Design Shortfall:
Sometimes even the best designs can have shortfalls. Recently I observed this roof scupper in action. The design is aesthetic and made from corrosion resistant stainless steel. It carries water from the roof at right to the drainage basin below.
Problem is the water seems to fall outside the basin…
Evaluation:
So why does this happen?
Wind? Coriolis Effect?
Nope: The design failed to consider a basic property of water:
Water is adhesive. Think water droplets sticking to blades of grass. At lower flow velocities, the roof runoff does not have enough speed to break the adhesion with the scupper. Runoff then follows the shape of the scupper around the tip and back toward the structure. As droplet speed increases and the water finally separates from the scupper. Runoff travels in a direction opposite of what was intended.
Improvements:
What could be improved:
- Improve Geometry: Revising the angle of the scupper to a steeper angle will ensure the runoff drops in the basin below.
- Add a Local Low Point: Water runs downhill. The addition of a local low point at the tip of the scupper will prevent water from running down the underside of the scupper.
Read more regarding the unique properties of water at https://water.usgs.gov/edu/adhesion.html
