What material is effective in shedding water that gets under the shingles?

Felt paper, often referred to as tar paper, is an effective underlayment material used in roofing systems to help shed water that may penetrate or get under the shingles. This material acts as a barrier, preventing water from reaching the roof deck and causing potential damage. Felt paper is designed to be water-resistant and is typically placed under the shingles as a protective layer. This characteristic is vital in roofing to ensure that even if water gets under the shingles due to wind or rain, it is effectively redirected away from the roof structure. The other materials listed do not serve the same purpose in roofing systems. Wood shakes, while serving as a roofing material themselves, do not offer effective moisture protection beneath them. Polyethylene sheeting, primarily used as a vapor barrier in other construction applications, is not suitable for roofing underlayment. Fiberglass mesh, while useful in various applications, does not provide the same water-shedding capabilities as felt paper in this context.