Daniel needs a flood sensor network for his in-laws' Connecticut basement, but two concrete slabs between the basement and top floor kill ZigBee signals. We break down the physics of radio attenuation through concrete, explain why LoRaWAN at 868 MHz handily outperforms 2.4 GHz ZigBee in these conditions, and walk through a complete bill of materials — gateway, sensors, Raspberry Pi, and local alerting — for about $320. Plus real-world data from a Vermont vacation home deployment that detected a flood within three minutes with zero false alarms over two winters.