Some building codes have specific requirements for underlayment and ice dam protection. The requirements of these codes must be followed. No roof is better than the quality of its installation.
Asphalt roofing materials are no exception. They are designed to give years of service when applied carefully and correctly.
Underlayment material used beneath roofing shingles is recommended to comply with one of the following: ASTM D226, ASTM D4869, ASTM D6757, ASTM D1970, or as recommended by the individual shingle manufacturer.
Asphalt shingles may be used on slopes ranging from 2”-4” per foot if special procedures are followed. Never use shingles on slopes lower than 2” per foot. Low slopes can lead to problems because water drains slowly from these slopes, creating the greater possibility of water backup and damage from ice dams. The special application method described below for applying shingles on low slopes ensures that the roof remains weathertight.
On low slope applications, cover the deck with two layers of non-perforated asphalt saturated felt or two layers of a synthetic underlayment. One layer of an appropriate self-adhering modified bitumen membrane can also be used in place of a double layer of felt or synthetic.
When using a double layer of felt or synthetic the starter coursed should be one half of the width of the underlayment plus 1”. Example: a 37” wide roll of felt will have a starter course of 19” while a 48” wide roll of synthetic will have a starter course of 25”. Place a full width sheet over the starter, with the long edge placed along the eaves and completely overlapping the initial starter course. All succeeding courses will be a minimum of the full roll wide and should be positioned to overlap the preceding course by one half the roll width plus 1”. Secure each course by using only enough fasteners to hold it in place until the shingles are applied. End laps should be 12” wide and located at least 6 feet from end laps in the preceding course. Instructions are different for self-adhering products because they are not required to be installed in two layers.