The louvre face provides directional airflow and offers some resistance to driven rain entering the subfloor void, making it a practical choice for exposed elevations or properties in areas with significant prevailing wind and rainfall. The square-grid (school board) pattern offers a larger open ventilation area but less weather protection.
![Mech 416 Cast Iron Soil Pipe System-BSEN1561/BSEN1563-87.5deg Long Radius Bend [MS]](https://tuscanfoundry.com/wp-content/uploads/2018/11/ms4008m-87.5deg-long-radius-bend.jpg)