Purpose of Roofing Felt
Roofing felt is laid between the wooden deck of the roof and the first layer of asphalt or fiberglass shingles. The felt serves a two-fold purpose. First it acts as a sealed vapor barrier so moisture cannot move from the outdoor elements into attics or upper floor crawl spaces. Secondly it acts as a backup to the shingles in case the shingles pull free and expose the roofing area. This backup is only a temporary measure during storms when most damage to roofs can occur.
Impregnated Tar and Weight
Tar is impregnated into the rolls of roofing felt. This tar enables the roofing nails or staples to seal any openings that occur during the fastening stage. Generally the rolls overlap each other when laid out in a horizontal pattern over the roof. A double or triple layer of felt maybe laid down in valleys and roof peak areas. This ensures that wind damage will not open the roof peak area. Multiple layers placed in the valleys of roofs ensure that any ice build up will not force its way into attic spaces. In most cases the heavier weight of felt is utilized on roofs with severe climates. This can include homes in northern climates with heavy snows. Other coastal regions may also use a heavy felt paper due to heavy rains from tropical storms and hurricanes.
Synthetic and Ice Barriers
Synthetic roofing underlayments are becoming more widespread due to its lighter weight and resistance to fire. Some local municipalities require the use of special underlayments as fire retardants for some roofing systems. In some cases the synthetic roofing underlayments cover the entire roof area with one large sheet. This material is less prone to tearing and has a moisture release system that allows the roof to "breathe" out moisture. Rubber membrane ice barriers are now used near the lower edges of roofs and in the valley areas. The thicker membrane resists the ice from working underneath the shingles. Many local regulations now require custom underlayments for specialized roofing systems. Not all regulations are consistent across the country. Check with your local building inspection department for any rules regarding roofing systems and felt underlayment.