Roofing felt is required to overlap 2 inches over the lower felt course. The overlapping felt creates a secondary barrier for moisture and wicks water away from the shingles above and the sheathing below. Therefore, a 2-inch overlap provides redundancy at the edges where the barrier is weakest.
Felt Layer Preferences
Felt underlayment can overlap more than 2 inches to provide an adequate barrier. Overlapping up to 6 inches can provide a superior felt layer; however, the additional overlap may alter the spacing of the felt courses relative to the sheathing, creating problematic conditions for fastening the felt to the sheathing.
Although overlapping felt more than 2 inches may provide extra protection, the 2-inch overlap is adequate for most applications and will provide an excellent roof. The additional overlap may only create problems when installing the shingles and felt over the roof structure. Therefore, use a 2-inch overlap for roof felt.