The shingles on your roof get all the publicity because they're easily seen, but the underlayment is the real hero in preventing water intrusion. Your roof can get by for a little while with some missing or damaged shingles, but if your underlayment is damaged, a roofing contractor should make a repair immediately to prevent possible water penetration.

Underlayment and your Roof System

The underlayment on your roof is installed over the sheathing and under the shingles to provide another barrier against water intrusion for your roof system. It wasn't that long ago that using underlayment was optional, but now just about all building codes require it to be installed. There are several types of underlayment commonly used in residential roofing and depending on the age of your home and whether you have ever had a new roof installed can determine which you have.

Asphalt felt--This underlayment is paper treated with asphalt for water resistance and it's available in several thicknesses. Most roofers use 15 pound felt as an underlayment, but the thicker 30 pound is also used. Felt paper can be purchased in rolls of various lengths depending on the thickness of the paper
Synthetic--Synthetic underlayment is more resistant to tearing than felt paper and many contractors feel it's easier to install. A big advantage is that it doesn't trap water beneath it as felt paper can, but instead allows it to pass through from underneath to avoid the possibility of moisture damage to the sheathing
Rubber membrane--This is normally self-adhering and is a completely waterproof underlayment that's ideal for areas prone to water leaks such as valleys or eaves

A roofing contractor can inspect your underlayment to ensure it's functioning as designed and if any repairs are needed, they should be done before any water damage occurs.