Let’s start with the basics. What is roofing felt? When installing a roof, the order goes with your wood panels on the bottom, then roofing felt, then shingles on top of that.
The purpose is for it to act as an extra barrier to protect the wood, in case something happens to the top shingles. Everything on your roof is there to prevent the outside (rain water, debris, etc) from getting inside.
The common felt installed on most houses when they are built is also known as tar paper. It’s a black paper that’s soft and malleable under force.
If we’re talking about standard tar paper, the answer is a soft yes. It can get wet, but only to a certain extent. If you have tar paper on your house for a few days without the shingles there and you experience rain, don’t worry! The roofing felt is fine. However, if the roofing felt remains uncovered for a few weeks though, you might start to run into problems.
Tar paper is water repellent, but it’s not as good as synthetic models in terms of durability and how it sheds water. Consistent exposure to the elements can cause it to rip and damage it’s integrity.
You can, technically, but it’s recommended you let the roofing felt dry first. This will prevent any unwanted surprises, such as moisture in the felt turning into mildew/ mold or causing the OSB wood to buckle or warp.
There are! You can ask for a synthetic underlayment. Some of these products are designed to withstand 6 months of exposure. Though there should never be a situation where your roof has only a synthetic underlayment on it for 6 months without shingles covering it, it certainly makes a strong argument for its protective abilities.
It is also helpful for if you ever have any unknown shingle damage. So let’s say a wind storm comes through and pulls up a few shingles. Unglued shingles in storms make it easier for rain water to creep down through the exposed nail beds. With synthetic roofing materials, you have the added waterproof protection so you don’t have to worry about it seeping through to the wood. They’re also much harder to tear than regular tar paper, so you don’t have to worry about storm winds damaging the underlayment while the shingles are being lifted. Overall, you are much less likely to get a leak.