When lapping siding, which part is the critical part for ensuring water does not penetrate?

The bottom edge of lapped siding is critical for preventing water penetration. This is because the bottom edge is where water is most likely to accumulate or be directed during rainfall. Proper lapping ensures that water runs down the siding and away from the structure rather than being trapped or seeping behind the siding.

When installed correctly, each course of siding overlaps the course below it, allowing any water to flow down and exit at the bottom rather than penetrate through the seams. The top edge and side edges, while important in the overall installation, do not experience the same risk of water pooling or running as the bottom edge does. Therefore, proper sealing and positioning at the bottom edge are crucial for an effective barrier against moisture intrusion, protecting the underlying structure from damage over time.