A shingle subfloor is used to protect roof structures and the interior from water penetration. Sweep the roof cover to remove dirt and debris. A felt base layer provides a barrier between the roof cover and the shingles. It helps repel water and, at the same time, provides some air to pass through.
This prevents the wood from being damaged and basically gives it a more attractive and durable roof. Felt also provides a safer work environment for the crew, as it provides them with better traction and helps prevent them from slipping.