatherproof your roof.
Roofing in the rain not only means you or your roofing experts will be extremely wet uncomfortable and exposed to more risk but it also means that your new roof could have less integrity than if it were installed in dry conditions.
Nature has a mind of its own.
Shingles will not stick to a wet roof.
Roofing felt that has gotten wet can be covered with shingles as long as it is allowed to dry out first the surface underneath did not get wet and it did not wrinkle bubble or rip.
If it starts to rain or drizzle the roofers should act quickly to cover your osb most likely with a tarp.
Shingles and roofing materials may not adhere properly to slick surfaces with high humidity.
If the sheathing or wood is wet during the time of installation it can trap moisture inside of your roof leading to mold rot or other severe problems.