What can impede the drainage of water on a roof?

Chimneys and skylights can impede the drainage of water on a roof due to their structural presence and the way they interact with the flow of water. When roofing elements such as chimneys are installed, they can create obstructions that prevent the free flow of water towards the gutters or drainage areas. Similarly, skylights, if not properly installed or flashed, can accumulate water around their perimeters, leading to potential pooling and drainage issues.

In contrast, wide soffits may help to ventilate and direct airflow but do not themselves create significant obstacles to water drainage. Flat roofs might have unique drainage challenges due to their design, but this is not limited to just flat roofs—pitfalls can occur on sloped roofs as well when structures like chimneys and skylights are involved. As for new roofing materials, while they can play a role in overall roof performance and drainage efficiency, they are not inherently a factor that blocks water from draining effectively.