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.
![Halifax Cast Iron Drain System BSEN877:1999-Ductile Iron Coupling [HD]](https://tuscanfoundry.com/wp-content/uploads/2018/11/hd4012-ductile-iron-coupling.jpg)